Species, Varieties, and Individual Differences

Before applying the principles reached in the last chapter to organic beings in a state of nature, we must briefly discuss whether these beings are subject to any variation. To treat this subject at all properly, I would need to give a long catalog of dry facts, but I will reserve these for my future work. I also will not discuss here the various definitions that have been given of the term species. No single definition has so far satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of a species. Generally, the term includes the unknown element of a distinct act of creation. The term “variety” is almost equally difficult to define, but here common descent is almost universally implied, though it can rarely be proved. We also have what are called monstrosities, but they grade into varieties. A monstrosity, I presume, means some considerable deviation of structure in one part, either injurious or not useful to the species, and not generally propagated. Some authors use the term “variation” in a technical sense, to mean a modification directly due to the physical conditions of life, and “variations” in this sense are supposed not to be inherited. But who can say that the dwarfed condition of shells in the brackish waters of the Baltic or of plants on Alpine summits, or the thicker fur of an animal from the far north, would not in some cases be inherited for at least a few generations? In that case, I presume, the form would be called a variety.

Again, we have many slight differences that may be called individual differences, such as those known to appear frequently in the offspring of the same parents, or those that may be presumed to have arisen in this way because they are frequently observed in individuals of the same species inhabiting the same confined locality. No one supposes that all the individuals of the same species are cast in exactly the same mold. These individual differences are highly important for us, because they provide materials for natural selection to accumulate, just as man can accumulate, in any given direction, individual differences in his domesticated animals and plants. These individual differences generally affect what naturalists consider unimportant parts, but I could show by a long catalog of facts that parts that must be called important, whether viewed from a physiological or a classificatory point of view, sometimes vary among the individuals of the same species. I am convinced that the most experienced naturalist would be surprised at the number of cases of variability, even in important parts of structure, that he could collect on good authority, as I have done over a period of years. It should be remembered that systematists are far from pleased to find variability in important characters, and that there are not many men who will laboriously examine internal and important organs and compare them in many specimens of the same species. I would never have expected that the branching of the main nerves close to the great central ganglion of an insect would be variable within the same species; I would have expected that changes of this nature could be effected only by slow degrees. Yet quite recently Mr. Lubbock has shown a degree of variability in these main nerves in Coccus that may almost be compared to the irregular branching of the stem of a tree. This thoughtful naturalist, I may add, has also quite recently shown that the muscles in the larvae of certain insects are very far from uniform. Authors sometimes argue in a circle when they state that important organs never vary, for in practice these same authors rank a character as important if it does not vary (as a few naturalists have honestly confessed); and from this point of view, no instance of an important part varying will ever be found. But from any other point of view, many instances can certainly be given.