THE HISTORY OF ANIMAL LIFE 



407 



evidence for organic evolution that has been preserved by the sur- 

 face of the earth as it has undergone geological changes, (i) The 

 oldest rocks contain the most simple forms and do not contain the 

 remains of more complex animals found in other rocks of more 

 recent date. (2) New forms that are found in later periods are 

 present together with the nearest related forms of older periods. 



Fig. 225. — Stages in the evolution of the foot of the modern horse. A, B, C, and D 

 were North American forms. E migrated to Asia. F is the modern type. (After 

 Kiihn: Grundriss der allgemcinen Zoologie, published by Georg Theime, Leipzig.) 



(3) The sequence in time in which the various phyla, classes, and 

 other groups appear is exactly the sequence that appears when 

 present-day animals are arranged in order of increasing complexity. 

 From the paleontological facts at hand our appreciation of the 

 capacity of living matter to give rise to varieties of animal forms 

 is greatly enlarged. Moreover, there is no reason to believe that all 

 possible types of animals have appeared, that modern types taken 

 together with those that are extinct represent all that protoplasm 

 is capable of developing. In other words, there is no reason to sup- 



