WHAT IS A MAMMAL? 



that this particular subspecies or variety served as the type 

 for the species. Tamias striatus fisheri is a variety which 

 differs sHghtly from typical striatus but is so closely related 

 to it that it interbreeds with it and we find examples where the 

 ranges of the two forms meet which are just as much typical 

 striatus as they are fisheri. This intergrading may extend 

 through a chain of several subspecies where subspecies A inter- 

 grades with B, B with C, C with D, et cetera. Although the 

 geographical range of subspecies A may not touch that of 

 subspecies D, so that A and D never actually intergrade 

 directly, nevertheless the subspecific relationship is well shown 

 by the intermediate members of the series and A, B, C, and 

 D are all to be considered as subspecies of the same species. 



Variability of Mammals 



Mammals vary in size, coloration, and proportion of parts. 

 This variation is usually within close limits, but sometimes 

 variation in color may cover a wide range. The descriptions 

 in this field book are intended to apply, as far as possible, to 

 the average individual of any given species and the measure- 

 ments, colors, etc., must not be regarded as narrowly restrictive. 

 Individuals may vary as much as ten per cent or more in size, 

 from the figures given, and when identifying a mammal due 

 consideration must be made for individual variation. Atten- 

 tion is generally directed to the most variable characters, in 

 the synopsis. 



Life-Histories of Mammals 



The study of the life-histories of mammals is a fascinating 

 subject and one that is by no means exhausted. While we 

 know many interesting facts about the behavior of mammals, 

 there are many details which we can only suspect, and 

 probably as many more of which we do not even have a 

 suspicion. Nor does one need to go to a far frontier to look 

 for these facts; some of our commonest mammals are today 

 only superficially known. The Mole, secreted in its subter- 

 ranean fortress, the Shrew, favored by small size and under- 

 cover activity, the Flying Squirrel, coming forth only at night, 

 and many other nocturnal wanderers have successfully con- 

 cealed many intimate details of behavior. Bats are an enigma 



