On the Origin of Species

MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY: EMBRYOLOGY: RUDIMENTARY ORGANS.

Classification: groups subordinate to groups. Natural system. Rules and difficulties in classification, explained on the theory of descent with modification. Classification of varieties. Descent always used in classification. Analogical or adaptive characters. Affinities: general, complex, and radiating. Extinction separates and defines groups. Morphology: between members of the same class and between parts of the same individual. Embryology: its laws explained by variations not supervening at an early age and being inherited at a corresponding age. Rudimentary organs: their origin explained. Summary.

From the first dawn of life, all organic beings are found to resemble each other in descending degrees, so that they can be classed in groups under groups. This classification is evidently not arbitrary like the grouping of the stars in constellations. The existence of groups would have had a simple significance if one group had been exclusively fitted to inhabit the land and another the water, one to feed on flesh and another on vegetable matter, and so on; but the case is widely different in nature, for it is well known how commonly members of even the same subgroup have different habits. In our second and fourth chapters, on variation and on natural selection, I have attempted to show that it is the widely ranging, much diffused, and common species—that is, the dominant species—belonging to the larger genera that vary most. The varieties, or incipient species, thus produced ultimately become converted, as I believe, into new and distinct species; and these, on the principle of inheritance, tend to produce other new and dominant species. Consequently, the groups that are now large, and that generally include many dominant species, tend to go on increasing indefinitely in size. I further attempted to show that, because the varying descendants of each species try to occupy as many and as different places as possible in the economy of nature, there is a constant tendency in their characters to diverge. I supported this conclusion by looking at the great diversity of the forms of life that, in any small area, come into the closest competition, and by looking at certain facts of naturalization.

I also attempted to show that there is a constant tendency in the forms that are increasing in number and diverging in character to supplant and exterminate the less divergent, the less improved, and the preceding forms. I ask the reader to turn to the diagram illustrating the action, as explained earlier, of these several principles; he will see that the inevitable result is that the modified descendants proceeding from one progenitor become broken up into groups subordinate to groups. In the diagram, each letter on the uppermost line may represent a genus including several species, and all the genera on this line together form one class, for all have descended from one ancient but unseen parent and consequently have inherited something in common. But the three genera on the left have, on this same principle, much in common and form a subfamily, distinct from the one including the next two genera on the right, which diverged from a common parent at the fifth stage of descent. These five genera also have much, though less, in common, and they form a family distinct from the one including the three genera still farther to the right, which diverged at a still earlier period. And all these genera, descended from (A), form an order distinct from the genera descended from (I). Here, then, we have many species descended from a single progenitor grouped into genera, and the genera are included in, or subordinate to, subfamilies, families, and orders, all united into one class. Thus, the grand fact in natural history of the subordination of group under group, which, from its familiarity, does not always sufficiently strike us, is in my judgment fully explained.

Naturalists try to arrange the species, genera, and families in each class on what is called the natural system. But what is meant by this system? Some authors regard it merely as a scheme for arranging together those living objects that are most alike and for separating those that are most unlike, or as an artificial means of enunciating general propositions as briefly as possible—that is, of giving in one sentence the characters common, for instance, to all mammals, in another those common to all the Carnivora, in another those common to the dog genus, and then, by adding a single sentence, of giving a full description of each kind of dog. The ingenuity and utility of this system are indisputable. But many naturalists think that something more is meant by the natural system; they believe that it reveals the plan of the Creator. But unless it is specified whether order in time or in space, or something else, is meant by the plan of the Creator, it seems to me that nothing is thus added to our knowledge. Such expressions as that famous one of Linnaeus, which we often meet with in a more or less concealed form—that the characters do not make the genus, but that the genus gives the characters—seem to imply that something more than mere resemblance is included in our classification. I believe that something more is included, and that propinquity of descent—the only known cause of the similarity of organic beings—is the bond, hidden as it is by various degrees of modification, that is partially revealed to us by our classifications.

Let us now consider the rules followed in classification and the difficulties that are encountered on the view that classification either expresses some unknown plan of creation or is simply a scheme for enunciating general propositions and for placing together the forms most like each other. It might have been thought (and in ancient times it was thought) that those parts of the structure that determined the habits of life and the general place of each being in the economy of nature would be of very high importance in classification. Nothing can be more false. No one regards the external similarity of a mouse to a shrew, of a dugong to a whale, or of a whale to a fish as of any importance. These resemblances, though so intimately connected with the whole life of the being, are ranked as merely “adaptive or analogical characters”; but we will have to return to the consideration of these resemblances. It may even be given as a general rule that the less any part of the organization is concerned with special habits, the more important it becomes for classification. As an instance, Owen, in speaking of the dugong, says, “I have always regarded the generative organs, being those that are most remotely related to the habits and food of an animal, as affording very clear indications of its true affinities. In the modifications of these organs, we are least likely to mistake a merely adaptive character for an essential one.” So with plants, how remarkable it is that the organs of vegetation, on which their whole life depends, are of little significance except in the first main divisions, whereas the organs of reproduction, with their product the seed, are of paramount importance!

We must not, therefore, in classifying, trust to resemblances in parts of the organization, however important they may be for the welfare of the being in relation to the outer world. Perhaps it is partly for this reason that almost all naturalists lay the greatest stress on resemblances in organs of high vital or physiological importance. No doubt this view of the classificatory importance of organs that are important is generally, but by no means always, true. But their importance for classification, I believe, depends on their greater constancy throughout large groups of species; and this constancy depends on such organs having generally been subjected to less change in the adaptation of the species to their conditions of life. That the mere physiological importance of an organ does not determine its classificatory value is almost shown by the single fact that, in allied groups in which the same organ, as we have every reason to suppose, has nearly the same physiological value, its classificatory value is widely different. No naturalist can have worked on any group without being struck by this fact, and it has been most fully acknowledged in the writings of almost every author. It will suffice to quote the highest authority, Robert Brown, who, in speaking of certain organs in the Proteaceae, says that their generic importance, “like that of all their parts, not only in this but, as I believe, in every natural family, is very unequal, and in some cases seems to be entirely lost.” Again, in another work he says that the genera of the Connaraceae “differ in having one or more ovaries, in the existence or absence of albumen, in the imbricate or valvular estivation. Any one of these characters singly is frequently of more than generic importance, though here, even when all are taken together, they appear insufficient to separate Cnestis from Connarus.” To give an example among insects: in one great division of the Hymenoptera, the antennae, as Westwood has remarked, are most constant in structure; in another division they differ much, and the differences are of quite subordinate value in classification; yet probably no one will say that the antennae in these two divisions of the same order are of unequal physiological importance. Any number of instances could be given of the varying importance for classification of the same important organ within the same group of beings.

Again, no one will say that rudimentary or atrophied organs are of high physiological or vital importance; yet, undoubtedly, organs in this condition are often of high value in classification. No one will dispute that the rudimentary teeth in the upper jaws of young ruminants, and certain rudimentary bones of the leg, are highly serviceable in exhibiting the close affinity between ruminants and pachyderms. Robert Brown has strongly insisted on the fact that the rudimentary florets are of the highest importance in the classification of the grasses.

Numerous instances could be given of characters derived from parts that must be considered of very trifling physiological importance but that are universally admitted to be highly serviceable in the definition of whole groups. Such characters include, for instance, whether or not there is an open passage from the nostrils to the mouth, which, according to Owen, is the only character that absolutely distinguishes fishes and reptiles; the inflection of the angle of the jaws in marsupials; the manner in which the wings of insects are folded; mere color in certain Algae; mere pubescence on parts of the flower in grasses; and the nature of the dermal covering, such as hair or feathers, in the Vertebrata. If the Ornithorhynchus had been covered with feathers instead of hair, this external and trifling character would, I think, have been considered by naturalists as important an aid in determining the degree of affinity of this strange creature to birds and reptiles as an approach in structure in any one internal and important organ.

