EVOLUTION. 329 EVOLUTION. Now, as the order of Crustacea is founded in pari 'in the nature of the carapace, or of the limbs, whether adapted for walking, swimming, in- biting, etc., the ordinal characters are evi dently due to the differenl uses to which these parts are adapted. It is so with the classes of mollusks; thf bivalves arc secondary forms which by change of habits gradually evolved from some worm-like ancestor. The gastropods, with their iinsymmetrical shells, and the creeping or swim- ming cephalopods, with their closely coiled shells (when a shell is present), are clearly the result of tin use of certain parts, the disuse of others. So it is with the orders of mammals and birds, ami the form of man is mainly due to the disuse of his feet in climbing, to his erect position, and to the use and exercise of his brain. Cooperative Evidences of Evolution. That the general theory of evolution represents the truth of history and existing facts is supported by evidence from all departments of biology. The basis of morphology is anatomy and embryology. As soon as anatomists studied the mode of de- velopment of organs and traced their history from the germ, it was found that organs of the most diverse shape and use had had a common origin. Thus the arm of man, the fore leg of the lion, the flipper of the seal, the paddle of the whale, and a fish's fin were found to be morphologically identical — the same in origin and fundamental structure — while the wings of a bird and an in- sert were perceived to be simply analogous. Thus, what seemed the most diverse organs were found to have a common origin. Also cell studies proved that the cell is the unit of organic life. Classification also yields evidence. It is now recognized that the plant and animal kingdoms may each be represented by a genealogical tree; that the members of different classes, orders, fam- ilies, genera, species, and varieties are blood rela- tions which have had a common descent from some primitive form, and that ultimately the vege- table and animal kingdoms have descended from a common ancestor. Taxonomy is an attempt to unravel these lines of descent. In classifying animals of any group (see Classification*), we are constructing a phylogeny or genealogical tree. Embryology furnishes an argument. The mode of development of an animal throws light on its affinities. Thus, the barnacle (q.v.) was sup- posed to be a mollusk until its development from a nauplius young, very similar to that of certain erustacea, proved that it is a member of that class. Mere resemblance between the young of members of different classes points unerringly to their common origin. Embryology (q.v.) teaches that all plants and animals have originated from a one-celled form. At one stage the fish, amphibian, reptile, bird, mammal, and even man are indis- tinguishable from each other, and the resem- blance of the early embryo points to the origin of all vertebrates from some worm-like form. From his studies on the embryology of vertebrates, Von Baer first indicated the 'recapitulation the- ory' —i.e. that the different stages of develop- ment of a highly specialized animal constitute an epitome or recapitulation of that of the class or t j pe t.i w hich it belongs. As stated by Von Baer, and afterwards more fully by Agassiz, the law expresses a genera] fact. It was Fritz Muller who, in 1864, explicit- ly pointed out its evolutional or phylogenetic bearings, and in 1866 Haeckel restated the doe- t rine in the follow in" i li pmental in torj (ontogeny) of an individual animal briel ly recapil ulates t he hi ' r; thi race i phy- logenj i i.e. I he mosl impoi tai gani zation which its ancestors have passed through
- i 1 1 1 k
- i r again, even it somew hat modifli
i|i elopmeni of indi ulna I animals." I hanks to recenl adt ance in mi i pholoj embryology, and particularly to the study of vestigial structures, we are in a po to work out thr phylogenj of the animal kingdom, or any group of it, with some approximation to exacti t ude. f. 'ai « it bin the limit a ol a genu il is in some ease- | k i — i 1 ■ I « - to deteel vestiges of what were primitive characters, and thus to arran in genealogical order the different species. 1 1 i this sort of work which gives new life, dignity, and importance to classification. Vim n,i i. Stiii ctubeS. The study of vestigial characters in highly specialized animals gives the clue to their ancestry. Thus, man has in his body about seventy vestigial structures which appear to be of no use to him ; Borne, as t be cs i e appendage (see Vermiform APPENDIX ), 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. In- 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. Geological Evidence. Our earth has had a history. Its age is approximately estimated to be about 50.000,000 years. Its history is divided by geologists into 'ages,' 'periods.' 'epochs.' etc. It is roughly estimated that about 30,000.000 years have elapsed since the deposition of the lowest fossiliferous rocks — those of Cambrian age. It is believed that this amount of time is suffi- cient for the origin and development of all the form-, of life with which we are thus far acquaint- ed. The stratified rocks are supposed to be about '20 miles thick, the earth's crust about 100 miles in average thickness. During the enormous space of time since the Cambrian, the forces of life and nature have
