176 ORIGIN OF SPECIES 



inherited, and have not differed within this same period. In 

 these remarks we have referred to special parts or organs 

 being still variable, because they have recently varied and 

 thus come to dift'er; but we have also seen in the second 

 chapter that the same principle applies to the whole indi- 

 vidual; 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 amongst 

 these species, ws 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 gener- 

 ally 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 vari- 

 able 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, 

 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 com- 

 mon 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 modifications may 

 not arise from reversion and analogous variation, such modi- 

 fications will add to the beautiful and harmonious diversity 

 of nature. 



Whatever the cause may be of each slight difference be- 



