THE ANIMALS OF THE PAST 



425 



the newest or latest animals. An examination of a whole 

 series of strata and their fossils shows that what we call 

 the most specialized or most highly organized animals did 

 not exist in the earliest epochs of the earth's history, but 



FIG. 246. An Ostracoderm (Pterichyodes milleri), Lower Devonian of Scotland. 

 After TRAQUAIR. (The jointed appendage on the head is not a limb.) 



that the animals of these epochs were all of the simpler or 

 lower kinds. For example, in the earlier stratified rocks 

 there are no fossil remains of the backboned or vertebrate 

 animals. When the vertebrates do appear, through several 

 geological epochs they are fishes only, members of the low- 

 est group of backboned animals. More than this, they 

 represent generalized types of fishes which lack many of 



FIG. 247. An Arthrodire (Coccosteus decipiens], Lower Devonian of Scotland. After 



WOODWARD. 



the special adaptations to marine life that modern fishes 

 show. For this reason, they bear a greater resemblance to 

 the earlier reptiles than do the fishes of to-day. In other 

 words, they were a generalized type, showing the begin- 

 nings of characters of their own and other types. It is 

 always through generalized types that great classes of 

 animals approach each other. 

 28 



