Chap. IV.] RESULTS OF NATURAL SELECTION. 81 



birth, but which wander little and can increase at a rapid rate, a 

 new and improved variety might he quickly formed on any one 

 spot, and might there maintain itself in a body and afterwards 

 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 

 thus lessened. 



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 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 pah 

 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 occasional intercrosses take place with 

 all animals and plants. Even if these take place only at long inter- 

 vals of time, the young thus produced 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 crosses, even 

 at rare intervals, will be great. With respect to organic being*, 

 extremely low in the scale, which do not propagate sexually, nor 

 conjugate, and which cannot possibly intercross, uniformity of cha- 

 racter can be retained by them under the same conditions of life, 

 only through the principle of inheritance, and through natural selec- 

 tion which will destroy any individuals departing from the proper 

 type. If the conditions of life change and the form undergoes modifi- 

 cation, uniformity of character can be given to the modified offspring, 

 solely by natural selection preserving similar favourable variations. 



Isolation, also, is an important element in the modification of 

 species through natural selection. In a confined or isolated area, if 

 not very large, the organic and inorganic conditions of life will 

 generally be almost uniform ; so that natural selection will tend to 

 modify all the varying individuals of the same species in the same 

 manner. Intercrossing with the inhabitants of the surrounding dis- 

 tricts will, also, be thus prevented. Moritz Wagner has lately pub- 

 lished an interesting essay on this subject, and has shown that the 

 service rendered by isolation in preventing crosses between newly- 



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