EVOLUTION AND HEREDITY 343 



fossil records are lacking, but as far back as they can be read, 

 they clearly indicate that the simplest, or most primitive, 

 animals appeared first. Though many primitive types 

 often persisted for long ages, there has been in general a 

 progressive change to new and often improved forms which 

 better met changing conditions of environment. In spite 

 of the imperfection of the fossil record, there are a number 

 of indubitable cases where series of stages which follow 

 each other in order are known (horse, elephant, etc.). 

 Thomson and Geddes say, for example: " What seems clear 

 is this, that in early Eocene times there lived small five- 

 toed hoofed quadrupeds of generalized type that the de- 

 scendants of these were gradually specialized throughout 

 long ages along similar but by and by divergent lines, that 

 they lost toe after toe until only the third remained, that 

 they gained longer necks, more complex teeth and larger 

 brains. So from the short-legged splay-footed plodders of 

 the Eocene marshes there were evolved light-footed horses 

 running on tiptoe on the dry plains. " 



Modern animals for the most part fall readily into a small 

 number of phyla, which are rather sharply set off from each 

 other. Only a few types have come down from the past, 

 but there are some fossils (Archseopteryx, Fig. 102) which fill 

 in certain gaps and indicate what the evolutionary changes 

 have been. We also have living a few "links," like Peripa- 

 tus (Fig. 33) and the duckbill (Fig. 107) which suggest re- 

 lationships that would have remained unsuspected in their 

 absence. Palaeontology tells a story which is fragmentary 

 but may be interpreted only in the light of evolutionary 

 theories, and the story has no events that seriously inter- 

 fere with such an interpretation. 



Groups of modern animals in many instances are known 

 to have had their past evolution in localities where they 

 are now abundant. Each continent has certain types that 

 are characteristic, both as fossils and as animals now living. 

 Furthermore, groups which are known to have originated 

 in rather recent geological ages have limited ranges of 



