^1^ THE NEW EVOLUTION "^^^^ 



groups. Now let us see what may be ascertained by 

 an analysis of animal forms. 



From the study of the development of animal forms 

 as they are preserved in successive geological deposits 

 we learn that after a special line of development begins 

 the types involved always increase in their degree of 

 specialization. They divide into various subtypes 

 each of which becomes more and more specialized, 

 and these may again divide. Finally the end branches 

 one after another, or sometimes simultaneously, come 

 to an end and the type becomes extinct. 



An excellent illustration of this is seen in the horses, 

 the tree-like figure rising from the little Eohippus as 

 a base and giving off a great number of increasingly 

 specialized branches ending in still more specialized 

 twigs about ten of which (the wild horse, the wild 

 donkeys and the zebras) still are to be found in Africa 

 and in Asia while the rest are all extinct. 



At the base of every branching line of progressive 

 specialization — or developmental tree — we find a type 

 or form which in its structure includes all of the 

 features found collectively in all the later types. 



Specialization, or developmental progress, is a 

 matter of subtraction and modification — never of addi- 

 tion. No structure lost ever is replaced. 



Thus Eobippus, with four toes on the fore feet and 

 three on the hind feet, was succeeded by other horses 

 with three toes on all the feet. In still later horses 

 the middle toe increased, while the lateral toes de- 

 creased in size. Finally we have the living horses 

 with a single toe on all the feet. 



[2.04] 



