EVOLUTION AS SEEN IN THE CLASSIFICATION OF ANIMALS 321 



and west of the latter another subspecies is found: Peromyscus mcmiculatus 

 bairdi. North of Lake Superior and on into Canada is found the subspecies 

 from which came the first specimen of Peromyscus mcmiculatus known to 

 science. This specimen is called the "type specimen," and its subspecies is 

 the type subspecies, being given the name Peromyscus mcmiculatus mani- 

 culatus. 



Returning momentarily to our earlier analogy between scientific names 

 and human names, we see that the name of the subspecies corresponds in 

 a sense to a "middle name." Thus in a large city there may be so many 

 John Smiths that it is necessary to distinguish between them in some way. 

 Accordingly a metropolitan directory may list: "Smith, John Gregory; 

 Smith, John Stuart; Smith, John Wilber," and so on. 



One of the most distinctive attributes of subspecies is the fact that they 

 occupy distinct territories, hence the appropriateness of the name geo- 

 graphic race. For example, the species Peromyscus mcmiculatus is divided 

 into many subspecies scattered over the face of North America in such 

 manner that a map of their distribution resembles a patchwork quilt. 

 Fig. 14.3 shows the range of the subspecies gracilis mentioned above, to- 

 gether with portions of the ranges of other subspecies found in the north- 

 eastern part of this continent. We note that the map indicates certain regions 

 in which intergradation occurs. This is a common feature of areas in which 

 two subspecies come into contact. 



Ordinarily members of different subspecies within one species are com- 

 pletely interfertile and hybridize readily whenever they come into contact. 

 This condition stands in direct contrast to that noted previously as prevailing 

 in regions in which the territories of two different species come into contact 

 or overlap. Mechanisms of reproductive isolation which keep species sep- 

 arate have on the whole not developed between subspecies. Our discussion 

 of lack of reproductive isolation between human races will be recalled in 

 this connection (pp. 248-250). The principal factor operating to keep sub- 

 species separate is a geographic one; different subspecies live in separate 

 regions and hence for the most part do not come into contact with each 

 other. So long as this condition prevails each subspecies maintains its in- 

 tegrity. 



For the most part subspecies differ from one another in one or more 

 structural features. Usually the differences are less than those found between 

 two related species. Indeed, the visible differences may be so small that only 

 an expert can see any difference at all, and frequently an expert must have 

 a considerable number of specimens for comparison before he can be sure 



