lo POPULAR SCIENCE MONTHLY. 



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 generations would pro- 

 duce a marked effect, and adapt the form of the fox or dog to the catch- 

 ing of hares instead of rabbits, than that greyhounds can be improved 

 by selection and careful breeding. So would it be with plants under 

 similar circumstances. If the number of individuals of a species 

 with plumed seeds could be increased by greater powers of dissemina- 

 tion 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 

 vigourous and healthy m.ales, implying perfect adaptation, must gen- 

 erally gain the victory in their contests. This kind of selection, how- 

 ever, is less rigorous than the other; it does not require the death of 

 the less successful, but gives to them fewer descendants. The struggle 

 falls, moreover, at a time of the year when food is generally abundant, 

 and perhaps the effect chiefly produced would be the modification of 

 the secondary sexual characters, which are not related to the power of 

 obtaining food, or to defence from enemies, but to fighting with or 

 rivalling other males. The result of this struggle amongst the males 

 may be compared in some respects to that produced by those agricul- 

 turists who pay less attention to the careful selection of all their young 

 animals, and more to the occasional use of a choice mate. 



3. Abstract of a Letter from C. Daewin, Esq., to Professor Asa 

 Gkay^ Boston, U. 8., dated Down, September 5th, 1857. 



1. It is wonderful what the principle of selection by man, that is 

 the picking out of individuals with any desired quality, and breeding 

 from them, and again picking out, can do. Even breeders have been 

 astounded at their own results. They can act on differences inappre- 



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

 A-arieties of the cotton plant. — C. D., 1858. 



