CHAP. IV.] RESULTS OF NATURAL SELECTION. 75 



intending to alter the breed, have a nearly common standard of 

 perfection, and all try to procure and breed from the best animals, 

 improvement surely but slowly follows from this unconscious 

 process of selection, notwithstanding that there is no separation 

 of selected individuals. Thus it will be under nat'\re ; for within 

 a confined area, with some place in the natural polity not perfectly 

 occupied, all the individuals varying in the right direction, though 

 in different degrees, will tend to be preserved. But if the area be 

 large, its several districts will almost certainly present different 

 conditions of life ; and then, if the same species undergoes modifi- 

 cation in different districts, the newly -formed varieties will inter- 

 cross on the confines of each. But we shall see in the sixth 

 chapter that intermediate varieties, inhabiting intermediate 

 districts, will in the long run generally be supplanted by one of 

 the adjoining varieties. Intercrossing will chiefly affect those 

 animals which unite for each birth and wander much, and which 

 do not breed at a very quick rate. Hence with animals of this 

 nature, for instance, birds, varieties will generally be confined to 

 separated countries ; and this I find to be the case. With herma- 

 phrodite organisms which cross only occasionally, and likewise 

 with animals which unite for each birth, but which wander little 

 and can increase at a rapid rate, a new and improved variety 

 might be quickly formed on any one spot, and might there main- 

 tain itself in a body and afterwards spread, so that the individuals 

 of the new variety would chiefly cross together. On this principle, 

 nurserymen always prefer saving seed from a large body of plants, 

 as the chance of intercrossing is thus lessened. 



Even with animals which unite for each birth, and which do 

 not propagate rapidly, we must not assume that free intercrossing 

 would always eliminate the effects of natural selection ; for I can 

 bring forward a considerable body of facts showing that within the 

 same area, two varieties of the same animal may long remain dis- 

 tinct, from haunting different stations, from breeding at slightly 

 different seasons, or from the individuals of each variety preferring 

 to pair together. 



Intercrossing plays a very important part in nature by keeping 

 the individuals of the same species, or of the same variety, true 

 and uniform in character. It will obviously thus act far more 

 efficiently with those animals which unite for each birth ; but, as 

 already stated, we have reason to believe that occasional inter- 

 crosses take place with all animals and plants. Even if these take 

 place only at long intervals of time, the young thus produced will 

 gain so much in vigour and fertility over the offspring from long- 

 continued self -fertilisation, that they will have a better chance of 

 surviving and propagating their kind ; and thus in the long run 

 the influence of crosses, even at rare intervals, will be great. With 



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