UNITY AND DIVERSITY. 25 



least fitted to survive, there will usually remain a number of varieties 

 equally fitted to survive ; and that through the law of segregation con- 

 stantly operating in a species distributed in isolated groups over a consid- 

 erable area these varieties continue to diverge both in form and in habits 

 till separate species are fully established, though the external conditions 

 are the same throughout the whole area occupied by the diverging 

 forms. 



(2) Uniformity, on the other hand, is the result of community of 

 descent, and varies directly as the diffusion of consanguinity, or the 

 amount of evenly distributed intercrossing. Isolation and intergenera- 

 tion are opposing factors, the one tending to divergence of character, 

 the other to uniformity ; but the influence of natural selection may be 

 in either direction, according as its action is diverse in different parts of 

 the area or uniform throughout the whole area. When animal immi- 

 grants enter a new region in which not only the climate but the flora 

 and fauna differ widely from those found in the home of the species, 

 the probability is that they will succumb without leaving descendants 

 or that their descendants will diminish with each generation till they 

 disappear ; but if the struggle is not too severe, the species will survive, 

 and, if isolated, the divergence of character may be greatly accelerated 

 by the effects of natural selection; for the forms that will be best 

 fitted to succeed in life and to propagate their kind will differ in the 

 two regions according to the conditions under which they have to 

 compete; and the intermediate forms that are less fitted will be 

 weeded out, and their influence in crossing with the diverging kinds 

 that survive will be removed. It will be seen that natural selection 

 acts as a divergent, not by its own inherent power, but by removing 

 the intermediate varieties and thereby preventing their influence in 

 crossing; but if the competition is severe and uniform throughout 

 the area occupied by any species, its influence will be to lessen 

 divergence. 



That this double relation of natural selection to divergence on one 

 side, and to uniformity on the other, was partially apprehended by 

 Darwin, appears from his brief paragraph on Polymorphic Genera, 

 and his fuller statements concerning the extinction of intermediate 

 forms by means of natural selection; but the quotation given near 

 the beginning of this chapter shows that he did not reach the 

 conclusion which lay but one step beyond, and to which his facts so 

 clearly point. He observed that polymorphic genera are probably 

 most variable in the characters that are neither useful nor injurious 

 to the species, and are, therefore, free from the influence of natural 

 selection ; and again, in another place, he observes that large genera 



