OVERLAPPING FORMS i 47 



sarily happens that the collector finds one form in one locality 

 and another in a distinct locality, and there is no evidence as to 

 the behaviour which the two representative species might exhibit 

 if they came into touch with each other. In the most familiar 

 examples of such distinction each inhabits an island, completely 

 occupying it to the exclusion of any other similar form. It can 

 only be when the two representative species occupy parts of a 

 continental area connected with each other by regions habitable 

 for the organism in question, that there is a chance of seeing the 

 two forms in contact. Often also, even where this condition is 

 satisfied, the habits, social organisation, or some other special 

 cause may act as a barrier which prevents the distinguishable 

 forms from ever coming into such complete contact as to inter- 

 breed or to behave as a genetically continuous race. When 

 genetic continuity is ensured by a constant diffusion of the popu- 

 lation over the whole area which they inhabit there will mani- 

 festly be no formation of local races. The practical uniformity, 

 for example, of so many species of birds which inhabit widely 

 extended ranges of Western Europe is doubtless maintained by 

 such constant diffusion. When, as in the case of the Falcons, 

 many localities have peculiar forms, the fact may be taken as 

 conclusive evidence that there is little or no diffusion; and when 

 we find in such a species as the Goldfinch that in spite of mi- 

 gratory fluctuations there are nevertheless geographical races 

 fairly well differentiated, it may similarly be inferred that these 

 fluctuations habitually move up and down on paths which do 

 not intermingle. There are however a few examples of animals, 

 not given to much irregular wandering, which occupy a wide and 

 continuous range of diversified country and are differentiated as 

 local races in two or more districts, though the distinct races 

 meet in intervening areas. Of these the most notorious illus- 

 tration which has been investigated with any thoroughness is 

 that of the species of Colaptes (Woodpeckers) known in the United 

 States as Flickers. The study of the variations of these forms, 

 made by J. A. Allen 1 is an admirable piece of work, with which 



1 J. A. Allen, The North American Species of the Genus Colaptes, Considered with 

 Special Reference to the Relationships of C. auratus and C. cafer. Bull. Am. Mus. 

 Nat. Hist., IV, 1892. 



