66 ADAPTATION AND PROGRESS 
line of argument which will not be duplicated in a later part of 
this discussion. 
Concerning the causes which lead to the extinction of races of 
man, we are told that “unfavorable physical conditions appear to 
have had but little effect,” but that ‘‘ extinction follows chiefly 
from the competition of tribe with tribe, and race with race”’; 
also, that ‘‘ when civilized nations come into contact with bar- 
barous, the struggle is short, except where a deadly climate gives 
aid to the native race.” Among the specific causes for this ex- 
termination, new diseases and vices are mentioned as being among 
the most potent.! 
Next to natural selection, the doctrine of sexual selection is 
the great original contribution of Darwin, although in a sense it 
may be considered but a modification of the former. The prob- 
lem is to account for the development of secondary sexual dif- 
ferences, among other things for the fact that in most species of 
birds the males are more conspicuously beautiful than the 
females. In contrast to natural selection which has to do with 
the results of a life and death struggle for existence, sexual 
selection has to do merely with the process and results of mating 
whereby certain qualities are selected and transmitted. The 
more vigorous males or those better weaponed, secure possession 
of the desired females leaving the weaker males to mate with the 
females that are left over. The supposition is that the more 
vigorous pairs will leave the most numerous offspring. Or, 
again, the females are supposed to exercise choice and select the 
more brilliant or active males, with the same result. Such selec- 
tion is most easily secured when the males largely exceed the 
females in number, otherwise resort is made to the hypothesis 
that the more vigorous are ready to mate first either physiologi- 
cally, or by virtue of reaching first the breeding place, and so 
rear a more numerous progeny. 
1 Descent of Man, pp. 229 £. 2 Tbid., chs. VIII and XIII. 
3 Professor T. H. Morgan has formulated twenty objections to this doctrine, 
among others that “ there is no evidence that the more precocious females would 
rear a larger number of offspring than the more normal females, or even those that 
breed somewhat later.” Evolution and Adaptation, ch. VI; cf. Kellogg, op. cit., 
p. 118. 