The importance of trifling characters for classification depends mainly on their being correlated with several other characters of more or less importance. The value of an aggregate of characters is, indeed, very evident in natural history. Hence, as has often been remarked, a species may depart from its allies in several characters, both of high physiological importance and of almost universal prevalence, and yet leave us in no doubt where it should be ranked. Hence, also, it has been found that a classification founded on any single character, however important that character may be, has always failed, for no part of the organization is universally constant. The importance of an aggregate of characters, even when none are important, alone explains, I think, that saying of Linnaeus that the characters do not give the genus, but the genus gives the characters; for this saying seems founded on an appreciation of many trifling points of resemblance too slight to be defined. Certain plants belonging to the Malpighiaceae bear perfect and degraded flowers; in the latter, as A. de Jussieu has remarked, “the greater number of the characters proper to the species, to the genus, to the family, and to the class disappear, and thus laugh at our classification.” But when Aspicarpa produced in France, for several years, only degraded flowers, which departed so wonderfully from the proper type of the order in a number of the most important points of structure, M. Richard nevertheless sagaciously saw, as Jussieu observes, that this genus should still be retained among the Malpighiaceae. This case seems to me to illustrate well the spirit in which our classifications are sometimes necessarily founded.

In practice, when naturalists are at work, they do not trouble themselves about the physiological value of the characters that they use in defining a group or in placing any particular species. If they find a character nearly uniform, common to a great number of forms, and not common to others, they use it as one of high value; if it is common to some smaller number, they use it as one of subordinate value. Some naturalists have broadly acknowledged this principle to be the true one, and none more clearly than that excellent botanist Auguste de Saint-Hilaire. If certain characters are always found correlated with others, though no apparent bond of connection can be discovered between them, special value is set on them. Because in most groups of animals important organs, such as those for propelling the blood, for aerating it, or for propagating the race, are found to be nearly uniform, they are considered highly serviceable in classification; but in some groups of animals all these, the most important vital organs, are found to offer characters of quite subordinate value.

We can see why characters derived from the embryo should be of equal importance to those derived from the adult, for our classifications of course include all ages of each species. But it is by no means obvious, on the ordinary view, why the structure of the embryo should be more important for this purpose than that of the adult, which alone plays its full part in the economy of nature. Yet those great naturalists Milne-Edwards and Agassiz have strongly urged that embryonic characters are the most important of all in the classification of animals, and this doctrine has very generally been admitted as true. The same fact holds good for flowering plants, whose two main divisions have been founded on characters derived from the embryo—on the number and position of the embryonic leaves, or cotyledons, and on the mode of development of the plumule and radicle. In our discussion of embryology, we will see why such characters are so valuable on the view that classification tacitly includes the idea of descent.

Our classifications are often plainly influenced by chains of affinities. Nothing can be easier than to define a number of characters common to all birds; but in the case of crustaceans, such a definition has until now been found impossible. There are crustaceans at the opposite ends of the series that have hardly a character in common; yet the species at both ends, because they are plainly allied to others, and these to others, and so onward, can be recognized as unequivocally belonging to this class of the Articulata and to no other.

Geographical distribution has often been used in classification, though perhaps not quite logically, more especially in very large groups of closely allied forms. Temminck insists on the utility, or even necessity, of this practice in certain groups of birds, and it has been followed by several entomologists and botanists.

Finally, with respect to the comparative value of the various groups of species, such as orders, suborders, families, subfamilies, and genera, these values seem to be, at least at present, almost arbitrary. Several of the best botanists, such as Mr. Bentham and others, have strongly insisted on their arbitrary value. Instances could be given among plants and insects of a group of forms first ranked by practiced naturalists as only a genus and then raised to the rank of a subfamily or family; and this has been done not because further research has detected important structural differences, at first overlooked, but because numerous allied species, with slightly different grades of difference, have subsequently been discovered.

All the foregoing rules, aids, and difficulties in classification are explained, if I do not greatly deceive myself, on the view that the natural system is founded on descent with modification; that the characters that naturalists regard as showing true affinity between any two or more species are those that have been inherited from a common parent, and, to that extent, all true classification is genealogical; and that community of descent is the hidden bond that naturalists have been unconsciously seeking, and not some unknown plan of creation, or the enunciation of general propositions, or the mere putting together and separating of objects more or less alike.

But I must explain my meaning more fully. I believe that the arrangement of the groups within each class, in due subordination and relation to the other groups, must be strictly genealogical in order to be natural, but that the amount of difference in the several branches or groups, though they are allied in the same degree in blood to their common progenitor, may differ greatly, being due to the different degrees of modification that they have undergone; and this is expressed by the forms being ranked under different genera, families, sections, or orders. The reader will best understand what is meant if he will take the trouble to refer to the diagram in the fourth chapter. We will suppose that the letters A to L represent allied genera that lived during the Silurian epoch and that these have descended from a species that existed at an unknown earlier period. Species of three of these genera (A, F, and I) have transmitted modified descendants to the present day, represented by the fifteen genera (a14 to z14) on the uppermost horizontal line. Now all these modified descendants from a single species are represented as related in blood or descent to the same degree; they may metaphorically be called cousins to the same millionth degree; yet they differ widely and in different degrees from each other. The forms descended from A, now broken up into two or three families, constitute an order distinct from that formed by those descended from I, which are also broken up into two families. The existing species descended from A cannot be ranked in the same genus as the parent A, nor can those descended from I be ranked with the parent I. But the existing genus F14 may be supposed to have been only slightly modified, and it will then rank with the parent genus F, just as a few organic beings that are still living belong to Silurian genera. Thus, the amount or value of the differences between organic beings all related to each other in the same degree in blood has come to be widely different. Nevertheless, their genealogical arrangement remains strictly true, not only at the present time but at each successive period of descent. All the modified descendants from A will have inherited something in common from their common parent, as will all the descendants from I; so it will be with each subordinate branch of descendants at each successive period. If, however, we choose to suppose that any of the descendants of A or of I have been so much modified as to have more or less completely lost traces of their parentage, then their places in a natural classification will have been more or less completely lost, as sometimes seems to have occurred with existing organisms. All the descendants of the genus F, along its whole line of descent, are supposed to have been only slightly modified, and they still form a single genus. But this genus, though much isolated, will still occupy its proper intermediate position, for F was originally intermediate in character between A and I, and the several genera descended from these two genera will have inherited their characters to a certain extent. This natural arrangement is shown in the diagram, as far as is possible on paper, but in much too simple a manner. If a branching diagram had not been used, and only the names of the groups had been written in a linear series, it would have been still less possible to give a natural arrangement; and it is well known that the affinities we discover in nature among the beings of the same group cannot be represented in a series on a flat surface. Thus, on the view that I hold, the natural system is genealogical in its arrangement, like a pedigree; but the degrees of modification that the different groups have undergone have to be expressed by ranking them under different so-called genera, subfamilies, families, sections, orders, and classes.

It may be worthwhile to illustrate this view of classification by taking the case of languages. If we possessed a perfect pedigree of mankind, a genealogical arrangement of the races of man would afford the best classification of the various languages now spoken throughout the world; and if all extinct languages, and all intermediate and slowly changing dialects, had to be included, such an arrangement would, I think, be the only possible one. Yet it might be that some very ancient language had altered little and had given rise to few new languages, while others (owing to the spreading and subsequent isolation and the states of civilization of the several races descended from a common race) had altered much and had given rise to many new languages and dialects. The various degrees of difference in the languages from the same stock would have to be expressed by groups subordinate to groups; but the proper, or even the only possible, arrangement would still be genealogical; and this would be strictly natural, as it would connect together all languages, extinct and modern, by the closest affinities and would give the filiation and origin of each tongue.

