132 LAWS OF VARIATION. [Chap. V 



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 

 organisation has generally been taken advantage of in giving secon- 

 dary 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 extra- 

 ordinary 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 descen- 

 dants — which on our view 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 constitution from a common parent, and exposed to similar 

 influences, naturally 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 modifica- 

 tions 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 between the 

 offspring and their parents — and a cause for each must exist — we 

 have reason to believe that it is the steady accumulation of bene- 

 ficial differences which has given rise to all the more important 

 modifications of structure in relation tc the habits of each species. 



