CHAP. IV.] NATURAL SELECTIOX. 85 



importance of the principle of benefit derived from divergence of 

 character comes in ; for this will generally lead to the most 

 different or divergent variations (represented by the outer dotted 

 lines) being preserved and accumulated by natural selection. 

 When a dotted line reaches one of the horizontal 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 l . 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 here- 

 ditary ; 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 nume- 

 rous 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 thou- 

 sand 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 l is supposed to have produced two varieties, namely 

 ?/i 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 

 unaltered, and then again undergoes modification. Nor do I 



