PRODUCTION OF NEW FORMS 115 



that there is no separation of selected individuals. Thus it 

 will be under nature; for within a confined area, with some 

 place in the natural polity not perfectly occupied, all the in- 

 dividuals 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 con- 

 ditions of life; and then, if the same species undergoes modi- 

 fication in different districts, the newly-formed varieties will 

 intercross on the confines of each. But we shall see in the 

 sixth chapter that intermediate varieties, inhabiting inter- 

 mediate 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 hermaphrodite organisms which cross 

 only occasionally, and likewise with animals which unite for 

 each birth, btat 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 in- 

 tercrossing 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 distinct, from haunting differ- 

 ent stations, from breeding at slightly different seasons, or 

 from the individuals of each variety preferring to pair to- 

 gether. 



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 be- 

 lieve that occasional intercrosses take place with all animals 



