Chap. IV. TO NATURAL SELECTION. 103 



terbalancecl by natural selection always tending to mo- 

 dify all the individuals in each district in exactly the 

 same manner to the conditions of each ; for in a con- 

 tinuous area, the conditions will generally graduate 

 away insensibly from one district to another. The in- 

 tercrossing will most affect those animals which unite 

 for each birth, which wander much, and which do not 

 breed at a very quick rate. Hence in animals of this 

 nature, for instance in birds, varieties will generally be 

 confined to separated countries ; and this I believe to be 

 the case. In hermaphrodite organisms which cross only 

 occasional^, and likewise in animals which unite for 

 each birth, but which wander little and which can in- 

 crease at a very rapid rate, a new and improved variety 

 might be quickly formed on any one spot, and might 

 there maintain itself in a body, so that whatever inter- 

 crossing took place would be chiefly between the indi- 

 viduals of the same new variety. A local variety when 

 once thus formed might subsequently slowly spread to 

 other districts. On the above principle, nurserymen 

 always prefer getting seed from a large body of plants 

 of the same variety, as the chance of intercrossing with 

 other varieties is thus lessened. 



Even in the case of slow-breeding animals, which 

 unite for each birth, w r e must not overrate the effects 

 of intercrosses in retarding natural selection ; for I can 

 bring a considerable catalogue of facts, showing that 

 within the same area, varieties of the same animal can 

 long remain distinct, from haunting different stations, 

 from breeding at slightly different seasons, or from 

 varieties of the same kind preferring to pair together. 



Intercrossing plays a very important part in nature 

 in 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 



