Diiratiofi of the Process of Selection. 85 



case of a single individual. This is true in the case of 

 apples and pears and many other fruit trees; of tulips 

 and other bulbs, of dahlias, canna and so forth. Among 

 agricultural plants it is true of potatoes, among plants 

 from tropical countries, of bananas and of sugar cane 

 and so forth. 



Strictly speaking a variety of this kind cannot be 

 compared with that of a plant propagated by seed. It 

 is, on the contrary, analogous to the single plants chosen 

 to furnish seed for market stock. If we assume Gal- 

 ton's formula to be applicable to this case also we may 

 say that the results of selection are three times as great 

 when the plants are propagated vegetatively as when 

 they are propagated by seed. 



So that, if we adduce the splendid flowers of our 

 bulbs, the size of our potatoes and the excellence of our 

 fruit as examples of a high degree of variability and 

 of the perfection that can be attained by selection we 

 must never forget that, in the light of what we now know 

 on this head, the improvement would have to be dimin- 

 ished by % if the results were to be applied to the theory 

 of descent. And if we do make this reservation nothing 

 much is left, from the biological standpoint at any rate. 



§ 9- ON THE DURATION OF THE PROCESS OF SELECT- 



TION. 



The modern theory of selection rests on two unproved 

 hypotheses : 



1. The advance brought about by selection may in- 

 crease for an indefinite period. 



2. The result of selection can become independent 

 of selection. 



