1858.] THE LETTER TO DR. GRAY. 479 



I. It is wonderful what the principle of Selection by Man, 

 that is the picking out of individuals with any desired quality, 

 and breeding from them, and again picking out, can do. 

 Even breeders have been astonished at their own results. 

 They can act on differences inappreciable to an uneducated 

 eye. Selection has been methodically followed in Europe for 

 only the last half century. But it has occasionally, and even 

 in some degree methodically, been followed in the most 

 ancient times. There must have been also a kind of uncon- 

 scious selection from t e most ancient times, namely, in the 

 preservation of the individual animals (without any thought 

 of their offspring) most useful to each race of man in his par- 

 ticular circumstances. The " roguing," as nursery-men call 

 the destroying of varieties, which depart from their type, is a 

 kind of selection. I am convinced that intentional and oc- 

 casiona'l selection has been the main agent in making our 

 domestic races. But, however this may be, its great power 

 of modification has been indisputedly shown in late times. 

 Selection acts only by the accumulation of very slight or 

 greater varations, caused by external conditions, or by the 

 mere fact that in generation the child is not absolutely similar 

 to its parent. Man, by this power of accumulating variations, 

 adapts living beings to his wants he may be said to make 

 the wool of one sheep good for carpets, and another for 

 cloth, &c. 



II. Now, suppose there was a being, who did not judge by 

 mere external appearance, but could study the whole internal 

 organisation who never was capricious who should go on 

 selecting for one end during millions of generations, who will 

 say what he might not effect ! In nature we have some slight 

 variations, occasionally in all parts : and I think it can be 

 shown that a change in the conditions of existence is the 

 main cause of the child not exactly resembling its parents ; 

 and in nature, geology shows us what changes have taken 

 place, and are taking place. We have almost unlimited time : 

 no one but a practical geologist can fully appreciate this : 

 think of the Glacial period, during the whole of which the 



