to form Varielies. 



6297 



II. Abstract of a Letter from C. Darwin, Esq., to Prof. Asa Gray, Boston, U.S., 

 dated Down, Septennber oih, 1857. 



1. 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 

 astounded at their own results. They can act on differences inap- 

 preciable to an uneducated eye. Selection has been metltodically 

 followed in Europe for only the last half-century ; but it was occa- 

 sionally, and even in some degree methodically, followed in the most 

 ancient times. There must have been also a kind of unconscious 

 selection from a remote period, namely, in the preservation of the 

 individual animals (without any thought of their offspring) most use- 

 ful to each race of man in his particular circumstances. The 

 " roguing," as nurserymen call the destroying of varieties which 

 depart from their type, is a kind of selection. I am convinced that 

 intentional and occasional selection has been the main agent in the 

 production of our domestic races ; but however this may be, its great 

 power of modification has been indisputably shown in later times. 

 Selection acts only by the accumulation of slight or greater varia- 

 tions, caused by external conditions, or by the mere fact that in gene- 

 ration the child is not absolutely similar to its parent. Man, by this 

 power of accumulating variations, adapts living beings to his wants, — 

 may be said to make the wool of one sheep good for carpets, of 

 another for cloth, &c. 



2. Now suppose there were a being who did not judge by mere 

 external appearances, but who could study the whole internal organi- 

 zation, who was never capricious, and should go on selecting for 

 one object during millions of generations ; who will say what he might 

 not effect ? In nature we have some slight variation occasionally in 

 all parts; and I think it can be shown that changed 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 same species at least of 

 shells have existed; there must have been during this period millions 

 on millions of generations. 



3. I think it can be shown that there is such an unerring power at 

 work in Natural Selection (the title of my book), which selects exclu- 



XVI. 3 L 



