CHAPTER V 

 Laws of Variation 



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

 selection ; organs of flight and of vision — Acclimatisation — Cor- 

 related variation — Compensation and economy of growth — False 

 correlations — Multiple, rudimentary, and lowly organised struc- 

 tures variable — Parts developed in an unusual manner are highly 

 variable ; specific characters more variable than generic : second- 

 ary 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 acknowledge 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 structttre, as to make the child like its 

 parents. But the fact of variations and monstrosities oc- 

 curring 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 at- 

 tempted to show that changed conditions act in two ways, 

 directly on the whole organisation 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- 



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