62 NATURAL SELECTION. [CHAP. IV. 



whereas another disease attacks yellow-fleshed peaches far more 

 than those with other coloured flesh. If, with all the aids of art, 

 these slight differences make a great difference in cultivating the 

 several varieties, assuredly, in a state of nature, where the trees 

 would have to struggle with other trees and with a host of 

 enemies, such differences would effectually settle which variety, 

 whether a smooth or downy, a yellow or purple fleshed fruit, 

 should succeed. 



In looking at many small points of difference between species, 

 which, as far as our ignorance permits us to judge, seem quite 

 unimportant, we must not forget that climate, food, <fcc., have no 

 doubt produced some direct effect. It is also necessary to bear in 

 mind that, owing to the law of correlation, when one part varies, 

 and the variations are accumulated through natural selection, 

 other modifications, often of the most unexpected nature, will 

 ensue. 



As we see that those variations which, under domestication, 

 appear at any particular period of life, tend to reappear in the 

 offspring at the same period ; for instance, in the shape, size, and 

 flavour of the seeds of the many varieties of our culinary and 

 agricultural plants ; in the caterpillar and cocoon stages of the 

 varieties of the silkworm ; in the eggs of poultry, and in the colour 

 of the down of their chickens ; in the horns of our sheep and 

 cattle when nearly adult ; so in a state of nature natural selection 

 will be enabled to act on and modify organic beings at any age, 

 by the accumulation of variations profitable at that age, and by 

 their inheritance at a corresponding age. If it profit a plant to 

 have its seeds more and more widely disseminated by the wind, I 

 can see no greater difficulty in this being effected through natural 

 selection, than in the cotton-planter increasing and improving by 

 selection the down in the pods on his cotton-trees. Natural 

 selection may modify and adapt the larva of an insect to a score 

 of contingencies, wholly different from those which concern the 

 mature insect ; and these modifications may effect, through 

 correlation, the structure of the adult. So, conversely, modifica- 

 tions in the adult may affect the structure of the larva ; but in all 

 cases natural selection will ensure that they shall not be injurious : 

 for if they were so, the species would become extinct. 



Natural selection will modify the structure of the young in 

 relation to the parent, and of the parent in relation to the young. 

 In social animals it will adapt the structure of each individual for 

 the benefit of the whole community ; if the community profits by 

 the selected change. What natural selection cannot do, is to 

 modify the structure of one species, without giving it any 

 advantage, for the good of another species ; and though statements 

 to this effect may be found in works of natural history, I cannot 



