Chap. IV.] NATURAL SELECTION. 91 



sented by the outer dotted lines) being preserved and accumulated 

 •by natural selection. When a dotted line reaches one of the hori- 

 zontal lines, and is there marked by a small numbered letter, a 

 sufficient amount of variation is supposed to have been accumulated 

 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 diagram, 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 1 and m 1 . 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 heredi- 

 tary ; 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 prin- 

 ciple 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 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 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. 



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 unal- 

 tered, and then again undergoes modification. Nor do I suppose 



