94 Darwin- Wallace Celebration. 



selection on the one hand, and death on the other, go on for 

 a thousand generations, who will pretend to affirm that it 

 would produce no effect, when we remember what, in a few 

 years, Bakewell effected in cattle, and Western in sheep, by 

 this identical principle of selection ? 



To give an imaginary example from changes in progress on 

 an island : let the organization of a canine animal which 

 preyed chiefly on rabbits, but sometimes on hares, become 

 slightly plastic ; let these same changes cause the number of 

 rabbits very slowly to decrease, and the number of hares to 

 increase ; the effect of this would be that the fox or dog- 

 would be driven to try to catch more hares : his organization, 

 however, being slightly plastic, those individuals with the 

 lightest forms, longest limbs, and best eyesight, let the 

 difference be ever so small, would be slightly favoured, and 

 would tend to live longer, and to survive during that time of 

 the year when food was scarcest ; they would also rear more 

 young, which would tend to inherit these slight peculiarities. 

 The less fleet ones would be rigidly destroyed. I can see no 

 more reason to doubt that these causes in a thousand gener- 

 ations would produce a marked effect, and adapt the form of 

 the fox or dog to the catching of hares instead of rabbits, than 

 that greyhounds can be improved by selection and careful 

 breeding. So would it be with plants under similar circum- 

 stances. If the number of individuals of a species with 

 plumed seeds could be increased by greater powers of dis- 

 semination within its own area (that is, if the check to increase 

 fell chiefly on the seeds), those seeds which were provided 

 with ever so little more down, would in the long run be most 

 disseminated ; hence a greater number of seeds thus formed 

 would germinate, and would tend to produce plants inheriting 

 the slightly better-adapted down*. 



Besides this natural means of selection, by which those 

 individuals are preserved, whether in their egg, or larval, or 

 mature state, which are best adapted to the place they fill in 

 nature, there is a second agency at work in most unisexual 

 animals, tending to produce the same effect, namely, the 

 struggle of the males for the females. These struggles are 

 generally decided by the law of battle, but in the case of 

 birds, apparently, by the charms of their song, by their beauty 

 or their power of courtship, as in the dancing rock- thrush of 

 Guiana. The most vigorous and healthy males, implying 



* I can see no more difficulty in this, than in the planter improving 

 his varieties of the cotton plant. C. D. 1858, 



