CHAPTER VII 



LOCAL DIFFERENTIATION. Continued 



OVERLAPPING FORMS 



The facts of the distribution of local forms on the whole are con- 

 sistent with the view that these forms come into existence by the 

 sporadic appearance of varieties in a population, rather than by 

 transformation of the population as a whole. Of such sporadic- 

 ally occurring varieties there are examples in great abundance, 

 though by the nature of the case it can be but rarely that we are 

 able to produce evidence of a previous type being actually super- 

 seded by the variety. When the two forms are found co-existing 

 in the same area they are usually recorded as one species if inter- 

 grades are observed, and as two species if the intergrades are 

 absent. On the other hand when two forms are found occupying 

 separate areas, when, that is, the process of replacement is com- 

 pleted in one of the areas, then forthwith each is named separately 

 either as species or subspecies. Successive observations carried 

 out through considerable periods of time would be necessary to 

 establish beyond question that the history proceeds in one way 

 rather than another. Such continuity of observation has for 

 the most part never been attempted. The kind of information 

 wanted has indeed only been lately recognized, and really critical 

 collecting is a thing of only the last few decades. The methods of 

 the older collectors, who aimed at bringing together a few typical 

 specimens of all distinct forms, are of little service in this class 

 of inquiry, which is better promoted by the indiscriminate col- 

 lection of large numbers of common forms from many localities. 

 When this has been done on a comprehensive scale we shall be 

 in a position to form much more confident judgments as to the 

 general theory of evolution. 



Some little work of the kind has however been done and the 

 results are already of great value. Seeing that the differenti- 

 ation of local forms is only made possible by isolation, it neces- 



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