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THE AMERICAN NATURALIST [Vol. XLIII 



These several conditions alone render it probable that variability 

 of every kind is directly or indirectly caused by changed conditions 

 of life. Or, to put the case under another point of view, if it were 

 possible to expose all the individuals of a species during many genera- 

 tions to absolutely uniform conditions of life, there would be no 

 variability. 11 



We have good reason to believe, as shown in the first chapter, that 

 changes in the conditions of life give a tendency to increased variability, 

 and in the foregoing cases the conditions have changed, and this would 

 manifestly be favorable to natural selection, by affording a better 

 chance of the occurrence of profitable variations. 12 



Use and Disuse. 



It is notorious, and we shall immediately adduce proofs, that in- 

 creased use or action strengthens muscles, glands, sense-organs, etc., 

 and that disuse, on the other hand, weakens them. 13 



There can be no doubt that with our anciently domesticated animals, 

 certain bones have increased or decreased in size and weight owing to 

 increased or decreased use. With animals living a free life and 

 occasionally exposed to severe competition, the redaction would tend 

 to be greater, as it would be an advantage to them to have the develop- 

 ment of every superfluous part saved. 14 



Competition 



In spite of the importance Darwin assigned to com- 

 petition, for to him it usually comprises the whole process 

 of natural selection, he gave little thought to its analysis 

 and almost none to its investigation. His ideas of com- 

 petition were drawn largely from Lyell and Herbert, and 

 he was content to take it as a universal and fundamental 

 process among living things, without detailed inquiry 

 into its working or its precise relation to the origin of 

 new forms. Darwin's actual contribution rests chiefly 

 upon his formulation of the following law of competition, 

 which recent quantitative experiments indicate to be 



