7] TYPES AND AREAS OF NATURAL DISTRIBUTIONS 207 



which we dealt with earher in this chapter are examples of polytopy, 

 which we must now consider briefly from the point of view of the 

 origin of the areas involved. 



Excluding the old conception of special creation, it is yet believed 

 by some that approximately polytopic forms may have had an 

 independent origin in their existing plurality of areas whose popula- 

 tions are similar because of parallel descent from a common ancestor. 

 This would explain discontinuity on the basis of the species con- 

 cerned having evolved independently in two or more separate areas. 

 But whereas this seems possible where a common near-ancestry and 

 minor taxa are concerned (for example, through natural selection 

 acting on a similar set of mutants), it scarcely seems conceivable 

 for members of major species whose common ancestry was remote 

 and whose distinctive characters are numerous. Largely separate 

 is the contention of the differentiation hypothesis that different 

 species of the same genus may have ' crystallized out ' from an 

 ancestral complex in two or more areas independently. Some 

 students even hold that the polytopic populations in different areas 

 have had an independent origin from taxonomically different 

 ancestors and have arrived at their present similarity through con- 

 vergent evolution. However, in the words of Cain (in the v\ork 

 cited at the end of the last chapter) such polyphylesis ' for most 

 students of evolution, genetics, and taxonomy, represents only the 

 result of inadequate knowledge, or the forming of groups (genera, 

 families) for practical convenience, except in cases of hybrid 

 descent . . .' 



More widely accepted is the hypothesis that polytopic forms are 

 immediately related, the intervening tracts having been ' bridged ' 

 in the past either by a continuous series of populations or by long- 

 distance dispersal. However, as in other biological instances, it 

 seems likely that different explanations apply in different cases. 

 Thus concerning dispersal, it has been calculated that even if the 

 probability that some member of a population will cross a barrier 

 in any one year is virtually nil, during the course of a million years 

 the event will be probable, and in ten million years almost certain. 

 With regard to the disruption of areas that were previously con- 

 tinuous, though not necessarily at one and the same time, we have 

 already discussed, especially in the last chapter, such possible causes 

 as continental drift, land-bridges, and climatic and other change. 

 We have also seen how isolated centres of survival may become 

 centres of dispersal, upon the return of more favourable conditions 



