316 INTRODUCTION TO EVOLUTION 



white. One authority on these snails commented, 'This may seem liice a 

 trivial character on which to found a species, but it is the only constant one" 

 (Simpson, 1929). Great variety in color pattern is found within the white- 

 tipped species; the same is true within the pink-tipped one. The color of 

 the tip, then, is the distinctive characteristic which makes possible classifi- 

 cation of the varying forms into the two species. 



Turning to a very different type of animal, we have an example of species 

 differences between the two species of "deer mice" or "white-footed mice" 

 inhabiting Vermont. These familiar inhabitants of woods and fields have 

 large eyes and ears and long tails. Body and tail are dark-colored above but 

 white on the under surfaces; the feet are white. Both species belong to genus 

 Peromyscus. One belongs to species maniciilatus , the other to the species 

 leucopus. Although the two species are much alike there are a number of 

 small differences between them. The hair clothing the back of leucopus is 

 reddish brown, while the color of maniculatus is more predominantly gray, 

 mixed with brown. Compared to leucopus, maniculatus has a longer tail, 

 with a more sharply defined black stripe running the length of its upper sur- 

 face, a more slender skull, with smaller molar teeth. Thus we see again that 

 the characteristics distinguishing one species from its allies are usually small. 

 Magnitude is not the important point; it is suflficient that the characteristic, 

 though small, be possessed by all members of the group in question. 



As a second attribute of species we return to the matter of reproductive 

 isolation. As indicated in our discussion of human evolution (pp. 248-250), 

 there is great difference of opinion as to the importance to be accorded this 

 attribute. No statement we can make will be immune to criticism. Hence we 

 shall make the statement which seems most generally applicable, admitting 

 that there are exceptions to it. Members of separate species are usually re- 

 productively isolated from each other, meaning that they usually do not in- 

 terbreed or hybridize if and when they come into contact with each other. 

 The wording "do not" was chosen deliberately. In many cases members of 

 different species cannot interbreed. In the two species of Peromyscus inhab- 

 iting Vermont, for example, there is no evidence that it is possible for inter- 

 breeding to occur, either in a state of nature or when members of the two 

 species are kept together in cages. On the other hand, many cases are known 

 of species which can interbreed but do not do so under natural conditions. 

 Referring to Peromyscus again, we find that our species leucopus ranges 

 southward into Virginia. Another species, gossypinus, is found in the south 

 Atlantic states, ranging northward into Virginia. In the Dismal Swamp of 

 Virginia the ranges of the two species overlap so that members of both live 

 together there. From a study of the mouse population of that swamp Dice 