In confirmation of this view, let us glance at the classification of varieties, which are believed or known to have descended from one species. These are grouped under species, with subvarieties under varieties; and with our domestic plants and animals, several other grades of difference are requisite, as we have seen with pigeons. The origin of the existence of groups subordinate to groups is the same with varieties as with species, namely, closeness of descent with various degrees of modification. Nearly the same rules are followed in classifying varieties as in classifying species. Authors have insisted on the necessity of classing varieties on a natural instead of an artificial system; we are cautioned, for instance, not to class two varieties of the pineapple together merely because their fruit, though the most important part, happens to be nearly identical; no one puts the Swedish and common turnips together, though their edible, thickened stems are so similar. Whatever part is found to be most constant is used in classing varieties: thus the great agriculturist Marshall says that the horns are very useful for this purpose with cattle because they are less variable than the shape or color of the body, etc., whereas with sheep the horns are much less serviceable because they are less constant. In classing varieties, I believe that if we had a real pedigree, a genealogical classification would be universally preferred, and it has been attempted by some authors. For we might feel sure that, whether there had been more or less modification, the principle of inheritance would keep together the forms that were allied in the greatest number of points. In tumbler pigeons, though some subvarieties differ from the others in the important character of having a longer beak, all are kept together because they have the common habit of tumbling; but the short-faced breed has nearly or quite lost this habit. Nevertheless, without any reasoning or thinking on the subject, these tumblers are kept in the same group because they are allied in blood and alike in some other respects. If it could be proved that the Khoikhoi had descended from Black Africans, I think they would be classed under the Black African group, however much they might differ from Black Africans in color and other important characters.

With species in a state of nature, every naturalist has in fact brought descent into his classification, for he includes in his lowest grade, that of a species, the two sexes; and every naturalist knows how enormously these sometimes differ in the most important characters. Scarcely a single fact can be predicated in common of the adult males and hermaphrodites of certain cirripedes, and yet no one dreams of separating them. The naturalist includes as one species the several larval stages of the same individual, however much they may differ from each other and from the adult, and he likewise includes the so-called alternate generations of Steenstrup, which can be considered the same individual only in a technical sense. He includes monsters; he includes varieties, not solely because they closely resemble the parent form, but because they are descended from it. Anyone who believes that the cowslip is descended from the primrose, or the reverse, ranks them together as a single species and gives a single definition. As soon as three orchid forms (Monochanthus, Myanthus, and Catasetum), which had previously been ranked as three distinct genera, were known to be sometimes produced on the same spike, they were immediately included as a single species. But it may be asked: what ought we to do if it could be proved that one species of kangaroo had been produced, by a long course of modification, from a bear? Ought we to rank this one species with bears, and what should we do with the other species? The supposition is, of course, preposterous, and I might answer with the argumentum ad hominem and ask what should be done if a perfect kangaroo were seen to come out of the womb of a bear. According to all analogy, it would be ranked with bears; but then assuredly all the other species of the kangaroo family would have to be classed under the bear genus. The whole case is preposterous, for where there has been close descent in common, there will certainly be close resemblance or affinity.

Descent has universally been used in classing together the individuals of the same species, though the males, females, and larvae are sometimes extremely different, and it has been used in classing varieties that have undergone a certain, and sometimes a considerable, amount of modification. May not this same element of descent, then, have been unconsciously used in grouping species under genera, and genera under higher groups, though in these cases the modification has been greater in degree and has taken a longer time to complete? I believe it has thus been unconsciously used, and only in this way can I understand the several rules and guides that have been followed by our best systematists. We have no written pedigrees; we have to make out community of descent by resemblances of any kind. Therefore, we choose those characters that, as far as we can judge, are the least likely to have been modified in relation to the conditions of life to which each species has been recently exposed. On this view, rudimentary structures are as good as, or sometimes even better than, other parts of the organization. We do not care how trifling a character may be—whether it is the mere inflection of the angle of the jaw, the manner in which an insect’s wing is folded, or the covering of the skin by hair or feathers. If it prevails throughout many and different species, especially those having very different habits of life, it assumes high value, for we can account for its presence in so many forms with such different habits only by its inheritance from a common parent. We may err in this respect in regard to single points of structure, but when several characters, however trifling they may be, occur together throughout a large group of beings having different habits, we may feel almost sure, on the theory of descent, that these characters have been inherited from a common ancestor. And we know that such correlated or aggregated characters have special value in classification.

We can understand why a species or a group of species may depart from its allies in several of its most important characteristics and yet be safely classed with them. This may be safely done, and is often done, as long as a sufficient number of characters, however unimportant they may be, betray the hidden bond of community of descent. Two forms may not have a single character in common, yet if these extreme forms are connected together by a chain of intermediate groups, we may at once infer their community of descent, and we put them all into the same class. Because we find that organs of high physiological importance—those that serve to preserve life under the most diverse conditions of existence—are generally the most constant, we attach special value to them; but if these same organs are found to differ much in another group or section of a group, we at once value them less in our classification. We will later, I think, clearly see why embryological characters are of such high classificatory importance. Geographical distribution may sometimes be brought usefully into play in classing large and widely distributed genera, because all the species of the same genus that inhabit any distinct and isolated region have in all probability descended from the same parents.

We can understand, on these views, the very important distinction between real affinities and analogical or adaptive resemblances. Lamarck first called attention to this distinction, and he has been ably followed by Macleay and others. The resemblance in the shape of the body and in the finlike anterior limbs between the dugong, which is a pachydermatous animal, and the whale, and between both these mammals and fishes, is analogical. Among insects there are innumerable instances: thus Linnaeus, misled by external appearances, actually classed a homopterous insect as a moth. We see something of the same kind even in our domestic varieties, as in the thickened stems of the common and Swedish turnips. The resemblance between the greyhound and the racehorse is hardly more fanciful than the analogies that have been drawn by some authors between very distinct animals. On my view that characters are of real importance for classification only insofar as they reveal descent, we can clearly understand why analogical or adaptive characters, although of the utmost importance to the welfare of the being, are almost valueless to the systematist. For animals belonging to two very distinct lines of descent may readily become adapted to similar conditions and thus assume a close external resemblance; but such resemblances will not reveal—will rather tend to conceal—their blood relationship to their proper lines of descent. We can also understand the apparent paradox that the very same characters are analogical when one class or order is compared with another but reveal true affinities when the members of the same class or order are compared with one another. Thus, the shape of the body and the finlike limbs are only analogical when whales are compared with fishes, being adaptations in both classes for swimming through the water; but the shape of the body and the finlike limbs serve as characters exhibiting true affinity among the several members of the whale family, for these cetaceans agree in so many characters, great and small, that we cannot doubt that they have inherited their general shape of body and structure of limbs from a common ancestor. So it is with fishes.

Because members of distinct classes have often been adapted by successive slight modifications to live under nearly similar circumstances—to inhabit, for instance, the three elements of land, air, and water—we can perhaps understand how it is that a numerical parallelism has sometimes been observed between the subgroups in distinct classes. A naturalist struck by a parallelism of this nature in any one class could easily, by arbitrarily raising or lowering the value of the groups in other classes (and all our experience shows that this valuation has so far been arbitrary), extend the parallelism over a wide range; and in this way the septenary, quinary, quaternary, and ternary classifications have probably arisen.

Because the modified descendants of dominant species belonging to the larger genera tend to inherit the advantages that made the groups to which they belong large and their parents dominant, they are almost sure to spread widely and to seize on more and more places in the economy of nature. The larger and more dominant groups thus tend to go on increasing in size, and they consequently supplant many smaller and feebler groups. Thus we can account for the fact that all organisms, recent and extinct, are included under a few great orders, under still fewer classes, and all in one great natural system. As evidence of how few the higher groups are in number and how widely spread they are throughout the world, it is a striking fact that the discovery of Australia has not added a single insect belonging to a new order, and that in the vegetable kingdom, as I learn from Dr. Hooker, it has added only two or three orders of small size.

