616 



COLLEGE ZOOLOGY 



Figure 436. The distribution of the major vertebrate groups throughout geologic time is 

 shown here in graphic form. Changes in width of the black areas indicates the relative abundance 

 of each group through time; broken lines suggest possible sources and time of origin for certain 

 groups. Like all geologic tables, this one reads from the bottom upwards. (After Colbert. Evolu- 

 tion of the Vertebrates. Copyright 1955 by John Wiley & Sons.) 



Such a table shows that the invertebrates 

 appeared first, since their remains occur in 

 the oldest layers of rock, unaccompanied by 

 remains of vertebrates; that the inverte- 

 brates became more complex in the suc- 

 ceeding periods; that the fishlike ostraco- 

 derms (low in the scale of vertebrate life) 

 were the first fossil vertebrates to appear; 

 and that these were followed by the fishes, 

 amphibians, reptiles, birds, and mammals 

 in the order that would be expected from 

 a study of the structure of these vertebrates. 

 This succession of the groups of vertebrates 

 is even more dramatically shown in the ac- 

 companying diagram (Fig. 436). The most 



convincing evidence derived from paleontol- 

 ogy results from investigation of the lines 

 of descent of single groups, such as the 

 horses, camels, and elephants. 



Evolution of the horse 



One the best arguments in favor of the 

 principle of organic evolution is furnished 

 by our knowledge of the evolution of the 

 horse. The horses now living in America are 

 descendants of domesticated animals, which 

 were brought to this country by the early 

 settlers from Europe; but, in prehistoric 

 times, the ancestors of our modern horse 



