It is so recently that all structural differences and changes were supposed to be readily explicable upon the doctrine of final causes, that we naturally turn first in that direction. Some transformations certainly seem to relate very distinctly to the welfare of the individual, as when the caterpillar becomes a butterfly, and when the aquatic larvas of mosquito and dragon-fly change their forms with their habits and modes of life. So, among the vertebrates, it is obvious that the tadpole is by no means adapted to the necessities of the frog and the toad; and the intermediate stages, resulting from the gradual loss of the tail and the acquisition of legs, while perhaps not particularly suited to either aquatic or terrestrial locomotion, seem to be required in order to permit the development of the lungs and the accompanying disappearance of the gills.
But can the transformations of the gar-pike's tail be thus accounted for? According to present knowledge and justifiable inference, the Lepidosteus not only passes the whole of its life in the water, but is also, from first to last, an active, predaceous fish, requiring all possible advantages of form and fin in order to overtake its prey.
Since no marked change occurs in the general form of the body, we may perhaps assume that it is perfectly well adapted to the fish's needs; although this suggests the general inquiry as to the cui bono of the almost infinite variations from the ideal form supposed to be best suited to aquatic locomotion.
But do we know, or can we easily infer, any differences in the necessities or the manner of life of the Lepidosteus at different ages, which may account for its having a tail first like a lamprey's, then like a sturgeon's, and, finally, like that of Amia?
It may be suggested that the rapid and, at most, invisible vibrations of the filament enable the young gar to glide stealthily upon its prey. But the very young would seem to be even more in need of such precaution, and with them the tail is relatively as large as in the adult, although differently shaped. Finally, even if we conclude that the three distinct stages of the tail are perfectly adapted to certain hypothetically unlike necessities, what shall be said of the intermediate conditions? While growing, the infra-caudal lobe must be rather a hinderance than a help to the movements of the primitive tail; and while disappearing, the filament, being useless, must be, if anything, an incumbrance.
Shall we, then, conclude that these changes in the appearance of a single individual are for the sake of variety—as some would explain the great diversity of specific form and coloration among animals and plants?
At the present day, neither of the explanations above given is likely to wholly satisfy the large class of thinkers who, whether or not they accept any particular evolution doctrine, are inclined to believe that there is, in many cases, a more or less exact parallelism between
