90 CIRCUMSTANCES FAVORABLE TO THE 



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 sea- 

 sons, 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 occasional intercrosses take place with all 

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

 intervals of time, the young thus produced 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 organic beings extremely low in the 

 scale, which do not propagate sexually, nor conjugate, and 

 which cannot possibly intercross, uniformity of character 

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

 only through the principle of inheritance, and through 

 natural selection which will destroy any individuals depart- 

 ing from the proper type. If the conditions of life change, 

 and the form undergoes modification, uniformity of character 

 can be given to the modified offspring, solely by natural 

 selection preserving similar favorable 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 indi- 

 viduals of the same species in the same manner. Inter- 

 crossing with the inhabitants of the surrounding districts 

 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-formed varieties is probably greater even than 

 I supposed. But from reasons already assigned I can by no 

 means agree with this naturalist, that migration and isola- 

 tion are necessary elements for the formation of new species. 

 The importance of isolation is likewise great in preventing, 

 after any physical change in the conditions, such as of 

 climate, elevation of the land, etc., the immigration of better 



