two periods of great specialization, and bridges over the gap between them, and thus does away with a period of imperfect specialization to both modes of life.
As an instance of the opposite kind of modification, the simplification of an embryonic history by the loss of ancestral stages, we may take the life of the fresh-water crawfish. Young lobsters, and most of the other marine allies of the crawfish, leave the egg in a form which is quite unlike the adult, in structure as well as in habits, and the new-born young pass through a long series of stages of metamorphosis before the mature form is reached.
The larval stages of the marine long-tailed Crustacea bear such a resemblance to each other, and to certain lower Crustacea, that we must regard them as ancestral; and we must therefore believe that they were one time present in the life-history of the crawfish, although we find nothing of the kind now. These larval forms are adapted to a swimming life at the surface of the ocean, and we can understand that, when the ancestors of the crawfishes became adapted to a life in fresh water, the larval stages must either have been modified to correspond or else been got rid of, and, in the crayfish, the. latter has happened, and the new-born young is simply a very small image of the adult, the whole metamorphosis having been suppressed.
A person who is unfamiliar with morphology may fairly ask whether we are not entering upon treacherous ground, and why we are to regard the life-history of the lobster as the ancestral one, and that of the crawfish as a secondary modification, rather than the reverse. This feeling is not confined to unscientific thinkers, for many naturalists are inclined to reject this conception of the falsification of the embryonic record, and to say that, when we accept the evidence furnished by one species as a basis for phylogeny, and reject the evidence of a related species as of no taxonomic importance, we are actuated by mere caprice, or by a desire to establish some pet hypothesis, and that this method of reasoning can have no scientific value.
The most satisfactory answer to this objection would be a thorough analysis of a specific example, but this would involve technical comparisons and discussions which could not be adequately presented without a number of figures; and a sufficient answer for our present purpose may be found by a reference to the facts and conclusions of comparative anatomy.
A whale differs from all the ordinary mammals in quite a number of features in which it bears a close resemblance to fishes, and at the same time it differs from fishes in a number of points of resemblance to mammals. The attention of the earlier naturalists was attracted by the first set of resemblances and differences, and they placed the whale among the fishes; but later investigators have decided that the second set of resemblances alone give any evidence of systematic relationship, and that the whale is a mammal. Now, what