354 TS^ HISTORY OF CREATIOI^. 



primcBval home, or its natal place. This is a necessary 

 consequence of the relations of population and over-popula- 

 tion (pp. 161 and 256). The more an animal or vegetable 

 species increases, the less is its limited natal place sufficient 

 for its sustenance, and the fiercer the struggle for life ; the 

 more rapid the over-population of the natal spot, the more 

 it leads to eTYiigration. These migrations are common to all 

 organisms, and are the real cause of the wide distribution 

 of the different species of organisms over the earth's surface. 

 Just as men leave over-crowded states, so all animals and 

 plants migrate from their over-crowded primaeval homes. 



Many distinguished naturalists, especially LyelP^ and 

 Schleiden, have before this repeatedly drawn attention to 

 the great importance of these very interesting migrations of 

 organisms. The means of transport by which they are 

 effected are extremely varied. Darwin has discussed these 

 most excellently in the eleventh and twelfth chapters of 

 his work, which are exclusively devoted to " geographical 

 distribution," The means of transport are partly active, 

 partly passive; that is to say, the organism effects its 

 migration partly by free locomotion due to its own activity, 

 and partly by the movements of other natural bodies in 

 which it has no active share. 



It is self-evident that active migrations play the chief 

 part in animals able to move freely The more freely an 

 animal's organization permits it to allmove in directions, the 

 more easily the animal species can migrate, and the more 

 rapidly it will spread over the earth. Flying animals are of 

 course most favoured in this respect, among vertebrate animals 

 especially birds, and among articulated animals, insects. 

 These two classes, as soon as they came into existence, can 



