EVIDENCES FROM EMBRYOLOGY 17t 
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 —_ Fic. 38.—Metamorphosis of a 
mature stage of some tailed crustacean barnacle,Lepas. a, larva; b, adult. 
likea crayfish. A barnacle, which looks Grom Lenton ond Helloss) 
a 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. 
