150 THE ORIGIN OF SPECIES 



and likewise with ;uiiinals 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 afterward spread, so that the individuals of the new 



SiVariety 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. 

 i 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 considerable 

 /body of facts showing that within the same area two 

 i 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 



Vpreferring 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 ob- 

 viously 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 

 j^lace 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 re- 

 spect to organic beings extremely low in the scale, which 

 do not propagate sexually, nor conjugate, and whicb^can- 



