xvi PREFACE 



tions of each to the others, simply because nature does not 

 develop them in one line, but by an indefinitely varied sys- 

 tem of branching in all directions. Yet in a book it is neces- 

 sary to adopt some linear system — as we have adopted it in 

 dealing with the Bovidcs^ for instance — even though we well 

 know that this system is faulty. 



Moreover, in some forms the variation will be in one 

 set of characteristics, in some forms in a totally different set. 

 In certain African antelope — the eland for instance — color 

 is a more permanent characteristic than osteological detail. 

 On the other hand, the American black bear, without change 

 of color or external peculiarity, shows here and there such 

 changes in skull structure as to have given rise not only to 

 the erection of subspecies but even of species by certain 

 learned systematists. The lion and tiger differ more from 

 one another in color and external peculiarities than do any 

 other pair of the big cats; and yet their skeletons and skulls 

 are practically indistinguishable. 



Again, the presence or lack of interfertility between 

 allied forms may have no seeming reference to the degree of 

 variation in bodily structure. The differences between 

 horses, asses, and zebras are so purely differences of color 

 and external anatomy that it is difficult to tell from the 

 skeletons of fossil remains the type to which they belonged. 

 On the other hand, the common ox, the humped ox, the 

 bison, the yak, and the gaur are sundered by such marked 

 skeletal as well as external peculiarities that many system- 

 atists put them all in different genera, and there is never 

 the slightest difficulty in telling the fossil forms apart. 

 Yet the mules of the ass and horse are infertile, whereas the 

 various species of Bovidcs mentioned above, seemingly far 



