^68 THE HISTORY OF CREATION. 



o 



dispersion of these species over a greater or less portion of 

 the earth's surface. But these migrations are also of great 

 importance to the theory of development, because we can 

 perceive in them a very important means for the origin of 

 netu species. When animals and plants migrate they meet in 

 their new home, in the same way as do human emigrants, 

 with conditions which are more or less different from those 

 which they have inherited throughout generations, and to 

 which they have been accustomed. The emigrants must 

 either submit and adapt themselves to these new conditions 

 of life or they perish. By adaptation their peculiar specific 

 character becomes the more changed the greater the dif- 

 ference between the new and the old home. The new 

 climate, the new food, but above all, new neighbours in 

 the forms of other animals and plants, influence and tend 

 to modify the inherited character of the immigrant species, 

 and if it is not hardy enough to resist the influences, then 

 sooner or later a new species must arise out of it. In most 

 cases this transformation of an immigrant species takes 

 place so quickly under the influence of the altered struggle 

 for life, that even after a few generations a new species 

 arises from it. 



Migration has an especial influence in this way on all 

 organisms with separate sexes. For in them the origin of 

 new species by natural selection is always rendered difficult, 

 or delayed, by the fact that the modified descendants oc- 

 casionally again mix sexually with the unchanged original 

 form, and thus by crossing return to the first form. But 

 if such varieties have migrated, if great distances or 

 barriers to migration — seas, mountains, etc. — have separated 

 them from the old home, then the danger of a mingling 



