LAWS OF VARIATION. 1M* 



CHAPTER V. 



LAWS OF VARIATION. 



Effects of Changed Conditions — Use and Disuse, combined with Nat- 

 ural Selection ; Organs of Flight and of Vision — Acclimatization 



— Correlated Variation — Compensation and Economy of Growth. 



— False Correlations — Multiple, Rudimentary, and Lowly Organ- 

 ized Structures Variable — Parts developed in an Unusual Man- 

 ner are highly Variable: Specific Characters more Variable than. 

 Generic; Secondary Sexual Characters Variable — Species of f he 

 Same Genus vary in an Analogous Manner — Reversions to Long- 

 Tost Characters — Summary. 



I have hitherto sometimes spoken as if the variations — 

 bo common and multiform with organic beings under domes- 

 tication, 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 ignor- 

 ance 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 vari- 

 ability is generally related to the conditions of life to which 

 each species has been exposed during several successive gen- 

 erations. In the first chapter I attempted to show that 

 changed conditions act in two ways, directly on the whole or 

 ganizacion or on certain parts alone, and indirectly through 

 the reproductive system. In ail 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. 



