212 EVOLUTION [CHAP. Ill 



Down, Nov. 20th [1862]. 



Letter 146 Your last letter has interested me to an extraordinary 

 degree, and your truly parsonic advice, " some other wise and 

 discreet person," etc., etc., amused us not a little. I will put a 

 concrete case to show what I think A. Gray believes about 

 crossing and what I believe. If 1,000 pigeons were bred 

 together in a cage for 10,000 years their number not being 

 allowed to increase by chance killing, then from mutual 

 intercrossing no varieties would arise ; but, if each pigeon 

 were a self-fertilising hermaphrodite, a multitude of varieties 

 would arise. This, I believe, is the common effect of crossing, 

 viz., the obliteration of incipient varieties. I do not deny 

 that when two marked varieties have been produced, their 

 crossing will produce a third or more intermediate varieties. 

 Possibly, or probably, with domestic varieties, with a strong 

 tendency to vary, the act of crossing tends to give rise to 

 new characters ; and thus a third or more races, not strictly 

 intermediate, may be produced. But there is heavy evidence 

 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 conditions. 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 



