37 6 THE POPULAR SCIENCE MONTHLY. 



two periods of great specialization, and bridges over the gap between 

 them, and thus does away w T ith 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, al- 

 though 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 modi- 

 fied 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 craw- 

 fish 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 w T e 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 com- 

 parisons and discussions which could not be adequately presented with- 

 out a number of figures ; and a sufficient answer for our present pur- 

 pose 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 resem- 

 blance 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 de- 

 cided that the second set of resemblances alone give any evidence of 

 systematic relationship, and that the whale is a mammal. Now, what 



