Chap. IV. 



Natural Selection. 



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more from a certain disease than yellow plums ; whereas another 

 disease attacks yellow-fleshed peaches far more than those with 

 other coloured flesh. If, with all the aids of art, these slight differ- 

 ences 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 differ- 

 ences 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, &c, 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 diffi- 

 culty 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 affect, through correlation, the structure of the 

 adult. So, conversely, 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 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 

 o the whole community ;. if the community profits by the selected 

 change. A\ hat natural selection cannot do, is to modify the struc- 

 ture ot one species, without giving it any advantage, for the good of 



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