LAWS OF VARIATION 231 



have also seen in the second chapter that the same prin- 

 ciple applies to the whole individual; for in a district 

 where many species of a genus are found — that is, where 

 there has been much former variation and differentiation, 

 or where the manufactory of new specific forms has been 

 actively at work — in that district and among these spe- 

 cies we now find, on an average, most varieties. Sec- 

 ondary sexual characters are highly variable, and such 

 characters differ much in the species of the same group. 

 Yariability in the same parts of the organization has 

 generally been taken advantage of in giving secondary 

 sexual differences to the two sexes of the same species, 

 and specific differences to the several species of the same 

 genus. Any part or organ developed to an extraordinary 

 size or in an extraordinary manner, in comparison with 

 the same part or organ in the allied species, must have 

 gone through an extraordinary amount of modification 

 since the genus arose; and thus we can understand why 

 it should often still be variable in a much higher degree 

 than other parts; for variation is a long-continued and 

 slow process, and natural selection will in such cases not 

 as yet have had time to overcome the tendency to further 

 variability and to reversion to a less modified state. But 

 when a species with any extraordinarily-developed organ 

 has become the parent of many modified descendants — 

 which on our view must be a very slow process, requir- 

 ing a long lapse of time — in this case, natural selection 

 has succeeded in giving a fixed character to the organ, 

 in however extraordinary a manner it may have been 

 developed. Species inheriting nearly the same constitu- 

 tion from a common parent, and exposed to similar in- 

 fluences, naturally tend to present analogous variations, 



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