166 THE ORIGIN OF SPECIES 



lunount of variation is supposed to have been accumu- 

 lated to form it into a fairly well-marked variety, such 

 as would be thought worthy of record in a systematic 

 work. 



The intervals between the horizontal lines in the dia- 

 gram may represent each a thousand or more generations. 

 After a thousand generations, species (A) is supposed to 

 have produced two fairly well-marked varieties, namely 

 a' and m\ These two varieties will generally 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 circum- 

 stances are favorable 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' is supposed in the diagram to have] 

 produced variety a', which will, owing to the principle^ 

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

 Variety m' is supposed to have produced two varieties, 

 namely m"^ and s*, differing from each other, and more 

 considerabl}' 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* 



