6296 



Tendency of Species 



longer, and to survive during that time of the year when food vvas 

 scarcest ; they w^ould also rear more young, which would tend to 

 inherit these shght 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 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 care- 

 ful 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 dissemination 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 perfect adaptation, 

 must generally gain the victory in their contests. This kind of selec- 

 tion, however, 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 year when food is generally 

 abundant, and perhaps the eff'ect chiefly produced would be the mo- 

 dification of the secondary sexual characters, which arc 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 pro- 

 duced by those agriculturists who pay less attention to the careful 

 selection of all their young animals, and more to the occasional use of 

 a choice mate. 



* T can see no more difficulty in this than in ibe planter improving his varieties 

 of the cotton plant.— 6'. D., 1858. 



