THE STRUGGLE FOR EXISTENCE 27 



three. The spotted chickens were not so easily seen 

 from above. Only one of them was killed. Thus the 

 mere chance of coat color was a decisive factor in de- 

 termining which chickens should survive. In time it is 

 probable that more of the black and white chickens would 

 l)e killed by birds of prey and only the spotted chickens 

 would be left. Their oiTspring would tend to inherit 

 their spotted coat and hence survive. All offspring 

 which varied from this type in the direction of having a 

 white coat or a black coat would be likely to be killed and 

 leave no black or white coated progeny. Eventually we 

 might liave only a spotted variety of chickens in this 

 area. Tliis is an excellent illustration of the ]orinciple 

 of natural selection. 



Where the characters of an inhabiting species show 

 great variation we conclude that there has not been 

 rigorous selection with reference to that character. 

 That is, the trait in question is one which, at the time 

 being, is neither of great disadvantage nor great advan- 

 tage to those who possess it. It was originally acquired 

 or preserved l)eeause it was favorable, but some change 

 has been wrought which makes it of indifferent value. 

 On the other hand if any character shows very slight 

 variation as between a large number of the species, we 

 conclude lliat selection with reference to it has been 

 severe; that is, the trait in question gives positive ad- 

 vantage. Thus, before the coming of the crows, coat 

 color in chickens was of indifferent value for survival 

 and there was wide variation from white to black. But 

 with the coming of the birds of prey, conditions were 

 changed and coat color had a positive survival value, if 

 it were inconspicuous. The wide variation soon dis- 

 appeared (the black and white chickens were killed off) 



