CHAPTER VIII 



HISTORY OF THE PERISSODACTYLA 



In attempting to trace the evolutionary history of the various 

 mammahan groups, it is necessary to bear in mind the inevitable 

 limitations of work of this kind. Speaking of plants, Dr. 

 D. H. Scott says : "Our ideas of the course of descent must of 

 necessity be diagrammatic ; the process, as it actually went 

 on, during ages of inconceivable duration, was doubtless in- 

 finitely too complex for the mind to grasp, even were the whole 

 evidence lying open before us. We see an illustration, on a small 

 scale, of the complexity of the problem, in the case of domesti- 

 cated forms, evolved under the influence of man. Though 

 we know that our cultivated plants, for instance, have been 

 developed from wild species within the human period, and 

 often within quite recent years, yet nothing is more difficult 

 than to trace, in any given instance, the true history of a field- 

 crop or garden plant, or even, in many cases, to fix its origin 

 with certainty." ^ With some mammalian groups the task, 

 though difficult enough, is not so hopeless, because of more 

 complete records, yet in dealing with mammals a very trouble- 

 some complication is introduced by the existence within the 

 families, and even within the genera, of two or more parallel 

 phyla, or genetic series. Without complete and perfect mate- 

 rial it is impossible to make sure that we are not confusing 

 the different phyla with one another and placing in one series 

 species and genera that properly belong in a different one. 

 Thus, Osborn distinguishes no less than seven such phyla 



1 D. H. Scott, Studies in FossU Botany, London, 1900, pp. 524-525. 



288 