In the chapter on geological succession, I attempted to show, on the principle of each group having generally diverged much in character during the long-continued process of modification, how it is that the more ancient forms of life often present characters in some slight degree intermediate between existing groups. A few old and intermediate parent forms that have occasionally transmitted only slightly modified descendants to the present day will give us our so-called osculant or aberrant groups. The more aberrant any form is, the greater must be the number of connecting forms that, on my theory, have been exterminated and utterly lost. And we have some evidence that aberrant forms have suffered severely from extinction, for they are generally represented by extremely few species, and such species as do occur are generally very distinct from each other, which again implies extinction. The genera Ornithorhynchus and Lepidosiren, for example, would not have been less aberrant had each been represented by a dozen species instead of by a single one; but such richness in species, as I find after some investigation, does not commonly fall to the lot of aberrant genera. We can, I think, account for this fact only by regarding aberrant forms as failing groups conquered by more successful competitors, with a few members preserved by some unusual coincidence of favorable circumstances.

Mr. Waterhouse has remarked that when a member belonging to one group of animals exhibits an affinity to a quite distinct group, this affinity in most cases is general and not special. Thus, according to Mr. Waterhouse, of all rodents the bizcacha is most nearly related to marsupials; but in the points in which it approaches this order, its relations are general and not to any one marsupial species more than to another. Because the points of affinity of the bizcacha to marsupials are believed to be real and not merely adaptive, they are due, on my theory, to inheritance in common. Therefore, we must suppose either that all rodents, including the bizcacha, branched off from some very ancient marsupial, which will have had a character in some degree intermediate with respect to all existing marsupials, or that both rodents and marsupials branched off from a common progenitor and that both groups have since undergone much modification in divergent directions. On either view we may suppose that the bizcacha has retained, by inheritance, more of the character of its ancient progenitor than other rodents have; therefore it will not be specially related to any one existing marsupial but indirectly related to all or nearly all marsupials, because it has partially retained the character of their common progenitor or of an early member of the group. On the other hand, as Mr. Waterhouse has remarked, of all marsupials the phascolomys most nearly resembles not any one species but the general order of rodents. In this case, however, it may be strongly suspected that the resemblance is only analogical, owing to the phascolomys having become adapted to habits like those of a rodent. The elder de Candolle has made nearly similar observations on the general nature of the affinities of distinct orders of plants.

On the principle of the multiplication and gradual divergence in character of the species descended from a common parent, together with their retention by inheritance of some characters in common, we can understand the excessively complex and radiating affinities by which all the members of the same family or higher group are connected together. For the common parent of a whole family of species, now broken up by extinction into distinct groups and subgroups, will have transmitted some of its characters, modified in various ways and degrees, to all; and the several species will consequently be related to each other by circuitous lines of affinity of various lengths (as may be seen in the diagram so often referred to), mounting up through many predecessors. As it is difficult to show the blood relationship between the numerous kindred of any ancient and noble family even with the aid of a genealogical tree, and almost impossible to do so without this aid, we can understand the extraordinary difficulty that naturalists have experienced in describing, without the aid of a diagram, the various affinities that they perceive between the many living and extinct members of the same great natural class.

Extinction, as we have seen in the fourth chapter, has played an important part in defining and widening the intervals between the several groups in each class. We may thus account even for the distinctness of whole classes from each other—for instance, of birds from all other vertebrate animals—by the belief that many ancient forms of life have been utterly lost, through which the early progenitors of birds were formerly connected with the early progenitors of the other vertebrate classes. There has been less complete extinction of the forms of life that once connected fishes with batrachians. There has been still less in some other classes, as in that of the Crustacea, for here the most wonderfully diverse forms are still tied together by a long but broken chain of affinities. Extinction has only separated groups; it has by no means made them. For if every form that has ever lived on this earth were suddenly to reappear, it would be quite impossible to give definitions by which each group could be distinguished from other groups, as all would blend together by steps as fine as those between the finest existing varieties; nevertheless, a natural classification, or at least a natural arrangement, would be possible. We will see this by turning to the diagram: the letters A to L may represent eleven Silurian genera, some of which have produced large groups of modified descendants. Every intermediate link between these eleven genera and their primordial parent, and every intermediate link in each branch and sub-branch of their descendants, may be supposed to be still alive, and the links to be as fine as those between the finest varieties. In this case, it would be quite impossible to give any definition by which the several members of the several groups could be distinguished from their more immediate parents, or these parents from their ancient and unknown progenitor. Yet the natural arrangement in the diagram would still hold good, and, on the principle of inheritance, all the forms descended from A, or from I, would have something in common. In a tree we can specify this or that branch, though at the actual fork the two unite and blend together. We could not, as I have said, define the several groups; but we could pick out types, or forms, representing most of the characters of each group, whether large or small, and thus give a general idea of the value of the differences between them. This is what we would be driven to if we were ever to succeed in collecting all the forms in any class that have lived throughout all time and space. We will certainly never succeed in making so perfect a collection; nevertheless, in certain classes we are tending in this direction, and Milne-Edwards has recently insisted, in an able paper, on the high importance of looking to types, whether or not we can separate and define the groups to which such types belong.

Finally, we have seen that natural selection, which results from the struggle for existence and which almost inevitably induces extinction and divergence of character in the many descendants from one dominant parent species, explains that great and universal feature in the affinities of all organic beings, namely, their subordination in group under group. We use the element of descent in classing under one species the individuals of both sexes and of all ages, although they have few characters in common; we use descent in classing acknowledged varieties, however different they may be from their parent; and I believe this element of descent is the hidden bond of connection that naturalists have sought under the term natural system. On this idea that the natural system is, insofar as it has been perfected, genealogical in its arrangement, with the grades of difference between the descendants from a common parent expressed by the terms genera, families, orders, etc., we can understand the rules that we are compelled to follow in our classification. We can understand why we value certain resemblances far more than others; why we are permitted to use rudimentary and useless organs, or others of trifling physiological importance; and why, in comparing one group with a distinct group, we summarily reject analogical or adaptive characters and yet use these same characters within the limits of the same group. We can clearly see how it is that all living and extinct forms can be grouped together in one great system, and how the several members of each class are connected together by the most complex and radiating lines of affinities. We will probably never disentangle the inextricable web of affinities between the members of any one class; but when we have a distinct object in view, and do not look to some unknown plan of creation, we may hope to make sure but slow progress.

Morphology. We have seen that the members of the same class, independently of their habits of life, resemble each other in the general plan of their organization. This resemblance is often expressed by the term “unity of type” or by saying that the several parts and organs in the different species of the class are homologous. The whole subject is included under the general name of morphology. This is the most interesting department of natural history and may be said to be its very soul. What can be more curious than that the hand of a man, formed for grasping, that of a mole, formed for digging, the leg of the horse, the paddle of the porpoise, and the wing of the bat should all be constructed on the same pattern and should include the same bones in the same relative positions? Geoffroy Saint-Hilaire has strongly insisted on the high importance of relative connection in homologous organs: the parts may change to almost any extent in form and size, and yet they always remain connected together in the same order. We never find, for instance, the bones of the arm and forearm, or of the thigh and leg, transposed. Hence the same names can be given to the homologous bones in widely different animals. We see the same great law in the construction of the mouths of insects. What can be more different than the immensely long spiral proboscis of a sphinx moth, the curious folded one of a bee or bug, and the great jaws of a beetle? Yet all these organs, serving for such different purposes, are formed by infinitely numerous modifications of an upper lip, mandibles, and two pairs of maxillae. Analogous laws govern the construction of the mouths and limbs of crustaceans. So it is with the flowers of plants.

