646 LAND MAMMALS IN THE WESTERN HEMISPHERE 



are generally implicit and often apparently unconscious. 

 In the present state of knowledge, these postulates are, for 

 the most part, matters of judgment, incapable of definite 

 proof, and they appeal with very different force to different 

 minds ; what to one seems almost self-evident, another regards 

 as all but impossible. It will, however, be of service to examine 

 such of these postulates as are involved in mammahan history. 



It is quite impracticable to construct a genetic series with- 

 out making certain assumptions as to the manner in which the 

 developmental process operated and the kinds of modification 

 that actually did occur. In the preceding chapters, which 

 deal with the evolutionary history of various mammalian 

 groups, it was repeatedly stated that, of two contemporary 

 genera, one was to be taken as the ancestor of some later form 

 and the other regarded as a collateral branch, but it was also 

 pointed out that in certain cases, palaeontologists differed more 

 or less decidedly as to the proper interpretation of the facts ; 

 it is just this lack of agreement as to the modes and processes 

 of change that forms the root of the difficulty. 



There are instructive analogies between the history, aims 

 and methods of comparative philology, on the one hand, and 

 zoology, on the other. In both sciences the attempt is made to 

 trace the development of the modern from the ancient, to 

 demonstrate the common origin of things which are now widely 

 separated and differ in all obvious characteristics, and to de- 

 termine the manner in which these cumulative modifications 

 have been effected. At the present time zoology is still far 

 behind the science of language with regard to the solution of 

 many of these kindred problems and has hardly advanced 

 beyond the stage which called forth Voltaire's famous sneer : 

 " L'etymologie est une science ou les voyelles ne font rien et 

 les consonnes fort peu de chose." Many of the animal gene- 

 alogies which have been proposed have no better foundation 

 than the "guessing etymologies" of the eighteenth century, 

 and for exactly the same reason. Just as the old etymologists 



