54 EVOLUTION OFILIFE 
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 ex- 
istence, 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 
and 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 pre- 
pro 
served, as imperishable proof of their having lived. Such 
. . : S 
are known as fossils, and the study of these ancient remain 
