PRINCIPLES OF CLASSIFICATION 21 



species, Sciurus carolinensis leucotis (white-eared), in the 

 Northern States, larger than the true Sciurus carolinensis^ 

 with the dark band on the back narrower. In Minnesota 

 is another subspecies, Sciurus carolinensis tiypophmus, with 

 only a narrow streak of white on the belly. As animals 

 come to be better known we can recognize by name more 

 and more of these subspecies or geographical variations. 



Even in the same locality the members of a species vary 

 more or less, no two being exactly alike. The name variety 

 is applied to any sort of variation which can be recognized. 

 Usually varieties not having definite geographical range re- 

 ceive no scientific name. When forms in different geograph- 

 ical areas are found to intergrade or mix with one another 

 they are known as subspecies, the one first named being re- 

 garded as the original species. When they do not intergrade 

 they are called distinct species. The subspecies differ from 

 the species in degree only. When the range of a species is 

 crossed by an impassable barrier, the subspecies on either 

 side of the barrier usually becomes in time a distinct spe- 

 cies. Thus distinct species are said to be produced through 

 isolation. The plates which follow may serve as illustra- 

 tions of species and subspecies thus formed. 



