104 CIRCUMSTANCES TAYOKABLE TO THE CnAr. IV. 



character, will in llie lonrr-run generally be supplanted by one 

 of the varieties on either hand. Intercrossing will aflect those 

 animals most which unite for each birth and Avauder 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 hennaphrodite organisms which cross only occasionally, 

 and likewise with animals which unite for each l)irth,but which 

 wander little and can increase at a very rapid rate, a new and 

 iinpro\'ed variety might be quickly formed on any one spot, 

 and might there maintain itself in a body and afterward 

 spread, so that the crossing would be chiefly between the indi- 

 viduals of the new variety living together in the same place. 

 On this principle, nurserymen always prefer saving seed from 

 a large body of plants, as the chance of intercrossing is thus 

 lessened. 



Even in the case of animals which breed slowly and xmite 

 for each birtli, Ave must not assume that the effects of natural 

 selection Avill always be immediately oveq:)OAvered by free inter- 

 crossing ; for I can bring a considerable body of facts, showing 

 that, Avitljin the- same area, varieties of the same animal may 

 long remain distinct, from haunting different stations, from 

 breeding at slightly-tlifferent seasons, or from varieties of the 

 same kind preferring to pair together. 



Intercrossing plays a very important part in Nature in keep- 

 ing the individuals of the same species, or of the same \'ariety, 

 true and xmifonn in character. It will obviously thus act far 

 more elViciently with those animals Avhich unite for each birth ; 

 but as already stated we have reason to believe that occasional 

 intercrosses take place with all animals and with all plants. 

 Even if these take ]ilace only at long intcr\-als of time, the 

 young thus produced Avill gain so much in vigor and fertihty 

 over the offspring from long-continued self-fertilization, that 

 the}' Avill have a better chance of surviving and propagat- 

 ing their kind ; and thus, in the long-run, the influence of inter- 

 crosses, even at rare intervals, will be great. If there exist 

 organic beings which never intercross, uniformity of character 

 ran be retained among them, as long as their conditions of 

 life remain tho same, only through the principle of inheritance 

 and through natural selection destroying any which depart from 

 tlic proper type ; but if their conditions of life change and they 

 undergo modification, xmiformity of character can be given to 

 their modified offspring, solely b^' natural selection prescn-ing 

 similar favorable variations. 



