J64 LAWS OF VARIATION, [Chap, V. 



CHAPTEE V, 



LAWS OF TAEIATION. 



Effects of changed conditions — Use and disuse, combined with 

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

 tion — 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 — Reversions to 

 long-lost characters — Summary. 



I HAVE hitherto sometimes spoken as if the varia- 

 tions — 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 acknowl- 

 edge 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 pro- 

 duce individual differences, or slight deviations of struc- 

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

 fact of variations and monstrosities occurring much 

 more frequently under domestication than under na- 

 ture, 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 the 



