CHAPTER XVII. 

 MIGRATION. 



That the types put into the world by a cross, do not 

 always remain at the spot of their birth, but migrate, is 

 a generally known fact. 



That they can do so in an astonishingly extensive 

 way is known also, and that it is done sometimes — fre- 

 quently perhaps even — by the intermediancy of man, 

 such as by travelling man himself, by his beasts of bur- 

 den or by his trains or vessels, of course, abstracts not a 

 iota from the fact of such migration existing, but only 

 justifies the conclusion that this kind of transport can- 

 not have existed in times before man appeared on our 

 globe. 



It does not prove of course, that migration was less 

 extensive in former periods than now, because other 

 ways of migration then existed than now, f. i. land- 

 connections allowing intercourse between continents 

 now separated, more rainfall preventing the erection of 

 such formidable bars against migration as the Sahara- 

 desert, higher temperature of the sea, allowing ani- 

 mals and plants of the aequator to migrate further to- 

 wards the north and south, than is now possible, less 

 interference of man, by non-extermination of types 

 considered a nuisance by him etc. etc. So that we have 

 no measure for the relative degree of migration going 



