NATURAL SELECTION 99 



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, &c., have no doubt produced some direct effect. It is 

 also necessary to bear in mind that, owing to the law of cor- 

 relation, when one part varies, and the variations are accu- 

 mulated through natural selection, other modifications, often 

 of the most unexpected nature, will ensue. 



As we see that those variations which, under domestica- 

 tion, 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 oi 

 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 selec- 

 tion, 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 con- 

 cern the mature insect; and these modifications may effect, 

 through correlation, the structure of the adult. So, con- 

 versely, modifications 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 

 w'ould become extinct. 



Natural selection will modif)^ 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 



