126 



ZAW8 OF VARIATION. 



CHAPTER V. 



LAWS OF VARIATION. 



EfEects of clianged 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 

 organized 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 do- 

 mestication, and inalesser degree with those under nature 

 — were due to chance. This, of course is a wholly incorrect 

 expression, but it serves to acknowledge plainly our igno- 

 rance 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 difierences, or slight 

 deviations of structure, as to make the child like its parents. 

 But the fact of variations and monstrosities occuri-ing 

 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 con- 

 clusion that variability is generally related to the conditions 

 of life to which each species has been exposed during sev- 

 eral successive generations. In the first chapter I at- 

 tempted to show that changed conditions act in two waj-s, 

 directly on the whole organization 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 

 organization seems to become plastic, and we have much 



