118 NATURAL SELECTION. Chap. IV. 



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 modified forms, will tend to inherit those ad- 

 vantages which made [their common parent (A) more 

 numerous than most of the other inhabitants of the 

 same country ; they will likewise 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 circumstances 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}. Variety m x is supposed to have produced two 

 varieties, namely m 2 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 

 generations, 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 in- 

 creasing in number and diverging in character. In 

 the diagram the process is represented up to the ten- 

 thousandth generation, and under a condensed and sim- 

 plified 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. 



