OF NATURAL SELECTION. 101 



variety, such as would be thought worthy of record in a 

 systematic work. 



The intervals between the horizontal lines in the diagram 

 may represent each a thousand or more generations. After 

 a thousand generations, species (A) is supposed to have pro- 

 duced two fairly well-marked varieties, namely a 1 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 circumstances are favorable to the production of new 

 varieties. 



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

 gent 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 s 2 , 

 differing from each other, and more considerably from their 

 common parent (A). We may continue the process by simi- 

 lar 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 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 four- 

 teen-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, nor that 

 it goes on continuously ; it is far more probable that each 

 form remains for long periods unaltered, and then again 

 undergoes modification. Nor do I suppose that the mosfc 



