CHAPTER VIII 



LOCALLY DIFFERENTIATED FORMS. Continued 



CLIMATIC VARIETIES 



In this chapter we will examine certain cases which illustrate 

 phenomena comparable with those just considered, though as 

 I have already indicated, they form to some extent a special 

 group. The outstanding fact that emerges prominently from 

 the study of the local forms is that when two definite types, 

 nearly allied, and capable of interbreeding with production of 

 fertile offspring, meet together in the region where their dis- 

 tributions overlap, though intergrades are habitually found, 

 there is no normally or uniformly intermediate population oc- 

 cupying the area of intergradation. Such phenomena as these 

 must, I think, be admitted to have great weight in any attempt 

 to construct a theory of evolution. True we must hesitate in 

 asserting their positive significance, but I see no escape from the 

 conclusion that they throw grave doubt on conventional views. 

 Again and again the same question presents itself. If A and B 

 lately emerged from a common form why is that common form 

 so utterly lost that it does not even maintain itself in the region 

 of overlapping? Almost equally difficult is it, in the cases which 

 I have numerated, to apply concrete suggestions based on any 

 factorial scheme. We may see that in Heliconius erato the type 

 with the red mark on the hind wing probably contains a dominant 

 factor, and that where the red mark is absent the metallic colours 

 are exposed ; and that similarly the green metallic colour may have 

 another factor which distinguishes it from the blue. In this way 

 we can fairly easily represent the various types of erato on a 

 factorial system as the result of the various possible combinations 

 of two pairs of factors. But there we stop, and we are quite 

 unable to suggest any reason why one area should have the red 

 and the green type while another should have the blue also. So 

 again with Colaptes or the Warblers. By application of a fac- 



164 



