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 byits 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 
