Rarity, Competition, and the Extinction of Species

The whole subject of the extinction of species has been wrapped in the most gratuitous mystery. Some authors have even supposed that, just as the individual has a definite length of life, so species have a definite duration. No one, I think, can have marveled more at the extinction of species than I have. When I found in La Plata the tooth of a horse embedded with the remains of Mastodon, Megatherium, Toxodon, and other extinct monsters, which all coexisted with still-living shells at a very late geological period, I was filled with astonishment. Seeing that the horse, since its introduction by the Spaniards into South America, has run wild over the whole country and has increased in numbers at an unparalleled rate, I asked myself what could so recently have exterminated the former horse under conditions of life apparently so favorable. But how utterly groundless my astonishment was! Professor Owen soon perceived that the tooth, though so like that of the existing horse, belonged to an extinct species. Had this horse still been living, but rare to some degree, no naturalist would have felt the least surprise at its rarity, for rarity is the attribute of a vast number of species of all classes in all countries. If we ask ourselves why this or that species is rare, we answer that something is unfavorable in its conditions of life; but what that something is, we can hardly ever tell. On the supposition that the fossil horse still existed as a rare species, we might have felt certain, from the analogy of all other mammals, even of the slow-breeding elephant, and from the history of the naturalization of the domestic horse in South America, that under more favorable conditions it would in a very few years have stocked the whole continent. But we could not have told what the unfavorable conditions were that checked its increase—whether they were one or several contingencies—or at what period of the horse’s life, and in what degree, they each acted. If the conditions had gone on becoming less and less favorable, however slowly, we assuredly would not have perceived the fact; yet the fossil horse would certainly have become rarer and rarer and finally extinct, its place being seized by some more successful competitor.

It is most difficult to remember always that the increase of every living being is constantly being checked by unperceived injurious agencies, and that these same unperceived agencies are amply sufficient to cause rarity and finally extinction. We see in many cases in the more recent Tertiary formations that rarity precedes extinction, and we know that this has been the course of events with those animals that have been exterminated, either locally or wholly, through man’s agency. I may repeat what I published in 1845: to admit that species generally become rare before they become extinct—to feel no surprise at the rarity of a species and yet to marvel greatly when it ceases to exist—is much the same as to admit that sickness in the individual is the forerunner of death—to feel no surprise at sickness, but, when the sick man dies, to wonder and to suspect that he died by some unknown deed of violence.

The theory of natural selection is grounded on the belief that each new variety, and ultimately each new species, is produced and maintained by having some advantage over those with which it comes into competition; the consequent extinction of less-favored forms almost inevitably follows. It is the same with our domestic plants and animals: when a new and slightly improved variety has been raised, it at first supplants the less improved varieties in the same neighborhood; when much improved, it is transported far and near, like our Shorthorn cattle, and takes the place of other breeds in other countries. Thus the appearance of new forms and the disappearance of old forms, both natural and artificial, are bound together. In certain flourishing groups, the number of new specific forms that have been produced within a given time is probably greater than the number of old forms that have been exterminated; but we know that the number of species has not gone on increasing indefinitely, at least during the later geological periods, so that, looking at later times, we may believe that the production of new forms has caused the extinction of about the same number of old forms.