278 



The Study of Animal Life 



PART IV 



between order of appearance and structural rank is often 

 true in detail within the separate classes of animals. There 

 are some marvellously complete series of fos- 

 sils, especially, perhaps, that of the extinct 

 cuttlefishes, in which the steps of progressive 

 evolution are still traceable. Moreover, the 

 long pedigree of some animals, such as the 

 horse, has been worked out so perfectly that 

 more convincing demonstration is hardly pos- 

 sible. In Professor Huxley's American Ad- 

 dresses, or in that pleasant introduction to 

 zoology afforded by Professor W. H. Flower's 

 little book on the horse (Modern Science 

 Series, Lond., 1891), you will find the story 

 of the horse's pedigree most lucidly 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 dis- 

 appeared 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- 

 jj - v u « footed plodders of the Eocene marshes. Fin- 

 ally, there are many extinct types which link 



Fig. 62 



and hhid feet°of order to order and even class to class, such 

 the horse and' as that strange mammal Phenacodus, which 



some of its an- ° . . 



cestors, show- seems to occupy a central position in the 

 reduct1on radU in 1 ser i es j s0 numerous are its affinities, or such 

 the number of as those saurians which link crawling reptile 



digits. (From , . , 



Chambers'siTB- to soaring bird. 



cyclop, after Another historical argument of great im- 

 portance is that derived from the study of 

 the geographical distribution of animals, but this cannot be 

 appreciated without studying the detailed facts. These 

 suggest that the various types of animals have spread from 



