LETTERS OF CHARLES DARWIN. 393 



dence against new characters arising from crossing wild forms; only 

 intermediate races are then produced. Now, do you agree thus far? 

 if not, it is no use arguing; we must come to swearing, and I am 

 convinced I can swear harder than you, . . I am right. Q.E.D. 



If the number of 1,000 pigeons were prevented increasing not by 

 chance killing, but by, say, all the shorter-beaked birds being killed, 

 then the whole body would come to have longer beaks. Do you agree? 



Thirdly, if 1,000 pigeons were kept in a hot country, and another 

 1,000 in a cold country, and fed on different food, and confined in 

 different-size aviary, and kept constant in number by chance killing, 

 then I should expect as rather probable that after 10,000 years the 

 two bodies would differ slightly in size, colour, and perhaps other 

 trifling characters; this I should call the direct action of physical con- 

 ditions. By this action I wish to imply that the innate vital forces 

 are somehow led to act rather differently in the two cases, just as heat 

 will allow or cause two elements to combine, which otherwise would 

 not have combined. I should be especially obliged if you would tell 

 me what you think on this head. 



But the part of your letter which fairly pitched me head over 

 heels with astonishment, is that where you state that every single 

 difference which we see might have occurred without any selection. 

 I do and have always fully agreed; but you have got right round the 

 subject, and viewed it from an entirely opposite and new side, and 

 when you took me there I was astounded. When I say I agree, I must 

 make the proviso, that under your view, as now, each form long re- 

 mains adapted to certain fixed conditions, and that the conditions of 

 life are in the long run changeable; and second, which is more im- 

 portant, that each individual form is a self-fertilising hermaphrodite, 

 so that each hair-breadth variation is not lost by intercrossing. Your 

 manner of putting the case would be even more striking than it is 

 if the mind could grapple with such numbers it is grappling with 

 eternity think of each of a thousand seeds bringing forth its plant, 

 and then each a thousand. A globe stretching to the furthest fixed 

 star would very soon be covered. I cannot even grapple with the 

 idea, even with races of dogs, cattle, pigeons, or fowls; and here all 

 admit and see the accurate strictness of your illustration. 



Such men as you and Lyell thinking that I make too much of a 

 Deus of Natural Selection is a conclusive argument against me. Yet 

 I hardly know how I could have put in, in all parts of my book, 

 stronger sentences. The title, as you once pointed out, might have 

 been better. No one ever objects to agriculturists using the strongest 

 language about their selection, yet every breeder knows that he does 

 not produce the modification which he selects. My enormous dim- 



