LAWS OF VARIATION 153 



the organisation has generally been taken advantage 

 of in giving secondary sexual differences to the 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 pro- 

 cess, 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 my view must be a very slow process, 

 requiring a long lapse of time — in this case, natural 

 selection may readily have succeeded in giving a fixed 

 character to the organ, in however extraordinary a 

 manner it may be developed. Species inheriting nearly 

 the same constitution from a common parent and ex- 

 posed to similar influences will naturally tend to present 

 analogous variations, and these same species may occa- 

 sionally 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 

 in the offspring from their parents — and a cause for 

 each must exist — it is the steady accumulation, through 

 natural selection, of such differences, when beneficial to 

 the individual, that gives rise to all the more important 

 modifications of structure, by which the innumerable 

 beings on the face of this earth are enabled to struggle 

 with each other, and the best adapted to survive. 



