1G6 SUMMARY. Chap. V. 



org-ans bcin,;^ still variable, because they have recently viiried 

 and thus come lo differ; but vrc have also seen in the second 

 chapter that the same principle applies to the whole individual ; 

 for in a district Mhcre many species of any genus are found — 

 that is, where there has been much former variation and differ- 

 entiation, 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. Secondary 

 sexual characters are highly variable, and such characters differ 

 much in the species of the same group. Variability in the 

 same parts of the organization has generally been taken ad- 

 vantage of in giving secondary sexual differences to the sexes 

 of the same species, and specific differences to the several spe- 

 cies of the same genus. Any part or organ developed to an 

 extraordinary size or in an extraordinary manner, in compari- 

 son 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 Ave 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 natiu"al 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 Wew must be a 

 very slow process, requiring 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 influences, 

 natui-ally tend to present analogous variations, or these same 

 species may occasionally revert to some of the characters of 

 their ancient progenitors. Although new and important modi- 

 fications may not arise from reversion and analogous variation, 

 such modifications will add to the beautiful and harmonious 

 diversity of Nature. 



Whatever the cause may be of each slight difference be- 

 tween the offspring and their parents — and a cause for each 

 must exist — it is the steady accumulation, through natural se- 

 lection, of beneficial differences that has given rise to all those 

 modifications of structure which are the most important for the 

 welfare of each species. 



