EVOLUTION AS SEEN IN THE CLASSIFICATION OF ANIMALS 323 



cephalus. Further inland live mice which are still darker in color and are 

 named Peromyscus polionotus polionotus. 



"The Subspecies Versus the Cline" 



In the above example we note a regular progression in pigmentation: ( 1 ) 

 in the interior a dark-colored form; (2) on the beaches a lighter-colored 

 form; (3) on the neighboring island a still lighter form. As noted earlier 

 (p. 274), such a gradient, or progressive pattern of change, across the face 

 of the country is called a dine. Actually this cline has more than the three 

 steps indicated, since Sumner found that intermediate forms between race 

 polionotus and race albijrons occurred as he collected specimens nearer and 

 nearer the coast of the mainland. This being the case, can we really draw a 

 line between P. polionotus polionotus and P. polionotus albijrons? 



Two points of view are possible: (1) subspecies polionotus and sub- 

 species albijrons are realities; they interbreed when they come into contact, 

 producing intermediate forms; (2) Peromyscus polionotus is not divided 

 into subspecies but is characterized by ordered variability of hair color in 

 the form of a cline. In our earlier discussion of clines (pp. 274-275) we 

 noted the cHne exhibited by zebras in the striping of the legs (Fig. 12.6, 

 p. 275). The cline is an observable fact. The zebras are commonly con- 

 sidered to be divided into subspecies, partly upon the basis of this striping, 

 but the difficulty encountered in delimiting the subspecies is reflected in 

 the fact that no two investigators seem able to agree on how many subspecies 

 there are or what shall be considered to constitute each one (cf. Cabrera, 

 1936; Rzasnicki, 1951). This is an extreme case; the confusion arises in 

 part from the fact that no two individual zebras are alike in markings. 



Wilson and Brown (1953) have stressed the view that subspecies are 

 artificial creations of biologists' minds. Frequently a species with wide dis- 

 tribution may exhibit clines in a number of characteristics, e.g., coat color, 

 ear size, foot length, tail length. From locality to locality the variability in 

 one characteristic does not parallel variability in another one; the variabili- 

 ties are independent, not concordant as we should expect them to be if sub- 

 species (representing a certain coterie of characteristics) were realities. 

 Nevertheless, considerations of practical usefulness will doubtless continue 

 to cause biologists to designate as subspecies populations living in certain 

 territories and characterized on the average by certain "constellations of 

 characteristics." To some extent this involves putting nature in a strait 

 jacket and drawing lines where there really are none, a fact which is of it- 

 self eloquent of the occurrence of evolution. 



