RESULTS OF NATURAL SELECTION. 95 



then, if the same species undergoes modification in differ- 

 ent districts, the newly formed varieties will intercross on 

 the confines of each. But tc shall see in the sixth chap- 

 ter 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 countries; 

 and this I find to be the case. With hermaphrodite or- 

 ganisms which cross only occasionally, and likewise with 

 animals which unite for each birth, but which wander lit- 

 tle 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 afterward 

 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 thns lessened. 



Even with animals which nnite for each birth, and which 

 do not propagate rajDidly, 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 

 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 occesional intercrosses take 

 place with all animals and plants. Even if these take 

 place only at long intervals of time, the young thus pro- 

 duced will gain so much in vigor and fertility over the 

 offspring from long-continued self-fertilization, that they 

 will have a better chance of surviving and propagating their 

 kind; and thus in the long-run the influence of crosses, 

 even at rare intervals, will be great, With respect to or- 



