No. 580] ORIGIN OF SIXGLE CHARACTERS 2< '9 



to classify them other than as intergrades. Particularly difficult cases 

 are those in which the intergrades approximate the color of one " sub- 

 species " and the cranial characters of another, thus reducing the ques- 

 tion of definition to one of the relative importance of characters. 

 Classification becomes like the division of a spectrum and depends 

 largely upon the standards set, for theoretically at least the possibili- 

 ties of subdivisions are unlimited. 



Some of the principles of variation [and perhaps of hybridization, 

 H. F. O.] are as follows : 



(1) Fortuitous individual variation is greatest in specimens from 

 localities lying just between the ranges of two well-established forms. 



(2) Where two genuine "subspecies" inhabit the same area and 

 maintain themselves distinct, each may in certain cases be traced by 

 a definite geographic route through every degree of inteniradatmn io 

 one parent form. For example, P. arctieus lives side by side with P. 

 algidus in the upper Yukon, but both intergrade toward the south 

 with P. oreas (see Fig. 3). If from sudden or gradual natural 

 causes these intergrades between P. arctieus, P. oreas, and P. algidus 

 were to become extinct three entirely separate and distinct subspecies 

 would apparently be created. 



(4) The "species" arc fairly well characterized in cranial propor- 



throughout a series although they often afford average proportional 

 characters of considerable value. For example, among " species " that 



cephaly is sometimes found, and \ice versa. The teeth vary chiefly in 

 proportions but seldom to great extent. Some subspecies are 



(5) In color there is a range of seasonal, polychromatic, and local 

 or geographic variation. A complete intergradation between two color 

 extremes may often be found in localities lying just between the ranges 

 of two well-established forms. Color intensities are often extremely 

 local and doubtless are produced immediately upon contact with cer- 



