2^2 CLASSICS OF MODERN SCIENCE 



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 unim- 

 portant, we must not forget that climate, food, etc., 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 varia- 

 tions are accumulated through natural selection, other modifications, 

 often of the most unexpected nature, will ensue. 



As we see that those variations which, under domestication, ap- 

 pear 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 increas- 

 ing and improving by selection the down in the pods on his cotton 

 trees. Natural selection may modify and adapt the larva of an in- 

 sect 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, modi- 

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

 cases natural selection will insure 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 rela- 

 tion 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 find one case which 

 will bear investigation. A structure used only once in an animal's 



