IT2 



EVOLUTION, GENETICS, AND EUGENICS 



animals are related to each other through being descended from a 

 common ancestor, the first or oldest backboned animal. In fact, it is 

 because all these backboned animals — the fishes, the batrachians, the 

 reptiles, the birds, and the mammals — have descended from a common 

 ancestor that they all have a backbone. It is believed that the 

 descendants of the first backboned animal have in the course of many 

 generations branched off little by little from the original type until 

 there came to exist very real and obvious differences among the back- 

 boned animals — differences which among the living backboned animals 

 are familiar to all of us. The course of development of an individual 

 animal is believed to be a very rapid and evidently much condensed and 

 changed recapitulation of the history 

 which the species or kind of animal to 

 which the developing individual belongs 

 has passed through in the course of its 

 descent through a long series of gradually 

 changing ancestors. If this is true, then 

 we can readily understand why a fish 

 and a salamander, a tortoise, a bird, and 

 a rabbit, are all much alike, as they 

 really are, in their earlier stages of 

 development, and gradually come to 

 differ more and more as they pass 

 through later and later developmental 

 stages. A crab has a tail in one of its 

 developmental stages, so that at that 

 time it looks like and really is like the 

 mature stage of some tailed crustacean 

 like a crayfish. A barnacle, which looks 

 little like a crayfish or crab in its ma- 

 ture stage, is hardly to be distinguished in its immature life from a 

 young crab or lobster. Sacculina, which is a still more degenerate 

 crustacean, is only a sort of feeding sac with rootlet-like processes 

 projecting into the body of the host crab on which it lives as a 

 parasite, but the young free-swimming Sacculina is essentially like a 

 barnacle, crayfish, or crab in its young stage. 



However, it is obvious that this recapitulation or repetition of 

 ancestral stages is never perfect, and it is often so obscured and modi- 

 fied by interpolated adaptive stages and characters that but little of an 

 animal's ancestry can be learned from a scrutiny of its development. 



Fig. 28.— Metamorphosis of a 

 barnacle, Lepas. a-, larva; b, adult. 

 (From Jordan and Kellogg.) 



