SPECIES AND THEIR ORIGIN 301 



that close relatives on the whole look much alike, 

 or at least resemble one another much more than 

 do remote kin. It is for the same reason, in a larger 

 way, that the diverse types of Europeans are never- 

 theless more alike than are Europeans and Mon- 

 golians. The different races of mankind are usually 

 classed as one species for reasons' that we will not 

 enter upon here, but the same phenomenon is found 

 in all lower animals and plants. Individuals of 

 common descent thus constitute groups which in 

 very many cases are identical with those segregated 

 by the diagnostic method, on the basis of a common 

 possession of characters. Here, then, we have 

 another criterion of species, community of descent. 

 Species may be not only forms which share in com- 

 mon certain physical characters ; by the same token 

 they also share a common ancestry. . 



Polymorphism. - - The importance of the latter 

 factor becomes evident when we consider the phe- 

 nomenon of dimorphism and polymorphism. Many 

 species of insects are known in which one sex occurs 

 in more than one form. Thus, in one of our common 

 American butterflies, Papilio turnus, the " tiger 

 swallowtail," the male is brilliant yellow with 

 vertical stripes on the fore wings. In the northern 

 range of the species (Canada and the North Central 

 States), the female is similar to the male, but in the 

 Southern States, in addition to the yellow females, 

 occurs also a black form without the stripes, or with 

 faint indications of them. For a long time this 



