NATURAL SELECTION 81 



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 organisms 

 which cross only occasionally, and likewise for animals 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 variety would 

 chiefly cross together. On this principle nurserymen always prefer 

 saving seed from a large body of plants, as the chance of inter- 

 crossing 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 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 ef- 

 ficiently with those animals which unite for each birth; but, as 

 already stated, we have reason to believe that occasional inter- 

 crosses 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 in- 

 tercross, uniformity of character can be retained by them under 

 the same conditions of life, only through the principle of inherit- 

 ance, and through natural selection which will destroy any indi- 

 viduals departing from the proper type. If the conditions of life 

 change, and the form undergoes modification, uniformity of char- 

 acter can be given to the modified offspring, solely by natural selec- 

 tion 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, th/ organic and inorganic conditions of life will 



