CHAPTER V 



LAWS OF VARIATION 



Effects of external conditions — Use and disuse, combined with 

 natural selection ; organs of flight and of vision— Acclimatisa- 

 tion — Correlation of growth — Compensation and economy of 

 growth— False correlations— Multiple, rudimentary, and lowly 

 organised structures variable— Parts developed in an unusual 

 manner are highly variable: specific characters more variable 

 than generic : secondary sexual characters variable— Species of 

 the same genus vary in an analogous manner— Reversions to 

 long-lost characters— Summary. 



I have hitherto sometimes spoken as if the variations 

 — so common and multiform in organic beings under 

 domestication, and in a lesser degree in those in a state 

 of nature — had been due to chance. This, of course, is 

 a wholly incorrect expression, but it serves to acknow- 

 ledge plainly our ignorance of the cause of each par- 

 ticular variation. Some authors believe it to be as much 

 the function of the reproductive system to produce 

 individual differences, or very slight deviations of 

 structure, as to make the child like its parents. But 

 the much greater variability, as well as the greater 

 frequency of monstrosities, under domestication or 

 cultivation, than under nature, leads me to believe 

 that deviations of structure are in some way due to the 

 nature of the conditions of life, to which the parents 

 and their more remote ancestors have been exposed 

 during several generations. I have remarked in the 

 first chapter — but a long catalogue of facts which cannot 

 be here given would be necessary to show the truth of 

 the remark — that the reproductive system is eminently 



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