NATURAL SELECTION 107 



fairly well-marked varieties, namely, a 1 and m 1 . These 

 two varieties will generally continue to be exposed to 

 the same conditions which made their parents variable, 

 and the tendency to variability is in itself hereditary, 

 consequently they will tend to vary, and generally to 

 vary in nearly the same manner as their parents varied. 

 Moreover, these two varieties, being only slightly modi- 

 fied 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 like- 

 wise partake of those more general advantages which 

 made the genus to which the parent-species belonged, a 

 large genus in its own country. And these circum- 

 stances we know to be 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 ra a and s 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 gener- 

 ations, 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, proceeding from 

 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. 



But I must here remark that I do not suppose that 

 the process ever goes on so regularly as is represented 

 in the diagram, though in itself made somewhat 

 irregular. I am far from thinking that the most 

 divergent varieties will invariably prevail and multiply : 



