650 LAND MAMMALS IN THE WESTERN HEMISPHERE 



having a common origin may independently run through a 

 similar course of development and arrive at similar results. 

 Illustrations of this principle are given by the many phyla 

 of horses, rhinoceroses and camels, which persisted side by side 

 through several geological stages, following independent, but 

 parallel, courses of change. An even more striking case is 

 that of the two subfamilies of the cats, the true felines and the 

 fsabre-tooths. Whatever view may be taken of the relation- 

 ships of these two groups, it is clear that, at least from the 

 upper Oligocene to the Pleistocene, they were separate, but 

 kept remarkably even pace with each other in their advance 

 and specialization. 



By convergence is meant a similar result which is reached 

 by two or more independent lines having different starting 

 points, so that the descendants are more ahke than were the 

 ancestors, and is thus the opposite of divergence, the result 

 of which is to make the descendants of common ancestors 

 less and less alike with each succeeding stage. Either par- 

 alleUsm or convergence may be involved in the independent 

 acquisition of similar characters, of which these are so many 

 examples. It is obvious that this problem is fundamental and 

 that little real progress is possible until a solution is reached. 

 As to the correct solution, there is much difference of opinion 

 among naturalists. Some deny altogether the reaUty and im- 

 portance of these modes of development, but such are almost 

 exclusively concerned with the modern world ; others go to the 

 opposite extreme, and looking upon every large group as poly- 

 phyletic, consider parallel and convergent development to be 

 the rule of evolution. Few palaeontologists are disposed to 

 doubt that these modes of evolution are very frequent ; their 

 difficulty is to determine what limits can be drawn, and this 

 difficulty can be removed only by much wider and more exact 

 knowledge than we now possess. 



So far as single structures are concerned, the fossils demon- 

 strate unequivocally that they have been independently ac- 



