126 CIECOISTANCES FAYOTJEABLE TO THE [Chap. IV. 



varieties, inhabiting intermediate districts, will in the 

 long run generally be supplanted by one of the adjoin- 

 ing 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, varie- 

 ties will generally be confined to separated countries ; 

 and this I find to be the case. With, hermaphrodite 

 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 after- 

 wards spread, so that the individuals of the new variety 

 would chiefly cross together. On this principle, nursery- 

 men 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 

 distinct, 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 intercrosses take 

 place with all animals and plants. Even if these take 



