VESTIGIAL STRUCTURES. The study of vestigial characters in highly speeia hied animals gives the elite to their ancestry. Thus, man has in his body about seventy vestigial structures which appear to be of no use to him: some, as the ea-cal appendage (see VERAtwonm AnPExindx), a positive menace: and all these afford the strongest possi ble circumstantial evidence of his descent from an arboreal ancestor.
The study of the changes undergone by animals like the frog or butterfly after birth, or what we call 'metamorphosis,' is rich also in facts and suggestions which tend to prove that such won derful changes are due to the action of the primary factors of organic evolution. it is so also with the hypermetamorphosis of certain in sects. On the other hand, in groups of animals which normally undergo a metamorphosis de velopment may, with a changed environment, be direct or abridged. It is so with the lobster, cer tain crabs, some insects. and especially some of
the tree-toads of the West Indies and of South America. As examples, a Guadeloupe species of Hylodes (q.v.) is hatched in the form of the adult ; since there are no marshes on the island, the tadpole state is suppressed. or passed through in an abbreviated way in the embryo. On the island of Martinique the young are tadpoles. but they are carried on the parent's back. The Surinam toad (Pipa) has similar breeding habits, yet the young have small gills, which. however, are of no use to them, as the tadpoles do not enter the water, but are carried about in cavities On the back. where the young pass through an abridged metamorphosis.
We have also seen that parthenogenesis (q.v.) is due to differences in temperature and food, while the alternation of generations (q.v.) of the hydroids is directly conditioned by the environ ment.