Nothing can be more hopeless than to attempt to explain this similarity of pattern in members of the same class by utility or by the doctrine of final causes. The hopelessness of the attempt has been expressly admitted by Owen in his most interesting work on the ‘Nature of Limbs.’ On the ordinary view of the independent creation of each being, we can only say that so it is—that it has so pleased the Creator to construct each animal and plant.

The explanation is manifest on the theory of the natural selection of successive slight modifications—each modification being profitable in some way to the modified form but often affecting, by correlation of growth, other parts of the organization. In changes of this nature, there will be little or no tendency to modify the original pattern or to transpose parts. The bones of a limb might be shortened and widened to any extent and become gradually enveloped in thick membrane, so as to serve as a fin; or a webbed foot might have all its bones, or certain bones, lengthened to any extent, and the membrane connecting them increased to any extent, so as to serve as a wing; yet in all this great amount of modification there will be no tendency to alter the framework of bones or the relative connection of the several parts. If we suppose that the ancient progenitor of all mammals—the archetype, as it may be called—had its limbs constructed on the existing general pattern, for whatever purpose they served, we can at once perceive the plain significance of the homologous construction of the limbs throughout the whole class. So with the mouths of insects, we have only to suppose that their common progenitor had an upper lip, mandibles, and two pairs of maxillae, these parts being perhaps very simple in form, and then natural selection will account for the infinite diversity in structure and function of the mouths of insects. Nevertheless, it is conceivable that the general pattern of an organ might become so much obscured as to be finally lost, by the atrophy and ultimately the complete abortion of certain parts, by the soldering together of other parts, and by the doubling or multiplication of others—variations that we know to be within the limits of possibility. In the paddles of the extinct gigantic sea lizards, and in the mouths of certain suctorial crustaceans, the general pattern seems to have been obscured in this way to a certain extent.

There is another and equally curious branch of the present subject, namely, the comparison not of the same part in different members of a class but of the different parts or organs in the same individual. Most physiologists believe that the bones of the skull are homologous with—that is, correspond in number and in relative connection with—the elemental parts of a certain number of vertebrae. The anterior and posterior limbs in each member of the vertebrate and articulate classes are plainly homologous. We see the same law in comparing the wonderfully complex jaws and legs in crustaceans. It is familiar to almost everyone that in a flower the relative positions of the sepals, petals, stamens, and pistils, as well as their intimate structure, are intelligible on the view that they consist of metamorphosed leaves arranged in a spiral. In monstrous plants, we often get direct evidence of the possibility of one organ being transformed into another; and we can actually see in embryonic crustaceans and in many other animals, and in flowers, that organs that become extremely different when mature are exactly alike at an early stage of growth.

How inexplicable these facts are on the ordinary view of creation! Why should the brain be enclosed in a box composed of such numerous and such extraordinarily shaped pieces of bone? As Owen has remarked, the benefit derived from the yielding of the separate pieces in the act of parturition in mammals will by no means explain the same construction in the skulls of birds. Why should similar bones have been created in the formation of the wing and leg of a bat, used as they are for such totally different purposes? Why should one crustacean that has an extremely complex mouth formed of many parts consequently always have fewer legs, or, conversely, why should those with many legs have simpler mouths? Why should the sepals, petals, stamens, and pistils in any individual flower, though fitted for such widely different purposes, all be constructed on the same pattern?

On the theory of natural selection, we can satisfactorily answer these questions. In the Vertebrata, we see a series of internal vertebrae bearing certain processes and appendages; in the Articulata, we see the body divided into a series of segments bearing external appendages; and in flowering plants, we see a series of successive spiral whorls of leaves. An indefinite repetition of the same part or organ is the common characteristic (as Owen has observed) of all low or little-modified forms; therefore, we may readily believe that the unknown progenitor of the Vertebrata possessed many vertebrae, the unknown progenitor of the Articulata many segments, and the unknown progenitor of flowering plants many spiral whorls of leaves. We have seen earlier that parts many times repeated are eminently liable to vary in number and structure; consequently, it is quite probable that natural selection, during a long-continued course of modification, has seized on a certain number of the primordially similar elements, many times repeated, and has adapted them to the most diverse purposes. And because the whole amount of modification will have been effected by slight successive steps, we need not wonder at discovering in such parts or organs a certain degree of fundamental resemblance, retained by the strong principle of inheritance.

In the great class of mollusks, though we can homologize the parts of one species with those of another, distinct species, we can indicate only a few serial homologies; that is, we are seldom able to say that one part or organ is homologous with another in the same individual. And we can understand this fact, for in mollusks, even in the lowest members of the class, we do not find nearly so much indefinite repetition of any one part as we find in the other great classes of the animal and vegetable kingdoms.

Naturalists frequently speak of the skull as formed of metamorphosed vertebrae, of the jaws of crabs as metamorphosed legs, and of the stamens and pistils of flowers as metamorphosed leaves; but in these cases it would probably be more correct, as Professor Huxley has remarked, to speak of both skull and vertebrae, both jaws and legs, etc., as having been metamorphosed, not one from the other, but from some common element. Naturalists, however, use such language only in a metaphorical sense: they are far from meaning that during a long course of descent, primordial organs of any kind—vertebrae in the one case and legs in the other—have actually been modified into skulls or jaws. Yet so strong is the appearance that a modification of this nature has occurred that naturalists can hardly avoid employing language having this plain meaning. On my view, these terms may be used literally; and the wonderful fact is explained that the jaws of a crab, for instance, retain numerous characters that they would probably have retained through inheritance if they had really been metamorphosed during a long course of descent from true legs, or from some simple appendage.

Embryology. It has already been casually remarked that certain organs in the individual that become widely different when mature and serve for different purposes are exactly alike in the embryo. The embryos of distinct animals within the same class are also often strikingly similar: no better proof of this can be given than a circumstance mentioned by Agassiz, namely, that having forgotten to label the embryo of some vertebrate animal, he cannot now tell whether it is that of a mammal, a bird, or a reptile. The vermiform larvae of moths, flies, beetles, etc., resemble each other much more closely than do the mature insects; but in the case of larvae, the embryos are active and have been adapted for special lines of life. A trace of the law of embryonic resemblance sometimes lasts until a rather late age: thus birds of the same genus, and of closely allied genera, often resemble each other in their first and second plumage, as we see in the spotted feathers in the thrush group. In the cat tribe, most of the species are striped or spotted in lines, and stripes can be plainly distinguished in the lion cub. We occasionally, though rarely, see something of this kind in plants: thus the embryonic leaves of Ulex, or furze, and the first leaves of the phyllodineous acacias are pinnate or divided like the ordinary leaves of the Leguminosae.

The points of structure in which the embryos of widely different animals of the same class resemble each other often have no direct relation to their conditions of existence. We cannot, for instance, suppose that in the embryos of the Vertebrata the peculiar looplike course of the arteries near the branchial slits is related to similar conditions—in the young mammal that is nourished in the womb of its mother, in the egg of the bird that is hatched in a nest, and in the spawn of a frog under water. We have no more reason to believe in such a relation than we have to believe that the same bones in the hand of a man, the wing of a bat, and the fin of a porpoise are related to similar conditions of life. No one will suppose that the stripes on a lion cub, or the spots on the young blackbird, are of any use to these animals or are related to the conditions to which they are exposed.

The case, however, is different when an animal is active during any part of its embryonic career and has to provide for itself. The period of activity may come on earlier or later in life; but whenever it comes on, the adaptation of the larva to its conditions of life is just as perfect and as beautiful as in the adult animal. Because of such special adaptations, the similarity of the larvae or active embryos of allied animals is sometimes much obscured; and cases could be given of the larvae of two species, or of two groups of species, differing from each other quite as much as, or even more than, their adult parents do. In most cases, however, the larvae, though active, still obey more or less closely the law of common embryonic resemblance. Cirripedes afford a good instance of this: even the illustrious Cuvier did not perceive that a barnacle was, as it certainly is, a crustacean; but a glance at the larva shows this to be the case in an unmistakable manner. So again, the two main divisions of cirripedes, the pedunculated and the sessile, which differ widely in external appearance, have larvae that are barely distinguishable in all their several stages.

