94 ON THE ORIGIN OF SPECIES 



intercrossing took place would be chiefly between the 

 individuals 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, we 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 which unite for each birth ; but I have already 

 attempted to show that we have reason to believe that 

 occasional intercrosses take place with all animals and 

 with all plants. Even if these take place only at long 

 intervals, I am convinced that the young thus pro- 

 duced 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 intercrosses, even at rare intervals, will 

 be great. If there exist organic beings which never 

 intercross, uniformity of character can be retained 

 amongst them, as long as their conditions of life 

 remain the same, only through the principle of inherit- 

 ance, and through natural selection destroying any 

 which depart from the proper type ; but if their 

 conditions of life change and they undergo modification, 

 uniformity of character can be given to their modified 

 offspring, solely by natural selection preserving the 

 lame favourable variations. 



