ORGANIC EVOLUTION 63 



donkey. Existing horses have one toe on each 

 foot — the digit corresponding to the big middle 

 finger — and the ruins of two others in the form of 

 spHnts on the back of each ankle. In the embryo 

 of the horse these splints are segmented, each of 

 them, into three phalanges. Fossil remains repre- 

 senting all stages in the development of the horse 

 have been found in the regions about the upper 

 waters of the Missouri River. 



It is an important fact that the types of struc- 

 ture forming any series grow more and more 

 generalised as the distance from the present 

 increases, and that different lines of development, 

 when traced back into the past, often converge in 

 types which combine the main characters of 

 various existing groups. The horses, rhinoceroses, 

 and tapirs, great as are the differences among 

 them now, can be traced back step by step through 

 fossil forms, their differences gradually becoming 

 less marked, until 'the lines ultimately blend 

 together, if not in one common ancestor, at all 

 events into forms so closely alike in all essentials 

 that no reasonable doubt can be held as to their 

 common origin.' * The four chief orders of the higher 

 mammals — the primates, ungulates, carnivora, and 

 rodents — seem to be separated by profound gulfs, 

 when we confine our attention to the representa- 

 tives of to-day. But these gulfs are completely 

 closed, and the sharp distinctions of the four orders 

 are entirely lost, when we go back and compare 

 their extinct predecessors of the Cenozoic period, 

 who lived at least three million years ago. There 



