360 BIOLOGY AND ITS MAKERS 



The geological record of other families of mammals has 

 also been made out, but none so completely as that of the 

 horse family. The records show that the camels were native 

 in North America, and that they spread by migration from 

 the land of their birth to Asia and Africa, probably crossing 

 by means of land-connections which have long since become 

 submerged. 



The geological record, considered as a whole, shows that 

 the earlier formed animals were representatives of the lower 

 groups, and that when vertebrate animals were formed, for 

 a very long time only fishes were living, then amphibians, 

 reptiles, birds, and finally, after immense reaches of time, 

 mammals began to appear. 



Connecting Forms. — Interesting connecting forms be- 

 tween large groups sometimes are found, or, if not connecting 

 forms, generalized ones embracing the structural character- 

 istics of two separate groups. Such a form is the archa?op- 

 teryx (Fig. 108), a primitive bird with reptilian anatomy, 

 with teeth in its jaws, and a long, lizard-like tail covered with 

 feathers, which seems to show connection between birds and 

 reptiles. The wing also shows the supernumerary fingers, 

 which have been suppressed in modem birds. Another sug- 

 gestive type of this kind is the flying reptile or pterodactyl, 

 of which a considerable number have been discovered. 

 Illustrations indicating that animals have had a common line 

 of descent might be greatly multiplied. 



The Embryological Record and its Connection with Evo- 

 lution. — The most interesting, as well as the most compre- 

 hensive clues bearing on the evolution of animal life are 

 found in the various stages through which animals pass on 

 their way from the egg to the fully formed animal. All 

 animals above the protozoa begin their lives as single cells, 

 and between that rudimentary condition and the adult stage 

 every gradation of structure is exhibited. As animals de- 