The embryo in the course of development generally rises in organization: I use this expression, though I am aware that it is hardly possible to define clearly what is meant by the organization being higher or lower. But probably no one will dispute that the butterfly is higher than the caterpillar. In some cases, however, the mature animal is generally considered lower in the scale than the larva, as with certain parasitic crustaceans. To refer once again to cirripedes: the larvae in the first stage have three pairs of legs, a very simple single eye, and a probosciform mouth, with which they feed largely, for they increase much in size. In the second stage, corresponding to the chrysalis stage of butterflies, they have six pairs of beautifully constructed natatory legs, a pair of magnificent compound eyes, and extremely complex antennae; but they have a closed and imperfect mouth and cannot feed. Their function at this stage is to search out, by their well-developed organs of sense, and to reach, by their active powers of swimming, a proper place on which to become attached and to undergo their final metamorphosis. When this is completed, they are fixed for life: their legs are now converted into prehensile organs; they again obtain a well-constructed mouth; but they have no antennae, and their two eyes are now reconverted into a minute, single, and very simple eyespot. In this last and complete state, cirripedes may be considered either more highly or more lowly organized than they were in the larval condition. But in some genera the larvae develop either into hermaphrodites having the ordinary structure or into what I have called complemental males; and in the latter, the development has assuredly been retrograde, for the male is a mere sac that lives for a short time and is destitute of a mouth, a stomach, or any other organ of importance, except for reproduction.

We are so accustomed to seeing differences in structure between the embryo and the adult, and likewise a close similarity in the embryos of widely different animals within the same class, that we might be led to regard these facts as necessarily contingent in some manner on growth. But there is no obvious reason why, for instance, the wing of a bat or the fin of a porpoise should not have been sketched out with all the parts in proper proportion as soon as any structure became visible in the embryo. And in some whole groups of animals and in certain members of other groups, the embryo does not at any period differ widely from the adult: thus Owen has remarked in regard to cuttlefish that “there is no metamorphosis; the cephalopodic character is manifested long before the parts of the embryo are completed”; and again, in regard to spiders, that “there is nothing worthy to be called a metamorphosis.” The larvae of insects, whether adapted to the most diverse and active habits or quite inactive—being fed by their parents or placed in the midst of proper nutriment—nearly all pass through a similar wormlike stage of development; but in a few cases, as in that of Aphis, if we look at Professor Huxley’s admirable drawings of the development of this insect, we see no trace of the vermiform stage.

How, then, can we explain these several facts in embryology—namely, the very general, but not universal, difference in structure between the embryo and the adult; the fact that parts in the same individual embryo that ultimately become very unlike and serve for diverse purposes are alike at this early period of growth; the fact that embryos of different species within the same class generally, but not universally, resemble each other; the fact that the structure of the embryo is not closely related to its conditions of existence, except when the embryo becomes active at any period of life and has to provide for itself; and the fact that the embryo apparently sometimes has a higher organization than the mature animal into which it develops? I believe that all these facts can be explained, as follows, on the view of descent with modification.

It is commonly assumed, perhaps because monstrosities often affect the embryo at a very early period, that slight variations necessarily appear at an equally early period. But we have little evidence on this point—indeed, the evidence rather points the other way, for it is well known that breeders of cattle, horses, and various fancy animals cannot positively tell, until some time after the animal has been born, what its merits or form will ultimately turn out to be. We see this plainly in our own children; we cannot always tell whether the child will be tall or short, or what its precise features will be. The question is not at what period of life any variation has been caused but at what period it is fully displayed. The cause may have acted, and I believe generally has acted, even before the embryo is formed; and the variation may be due to the male and female sexual elements having been affected by the conditions to which either parent, or their ancestors, have been exposed. Nevertheless, an effect thus caused at a very early period, even before the formation of the embryo, may appear late in life, as when a hereditary disease that appears only in old age has been communicated to the offspring from the reproductive element of one parent. The same is true when the horns of crossbred cattle have been affected by the shape of the horns of either parent. For the welfare of a very young animal, as long as it remains in its mother’s womb or in the egg, or as long as it is nourished and protected by its parent, it must be quite unimportant whether most of its characters are fully acquired a little earlier or a little later in life. It would not matter, for instance, to a bird that obtained its food best by having a long beak whether or not it assumed a beak of this particular length, as long as it was fed by its parents. Hence, I conclude that it is quite possible that each of the many successive modifications by which each species has acquired its present structure may have supervened at a not very early period of life; and some direct evidence from our domestic animals supports this view. But in other cases it is quite possible that each successive modification, or most of them, may have appeared at an extremely early period.

I have stated in the first chapter that there is some evidence that renders it probable that, at whatever age any variation first appears in the parent, it tends to reappear at a corresponding age in the offspring. Certain variations can appear only at corresponding ages—for instance, peculiarities in the caterpillar, cocoon, or imago states of the silk moth, or, again, in the horns of almost full-grown cattle. But beyond this, variations that, for all we can see, might have appeared earlier or later in life tend to appear at a corresponding age in the offspring and the parent. I am far from meaning that this is invariably the case; and I could give a good many cases of variations (taking the word in the broadest sense) that have supervened at an earlier age in the child than in the parent.

These two principles, if their truth is admitted, will, I believe, explain all the leading facts in embryology specified above. But first let us look at a few analogous cases in domestic varieties. Some authors who have written on dogs maintain that the greyhound and bulldog, though they appear so different, are really very closely allied varieties and have probably descended from the same wild stock; hence I was curious to see how far their puppies differed from each other. Breeders told me that they differed just as much as their parents did, and, judging by the eye, this seemed almost to be the case; but on actually measuring the old dogs and their six-day-old puppies, I found that the puppies had not nearly acquired their full amount of proportional difference. So, again, I was told that the foals of cart horses and racehorses differed as much as the full-grown animals did; and this surprised me greatly, as I think it probable that the difference between these two breeds has been wholly caused by selection under domestication. But having had careful measurements made of the dam and of a three-day-old colt of a racehorse and of a heavy cart horse, I find that the colts have by no means acquired their full amount of proportional difference.

Because the evidence that the several domestic breeds of pigeon have descended from one wild species appears to me conclusive, I compared young pigeons of various breeds within twelve hours after they were hatched. I carefully measured the proportions (but will not give details here) of the beak, width of mouth, length of nostril and of eyelid, size of feet, and length of leg in the wild stock and in pouters, fantails, runts, barbs, dragons, carriers, and tumblers. Now some of these birds, when mature, differ so extraordinarily in the length and form of the beak that they would, I cannot doubt, be ranked in distinct genera had they been produced in nature. But when the nestling birds of these several breeds were placed in a row, though most of them could be distinguished from each other, their proportional differences in the several points specified above were incomparably less than in the full-grown birds. Some characteristic points of difference—for instance, that of the width of mouth—could hardly be detected in the young. But there was one remarkable exception to this rule, for the young of the short-faced tumbler differed from the young of the wild rock pigeon and of the other breeds, in all its proportions, almost exactly as much as in the adult state.

The two principles given above seem to me to explain these facts in regard to the later embryonic stages of our domestic varieties. Fanciers select their horses, dogs, and pigeons for breeding when they are nearly grown up: they are indifferent as to whether the desired qualities and structures have been acquired earlier or later in life, as long as the full-grown animal possesses them. And the cases just given, more especially that of pigeons, seem to show that the characteristic differences that give value to each breed, and that have been accumulated by man’s selection, have not generally first appeared at an early period of life and have been inherited by the offspring at a corresponding, not early, period. But the case of the short-faced tumbler, which when twelve hours old had acquired its proper proportions, proves that this is not the universal rule; for here the characteristic differences must either have appeared at an earlier period than usual or, if not, must have been inherited not at the corresponding age but at an earlier one.

