THE DOCTRINE OF EVOLUTION 107 



struggle between the individuals of one sex, generally the 

 males, for the possession of the other sex. The result is not 

 death to the unsuccessful competitor, but few or no offspring. 

 Sexual selection is, therefore, less rigorous than natural selec- 

 tion. Generally the most vigorous males, those which are best 

 fitted for their place in nature, will leave most progeny. But 

 in many cases victory depends not so much on general vigor 



FIG. 56. Stag-Beetle (male and female). Natural size 



as on having special weapons confined to the male sex. A 

 hornless stag or spurless cock would have a poor chance 

 of leaving numerous offspring." The greater brilliancy of 

 many males is accounted for by ascribing it to the choice 

 by the females, through countless generations, of the most 

 brilliantly colored and attractive males. The song of male 

 birds is accounted for in a similar manner. 



The Inheritance of Acquired Characters. Though Darwin 

 considered that species have arisen largely through the action 

 of natural selection on favorable variations, he admitted also 

 other factors in evolution, on which some naturalists to-day 

 lay great stress. It is a truism of our everyday life that the 

 use of an organ sooner or later affects its structure. Thus 

 the brawny arm of the blacksmith may be directly attributed 

 to the kind of work he does. There are many cases in the 

 animal kingdom where the characters of animals might be 

 interpreted as due to the use or disuse of organs. Among 

 the insects we may instance the enlarged fore legs of the 



