monkey, the wing of bat and bird, the pectoral fins of whale and Ichthyosaurus, the fore limb of horse and frog, one would not believe that they are all identical structures. Nevertheless, Comparative Osteology has shown that the fore limb in every Vertebrate is composed of the same bones, joined in the same way (Figs. 82 to 86), giving attachments to the same kind of muscles, though serving very different purposes, as in the cases just mentioned. There seems to be but one explanation for the existence of these similar parts with dissimilar uses, namely, that the Vertebrata have descended from one common ancestor, and that their posterity, subjected to different conditions of existence, have had their originally similar structures more or less modified.
Embryology has shown that the early conditions of all Vertebrata are alike, so much so that it is impossible to distinguish the young turtle, chicken, dog, and man (Figs. 178 to 181) from one another at certain stages of their existence; and that in proportion as the animals are alike when mature, the longer will their young resemble each other, whereas in those animals which are most unlike when adult, it will be found that their young early indicate difference; and that what is transitory, in the higher animals is retained permanently in the lower,—the higher animals representing at some time the lower. These facts can only be explained by the theory that the Vertebrata are the descendants of a common ancestor. Geology has shown that the earth has experienced great changes through past time,—the sea washing away the land, the land filling up the sea, together with other causes, changing entirely the conditions of existence. Some of the animals living at that day, not capable of resisting such changes, perished, in many cases leaving their skeletons well preserved, as imperishable proof of their having lived. Such are known as fossils, and the study of these ancient remains