Now let us apply these facts and the above two principles—the latter of which, though not proved true, can be shown to be in some degree probable—to species in a state of nature. Let us take a genus of birds that, on my theory, has descended from some one parent species and whose several new species have become modified through natural selection in accordance with their diverse habits. Then, because the many slight successive steps of variation have supervened at a rather late age and have been inherited at a corresponding age, the young of the new species of our supposed genus will manifestly tend to resemble each other much more closely than do the adults, just as we have seen in the case of pigeons. We may extend this view to whole families or even classes. The forelimbs, for instance, which served as legs in the parent species, may become adapted, by a long course of modification, in one descendant to act as hands, in another as paddles, and in another as wings; and on the above two principles—namely, of each successive modification supervening at a rather late age and being inherited at a correspondingly late age—the forelimbs in the embryos of the several descendants of the parent species will still resemble each other closely, for they will not have been modified. But in each individual new species, the embryonic forelimbs will differ greatly from the forelimbs in the mature animal, the limbs in the latter having undergone much modification at a rather late period of life and having thus been converted into hands, paddles, or wings. Whatever influence long-continued exercise or use on the one hand, and disuse on the other, may have in modifying an organ, such influence will mainly affect the mature animal, which has come to its full powers of activity and has to gain its own living; and the effects thus produced will be inherited at a correspondingly mature age. The young, by contrast, will remain unmodified, or be modified in a lesser degree, by the effects of use and disuse.

In certain cases the successive steps of variation might supervene, from causes of which we are wholly ignorant, at a very early period of life, or each step might be inherited at an earlier period than that at which it first appeared. In either case (as with the short-faced tumbler), the young or embryo would closely resemble the mature parent form. We have seen that this is the rule of development in certain whole groups of animals, as with cuttlefish and spiders, and with a few members of the great class of insects, as with Aphis. With respect to the final cause of the young in these cases not undergoing any metamorphosis, or closely resembling their parents from their earliest age, we can see that this would result from the following two contingencies: first, from the young, during a course of modification carried on for many generations, having to provide for their own wants at a very early stage of development; and second, from their following exactly the same habits of life as their parents. For in this case, it would be indispensable for the existence of the species that the child be modified at a very early age in the same manner as its parents, in accordance with their similar habits. Some further explanation, however, of the embryo not undergoing any metamorphosis is perhaps requisite. If, on the other hand, it profited the young to follow habits of life in any degree different from those of their parent, and consequently to be constructed in a slightly different manner, then, on the principle of inheritance at corresponding ages, the active young or larvae might easily be rendered different from their parents to any conceivable extent by natural selection. Such differences might also become correlated with successive stages of development, so that the larvae in the first stage might differ greatly from the larvae in the second stage, as we have seen to be the case with cirripedes. The adult might become fitted for sites or habits in which organs of locomotion or of the senses, etc., would be useless; and in this case the final metamorphosis would be said to be retrograde.

As all the organic beings, extinct and recent, that have ever lived on this earth have to be classed together, and as all have been connected by the finest gradations, the best, or indeed, if our collections were nearly perfect, the only possible, arrangement would be genealogical. Descent is, on my view, the hidden bond of connection that naturalists have been seeking under the term natural system. On this view we can understand how it is that, in the eyes of most naturalists, the structure of the embryo is even more important for classification than that of the adult. For the embryo is the animal in its less modified state, and to that extent it reveals the structure of its progenitor. If two groups of animals, however much they may at present differ from each other in structure and habits, pass through the same or similar embryonic stages, we may feel assured that they have both descended from the same or nearly similar parents and are therefore closely related to that degree. Thus, community in embryonic structure reveals community of descent. It will reveal this community of descent however much the structure of the adult may have been modified and obscured; we have seen, for instance, that cirripedes can at once be recognized by their larvae as belonging to the great class of crustaceans. As the embryonic state of each species and group of species partially shows us the structure of their less modified ancient progenitors, we can clearly see why ancient and extinct forms of life should resemble the embryos of their descendants—our existing species. Agassiz believes this to be a law of nature, but I am bound to confess that I only hope to see the law proved true in the future. It can be proved true only in those cases in which the ancient state, now supposed to be represented in many embryos, has not been obliterated, either by the successive variations in a long course of modification having supervened at a very early age or by the variations having been inherited at an earlier period than that at which they first appeared. It should also be borne in mind that the supposed law of resemblance of ancient forms of life to the embryonic stages of recent forms may be true but, because the geological record does not extend far enough back in time, may remain incapable of demonstration for a long period, or forever.

Thus, as it seems to me, the leading facts in embryology, which are second in importance to none in natural history, are explained on the principle that slight modifications, in the many descendants from some one ancient progenitor, do not appear at a very early period in the life of each, though they were perhaps caused at the earliest, and are inherited at a corresponding, not early, period. Embryology rises greatly in interest when we thus regard the embryo as a picture, more or less obscured, of the common parent form of each great class of animals.

Rudimentary, Atrophied, or Aborted Organs. Organs or parts in this strange condition, bearing the stamp of inutility, are extremely common throughout nature. For instance, rudimentary mammae are very general in the males of mammals; I presume that the “bastard wing” in birds may be safely considered a digit in a rudimentary state; in very many snakes one lobe of the lungs is rudimentary; in other snakes there are rudiments of the pelvis and hind limbs. Some of the cases of rudimentary organs are extremely curious: for instance, the presence of teeth in fetal whales, which when grown up do not have a tooth in their heads, and the presence of teeth that never cut through the gums in the upper jaws of our unborn calves. It has even been stated on good authority that rudiments of teeth can be detected in the beaks of certain embryonic birds. Nothing can be plainer than that wings are formed for flight; yet in how many insects do we see wings so reduced in size as to be utterly incapable of flight, and not rarely lying under wing cases that are firmly soldered together!

The meaning of rudimentary organs is often quite unmistakable: for instance, there are beetles of the same genus (and even of the same species) resembling each other most closely in all respects, one of which will have full-sized wings and another mere rudiments of membrane; and here it is impossible to doubt that the rudiments represent wings. Rudimentary organs sometimes retain their potentiality and are merely not developed: this seems to be the case with the mammae of male mammals, for many instances are on record of these organs having become well developed in full-grown males and having secreted milk. So again, there are normally four developed and two rudimentary teats in the udders of the genus Bos, but in our domestic cows the two sometimes become developed and give milk. In individual plants of the same species, the petals sometimes occur as mere rudiments and sometimes in a well-developed state. In plants with separate sexes, the male flowers often have a rudiment of a pistil; and Kölreuter found that, when such male plants were crossed with a hermaphrodite species, the rudiment of the pistil in the hybrid offspring was much increased in size, which shows that the rudiment and the perfect pistil are essentially alike in nature.

An organ serving two purposes may become rudimentary or utterly aborted for one, even the more important, purpose and remain perfectly efficient for the other. Thus in plants, the function of the pistil is to allow the pollen tubes to reach the ovules protected in the ovary at its base. The pistil consists of a stigma supported on the style; but in some Compositae, the male florets, which of course cannot be fecundated, have a pistil that is in a rudimentary state, for it is not crowned with a stigma. The style, however, remains well developed and is clothed with hairs, as in other Compositae, for the purpose of brushing the pollen out of the surrounding anthers. Again, an organ may become rudimentary for its proper purpose and be used for a distinct object: in certain fish the swim bladder seems to be rudimentary for its proper function of giving buoyancy but has become converted into a nascent breathing organ, or lung. Other similar instances could be given.

