OF NATURAL SELECTION. lO; 



after long intervals of time; nor are tliey all supposed to 

 endure for equal periods. Only those variations wliich arc 

 in some way profitable will be preserved or naturally 

 selected. And here the 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. AVhen 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 tlie 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 hered- 

 itary; 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 

 divergent of their variations will generally be preserved 

 during the next thousand generations. And after this 

 interval, variety cc^ is supposed in the diagram to have pro- 

 duced 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 

 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 genera- 

 tions, producing only a single variety, but in a more and 



