CHAPTER V 



Laws of Variation 



Effects of Changed Conditions — Use and Disuse, combined with Natural 

 Selection; Organs of Flight and of Vision — Acclimatization — 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 com- 

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

 the fact of variations and monstrosities occurring much more fre- 

 quently under domestication than under nature, and the greater 

 variability of species having wide ranges than of those with re- 

 stricted 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 first chapter 

 I attempted to show that changed conditions act in two ways, di- 

 rectly 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 fluctuating variability. 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 



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