xvii THE EVIDENCES OF EVOLUTION 343 



demonstration is hardly possible. The story has been 

 often told, how in early Eocene times there lived small 

 quadrupeds, about the size of sheep, that walked 

 securely upon five toes, how these animals lost, first the 

 inner toe, while the third grew larger, and then the fifth ; 

 how the third continued to grow larger and the second 

 and fourth to become smaller until they disappeared 

 almost entirely, remaining only as small splint bones ; 

 and how thus the light-footed runners on tiptoe of the dry 

 plains were evolved from the short-legged, splay-footed 

 plodders of the Eocene marshes (fig. 113). There are 

 many extinct types which link order to order and even class 

 to class, such as Phenacodiis (fig. 97), which seems to 

 occupy a central position in the mammalian series, so 

 numerous are its affinities, or such as Archceopteryx, which 

 links crawling reptile to soaring bird. 



Another historical argument of great importance is 

 that derived from the study of the geographical distribu- 

 tion of animals, but this cannot be appreciated without 

 studying the detailed facts. These suggest that the 

 various types of animals have spread from definite centres, 

 along convenient paths of diffusion, varying into species 

 after species as their range extended. 



But the history of the individual is even more instruc- 

 tive. In a general way, especially as regards the develop- 

 ment of particular organs, the individual proceeds step 

 by step along a path approximately parallel to the pre- 

 sumed progress of the race, so far as that is traceable 

 from the successive .grades of structure and from the 

 records of the rocks. Even in details such as the de- 

 velopment of antlers on stags the parallelism of racial 

 and individual history is hinted at (fig. 114). Of this 

 correspondence it is difficult to see any elucidation except 

 that the individual in its life-history in great part re- 

 treads the path of ancestral evolution. 



We have illustrated these evidences of evolution very 

 briefly, for they have been stated many times. The gist 

 of the matter is that the evolution idea is a modal inter- 

 pretation of the way in which the present state of things 

 has come to be ; it is the only scientific interpretation in 



