Page:Popular Science Monthly Volume 68.djvu/488

484 once that adaptive characters are older than the non-adaptive characters. This is true, even though the adaptive character itself be a relatively recent one. The flying-fishes flew before the group was split up into thirty species. Yet the enlargement of the fins of these fishes is one of the latest products in fish evolution.

The species of Hawaiian birds of the family of Drepanidse are among the latest results of bird development. Yet these birds were divided into genera characterized by the form of the bill, each type of bill being adapted to a special purpose long before the present actual species were themselves differentiated.

All the orioles build hanging nests, and all are adapted alike in structure and instinct to their mode of life. All the sparrows and finches have bills fitted for cracking seeds. And these features of adaptation preceded the subdivision of orioles or sparrows into their present genera and species. In general, the traits by which we distinguish species are non-adaptive characters, while the features of adaptation are most distinctly traceable in structures common to many species, the characters of genera or of families in zoology.

But in this we find certain paradoxes. In studying the characters of members of a zoological family, we find that the most distinctly adaptive characters are relatively recent. It is a truism that physiological characters have a lower systematic value than characters not related to the character of life processes. The traits that fit animals for a special kind of food or for a special kind of topography are recognized as of low value in taxonomy. In other words, special adaptations are of relatively recent origin. General adaptations are older than special adaptations. Hence they have a higher value in classification. A flying-fish is fitted to swim in the water before it is adapted to leap in the air. But all general adaptations began some time as special ones. General adaptations have become so ingrained in the life of animals that they are, in a sense, invisible to the systematist. He passes them by as matters of course, directing his attention to special adaptations or to special peculiarities which seem to be devoid of utility. The ancient adaptations are not even considered as adaptations at all, when we say that adaptive or physiological characters have a low value in classification. And it is from this fact that the seeming paradox arises.

Again the adaptive character in the race or species may appear to be of very recent origin. The Southdown sheep of England had tawny faces before they acquired their present traits of wool or mutton. The explanation of such cases is this: an adaptation is never finished, a more rigid selection may at any time enforce a more rigid adaptation. Natural selection acts as a constant influence. It may be varied or intensified under new circumstances, as when it is directed by the will of man, when it becomes artificial selection. And under the