1858.] THE LETTER TO DR. GRAY. 12 



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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 

 same species of shells at least have existed ; there must 

 have been during this period, millions on millions of 

 generations. 



III. I think it can be shown that there is such an unerring 

 power at work, or Natural Selection (the title of my book), 

 which selects exclusively for the good of each organic being. 

 The elder De Candolle, W. Herbert, and Lyell, have written 

 strongly on the struggle for life ; but even they have not 

 written strongly enough. Reflect that every being (even the 

 elephant) breeds at such a rate that, in a few years, or at most 

 a few centuries or thousands of years, the surface of the earth 

 would not hold the progeny of any one species. I have found 

 it hard constantly to bear in mind that the increase of every 

 single species is checked during some part of its life, or during 

 some shortly recurrent generation. Only a few of those 

 annually born can live to propagate their kind. What a 

 trifling difference must often determine which shall survive 

 and which perish ! 



IV. Now take the case of a country undergoing some 

 change ; this will tend to cause some of its inhabitants to vary 

 slightly ; not but what I believe most beings vary at all times 



