io6 Laws of Variation. Chap. v. 



CHAPTEE V. 



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 



Reversions 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 repro- 

 ductive 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 condi- 

 tions of life to which each species has been exposed during several 

 successive generations. In the first chapter I attempted to show that 

 changed conditions act in two ways, directly on the whole organisa- 

 tion or on certain parts alone, and indirectly through the reproductive 

 system. In all cases there are two factors, the nature of the 

 organism which is much the most important of the two, and the 

 nature of the conditions. The direct action of changed conditions 

 leads to definite or indefinite results. In the latter case the organi- 

 sation seems to become plastic, and we have much fluctuating vari- 

 ability. In the former case the nature of the organism is such 

 that it yields readily, when subjected to certain conditions, and all, 

 or nearly all the individuals become modified in the same way. 



It is very difficult to decide how far changed conditions, such as 

 of climate, food, &c, have acted in a definite manner. There is 