Rudimentary organs in the individuals of the same species are very liable to vary in degree of development and in other respects. Moreover, in closely allied species, the degree to which the same organ has been rendered rudimentary occasionally differs much. This latter fact is well exemplified in the state of the wings of the female moths in certain groups. Rudimentary organs may be utterly aborted, and this implies that we find in an animal or plant no trace of an organ that analogy would lead us to expect to find and that is occasionally found in monstrous individuals of the species. Thus in the snapdragon (Antirrhinum) we generally do not find a rudiment of a fifth stamen, but one may sometimes be seen. In tracing the homologies of the same part in different members of a class, nothing is more common, or more necessary, than the use and discovery of rudiments. This is well shown in the drawings given by Owen of the bones of the leg of the horse, ox, and rhinoceros.

It is an important fact that rudimentary organs, such as teeth in the upper jaws of whales and ruminants, can often be detected in the embryo but afterward wholly disappear. It is also, I believe, a universal rule that a rudimentary part or organ is of greater size relative to the adjoining parts in the embryo than in the adult, so that the organ at this early age is less rudimentary, or cannot even be said to be in any degree rudimentary. Hence, also, a rudimentary organ in the adult is often said to have retained its embryonic condition.

I have now given the leading facts with respect to rudimentary organs. In reflecting on them, everyone must be struck with astonishment, for the same reasoning power that tells us plainly that most parts and organs are exquisitely adapted for certain purposes tells us with equal plainness that these rudimentary or atrophied organs are imperfect and useless. In works on natural history, rudimentary organs are generally said to have been created “for the sake of symmetry” or in order “to complete the scheme of nature”; but this seems to me no explanation, merely a restatement of the fact. Would it be thought sufficient to say that because planets revolve in elliptic courses around the sun, satellites follow the same course around the planets for the sake of symmetry and to complete the scheme of nature? An eminent physiologist accounts for the presence of rudimentary organs by supposing that they serve to excrete matter that is in excess or injurious to the system; but can we suppose that the minute papilla that often represents the pistil in male flowers, and that is formed merely of cellular tissue, can act in this way? Can we suppose that the formation of rudimentary teeth that are subsequently absorbed can be of any service to the rapidly growing embryonic calf by the excretion of precious calcium phosphate? When a man’s fingers have been amputated, imperfect nails sometimes appear on the stumps; I could just as easily believe that these vestiges of nails have appeared, not from unknown laws of growth, but in order to excrete horny matter, as that the rudimentary nails on the fin of the manatee were formed for this purpose.

On my view of descent with modification, the origin of rudimentary organs is simple. We have plenty of cases of rudimentary organs in our domestic plants and animals—such as the stump of a tail in tailless breeds, the vestige of an ear in earless breeds, the reappearance of minute dangling horns in hornless breeds of cattle (more especially, according to Youatt, in young animals), and the state of the whole flower in the cauliflower. We often see rudiments of various parts in monsters. But I doubt whether any of these cases throws light on the origin of rudimentary organs in a state of nature, beyond showing that rudiments can be produced, for I doubt whether species in nature ever undergo abrupt changes. I believe that disuse has been the main agency, and that it has led in successive generations to the gradual reduction of various organs until they have become rudimentary—as in the case of the eyes of animals inhabiting dark caverns, and of the wings of birds inhabiting oceanic islands, which have seldom been forced to take flight and have ultimately lost the power of flying. Again, an organ useful under certain conditions might become injurious under others, as with the wings of beetles living on small and exposed islands; and in this case natural selection would continue slowly to reduce the organ until it was rendered harmless and rudimentary.

Any change in function that can be effected by imperceptibly small steps is within the power of natural selection, so that an organ rendered useless or injurious for one purpose during changed habits of life might easily be modified and used for another purpose. Or an organ might be retained for only one of its former functions. An organ, when rendered useless, may well be variable, for its variations cannot be checked by natural selection. At whatever period of life disuse or selection reduces an organ—and this will generally be when the being has come to maturity and to its full powers of action—the principle of inheritance at corresponding ages will reproduce the organ in its reduced state at the same age and consequently will seldom affect or reduce it in the embryo. Thus we can understand the greater relative size of rudimentary organs in the embryo and their lesser relative size in the adult. But if each step of the process of reduction were to be inherited, not at the corresponding age, but at an extremely early period of life (as we have good reason to believe to be possible), the rudimentary part would tend to be wholly lost, and we would have a case of complete abortion. The principle of economy, also, explained in an earlier chapter, by which the materials forming any part or structure, if not useful to the possessor, will be saved as far as possible, will probably often come into play; and this will tend to cause the entire obliteration of a rudimentary organ.

Because the presence of rudimentary organs is thus due to the tendency of every part of the organization that has long existed to be inherited, we can understand, on the genealogical view of classification, how it is that systematists have found rudimentary parts as useful as, or sometimes even more useful than, parts of high physiological importance. Rudimentary organs may be compared with the letters in a word that are still retained in the spelling but that have become useless in the pronunciation and that serve as a clue in seeking its derivation. On the view of descent with modification, we may conclude that the existence of organs in a rudimentary, imperfect, and useless condition, or quite aborted, far from presenting a strange difficulty, as it assuredly does on the ordinary doctrine of creation, might even have been anticipated and can be accounted for by the laws of inheritance.

Summary. In this chapter I have attempted to show that the subordination of group to group in all organisms throughout all time; the nature of the relationship by which all living and extinct beings are united by complex, radiating, and circuitous lines of affinities into one grand system; the rules followed and the difficulties encountered by naturalists in their classifications; the value set upon characters, if constant and prevalent, whether of high vital importance, of the most trifling importance, or, as in rudimentary organs, of no importance; the wide opposition in value between analogical or adaptive characters and characters of true affinity; and other such rules—all naturally follow from the view of the common parentage of those forms that naturalists consider allied, together with their modification through natural selection, with its contingencies of extinction and divergence of character. In considering this view of classification, it should be borne in mind that the element of descent has been universally used in ranking together the sexes, ages, and acknowledged varieties of the same species, however different they may be in structure. If we extend the use of this element of descent—the only certainly known cause of similarity in organic beings—we will understand what is meant by the natural system: it is genealogical in its attempted arrangement, with the grades of acquired difference marked by the terms varieties, species, genera, families, orders, and classes.

On this same view of descent with modification, all the great facts in morphology become intelligible, whether we look at the same pattern displayed in the homologous organs of the different species of a class, to whatever purpose they are applied, or at the homologous parts constructed on the same pattern in each individual animal and plant.

On the principle of successive slight variations, not necessarily or generally supervening at a very early period of life and being inherited at a corresponding period, we can understand the great leading facts in embryology, namely, the resemblance in an individual embryo of the homologous parts, which when matured will become widely different from each other in structure and function; and the resemblance in different species of a class of the homologous parts or organs, though they are fitted in the adult members for purposes as different as possible. Larvae are active embryos that have become specially modified in relation to their habits of life through the principle of modifications being inherited at corresponding ages. On this same principle—if we bear in mind that when organs are reduced in size, either from disuse or by selection, it will generally be at that period of life when the being has to provide for its own wants, and if we bear in mind how strong the principle of inheritance is—the occurrence of rudimentary organs and their final abortion present us with no inexplicable difficulties; on the contrary, their presence might even have been anticipated. The importance of embryological characters and of rudimentary organs in classification is intelligible on the view that an arrangement is natural only insofar as it is genealogical.

Finally, the several classes of facts that have been considered in this chapter seem to me to proclaim so plainly that the innumerable species, genera, and families of organic beings with which this world is peopled have all descended, each within its own class or group, from common parents, and have all been modified in the course of descent, that I would without hesitation adopt this view, even if it were unsupported by other facts or arguments.