HEREDITY AND EVOLUTION 187 



beings or to external conditions, but on a struggle between 

 the individuals of one sex, generally the males, for the pos- 

 session of the other sex. The result is not death to the un- 

 successful competitor, but few or no offspring. Sexual 

 selection is, therefore, less rigorous than natural selection. 

 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 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 ac- 

 tion of natural selection on favorable variations, he ad- 

 mitted also other factors in evolution on which some 

 naturalists today 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. Among 

 the insects we may instance the enlarged fore legs of the mole 

 cricket and mantid, the enlarged hind legs of the grass- 

 hopper, and the absence of eyes in certain cave-inhabiting 

 insects. With this principle are associated the names of 

 Erasmus Darwin, grandfather of Charles Darwin, and the 

 French naturalist Lamarck. 



The chief difficulty in the application of the principle of 

 the inheritance of acquired characters as a factor in evolu- 

 tion lies in the fact that we have little or no evidence that 

 the characters acquired by use or disuse during the life 

 of an individual are transmitted to its descendants. Many 



