NATURAL SELECTION 91 



slight, but of the most diversified nature; they are not supposed 

 all to appear simultaneously, but often after long intervals of time; 

 nor are they all supposed to endure for equal periods. Only those 

 variations which are 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 (rep- 

 resented by the outer dotted lines) being preserved and accumu- 

 lated 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 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 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 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 in- 

 herit those advantages which made their parent (A) more numer- 

 ous than most of the other inhabitants of the same country; they 

 v/ill 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 a 1 is sup- 

 posed 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 vari- 

 eties, namely m 2 and s 2 , differing from each other, and more con- 

 siderably from their common parent (A). We may continue the 

 process by similar steps for any length of time; some of the vari- 

 eties, after each thousand generations, producing only a single 

 variety, but in a more and more modified condition, some produc- 

 ing 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 chaf-, 

 acter. In the diagram the process is represented up to the ten- 



