142 RESULTS OF THE ACTION OF [Chap. IV. 



still be exposed to the same conditions which made 

 their parents variable, and the tendency to variability is 

 in itself hereditary; consequently they will likewise 

 tend to vary, and commonly in nearly the same manner 

 as did their parents. Moreover, these two varieties, 

 being only slightly modified forms, will tend to inherit 

 those advantages which made their parent (A) more 

 numerous than most of the other inhabitants of the same 

 country; they will also partake of those more general 

 advantages which made the genus to which the parent - 

 species belonged, a large genus in its own country. And 

 all these circumstances are favourable to the production 

 of new varieties. 



If, then, these two varieties be variable, the most 

 divergent of their variations will generally be preserved 

 during the next thousand generations. And after this 

 interval, variety a 1 is supposed in the diagram to have 

 produced variety a 2 , which will, owing to the principle 

 of divergence, differ more from (A) than did variety a 1 . 

 Variety m 1 is supposed to have produced two varieties, 

 namely m 2 and a 2 , differing from each other, and more 

 considerably from their common parent (A). We may 

 continue the process by similar steps for any length of 

 time ; some of the varieties, after each thousand genera- 

 tions, producing only a single variety, but in a more and 

 more modified condition, some producing two or three 

 varieties, and some failing to produce any. Thus the 

 varieties or modified descendants of the common parent 

 (A), will generally go on increasing in number and 

 diverging in character. In the diagram the process is 

 represented up to the ten-thousandth generation, and 

 under a condensed and simplified form up to the fourteen- 

 thousandth generation. 



