126 CIRCUMSTANCES FAVOURABLE TO THE [Chap. IV, 



that intermediate varieties, inhabiting intermediate dis- 

 tricts, 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 coun- 

 tries; and this I find to he the case. With hermaph- 

 rodite 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 maintain 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 consider- 

 able 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 ani- 

 mals which unite for each birth; but, as already stated, 

 we have reason to believe that occasional intercrosses 

 take place with all animals and plants. Even if these 



