Chap. IV. NATURAL SELECTION. 1 ] 5 



The intervals between the horizontal lines in the dia^ara, 

 may ix'prescnt each a thousand generations, or ten thousand. 

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

 produced two fairly well-marked varieties, namely a} and rn). 

 These two varieties will generally continue to be exposed to 

 the same conditions Avhich made their parents variable, and 

 the tendency to variability is in itself hereditary, consequently 

 tliey will tend to vary, and generally to vary in nearl}' the 

 same manner as their j)arents varied. Moreover, these two 

 varieties, being only slightl^'-niodilied forms, will tend to in- 

 herit those advantages which made their parent (A) more 

 numerous than most of the other inhabitants of the same coun- 

 try ; they will likewise partake of those more general advan- 

 tages v/hich made the genus to which the parent-species be- 

 longed a large genus in its ov,n country. And these circinn- 

 stanccs we know to bo favoral)le to tiic production of new 

 varieties. 



If, then, these two varieties be variable, the most divergent 

 of their variations will generally be ])reserved during the next 

 thousand generations. And after this interval, variety 0} is 

 supposed in the diagram to have pi'oduccd variety a^, which 

 will, owing to the principle of divergence, difler more from (A) 

 than did variety a*. Variety «^' is supposed to have produced 

 two varieties, namely Wi" and s", diflering from each other, and 

 more considerably from their common parent (A). Wc may 

 continue the process by similar steps for any length of time ; 

 some of the varieties, after each thousand generations, produ- 

 cing only a single variety, but in a more and more modi lied con- 

 dition, some producing two or three varieties, and some failing 

 to produce any. Thus the varieties, or modified descendants, 

 proceeding from the common ])arent (A), will generally go on 

 increasing in numljcr, and diverging in character. In the dia- 

 gram tlie process is represented up to the ten-thousandth gen- 

 eration, and under a condensed and siniplilled form up to the 

 fourteen-thousandth generation, 



IJut I must hen; remark that I do not suppose that the 

 process ever goes on so regularly as is represented in the dia- 

 gram, though in itself made somewhat irregular, nor that it 

 goes on continuously ; it is far more ])robable that each form 

 remains for long periods unaltered, and tlien again imdcr- 

 gocs modification. Nor do I suppose that the most di- 

 vergent varieties are invariably preserved: a medium form 

 may often long endure, and may or may not produce more 



