PREFACE xlil 



prevents the optical comparison of form or coloration when 

 the specimens are arranged side by side in a series, the eye 

 being incapable of taking within its area of vision at close 

 range more than one specimen, or part of one, at a time, so 

 that comparison by measurements must be resorted to. 

 The smaller mammals, on the other hand, can be compared 

 much more minutely because a large number of them can 

 be brought within the range of vision at the same time ; and 

 also, because of their smaller size, larger numbers of them 

 are available in museums for study. The specimens of large 

 mammals being limited in number, and their size being so 

 great, minute discrimination cannot be employed in deter- 

 mining their characters. 



A "species," as we have used it throughout the text, 

 may be defined as the most limited collection of individuals 

 showing some one structural or color character or combina- 

 tion of characters distinct from the character or characters 

 possessed by any other group of individuals. A subspecies 

 or race we may define as a closely related group of indi- 

 viduals occupying a certain portion of the area occupied 

 by the species to which it refers, and exhibiting intermedi- 

 ate characters in part or in all of the individuals occupying 

 the intermediate regions between the centre of development 

 of the race and the centre of development of any other race 

 of the same species. In other words, the subspecies is a 

 geographical race linked with any other subspecies of the 

 species to which it refers by individuals exhibiting inter- 

 mediate characters. We regard degree of difference as the 

 essential feature in the determination of species and hold 

 that geographical isolation is not of itself of specific 

 value if the individuals concerned show but slight, even if 



