164 LAWS OF VARIATION. [Chap. V. 



CHAPTEE Y. 



Laws of Variation. 



Effects of changed conditions — Use and disuse, combined with, 

 natural selection ; organs of flight and of vision — Acclimatisation 

 — Correlated variation — 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 — Eeversions to long-lost characters 

 — Summary. 



I have hitherto sometimes spoken as if the variations 

 — so common and multiform with organic beings under 

 domestication, and in a lesser degree with those under 

 nature — were 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 

 particular variation. Some authors believe it to be as 

 much the function of the reproductive system to 

 produce individual differences, or slight deviations of 

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

 the fact of variations and monstrosities occurring much 

 more frequently under domestication than under 

 nature, and the greater variability of species having 

 wide ranges than of those with restricted ranges, lead 

 to the conclusion that variability is generally related 

 to the conditions of life to which each species has been 

 exposed during several successive generations. In 



