276 THE INFLrENCE OF INANIMATE SUEKOUNDINGS. 



CHAPTBE IX. 



CURRENTS, VIEWED AS A MEANS OF EXTENDING OR HINDERING 

 THE DISTRIBUTION OF SPECIES. 



All the parasites tliat live witHn other animals are without 

 exception adapted to more or less extensive migrations if they 

 are to maintain their existence as a species in the struggle for 

 existence. If, for instance, a Trichina were incapable of living 

 in the stomach of any other animal than man, it would have a 

 bad chance of living any longer than the man in whom it had 

 accidentally originated ; for since its progeny imbed themselves 

 in the muscles of the individual in whose stomach the parent 

 produces them, any transmission to another individual, and con- 

 sequently its continuance as a species, could only be secured by 

 cannibahsm. If an ordinary fluke (Distoma) were obliged to 

 complete the whole cycle of its development in one and the 

 same individual, it would necessarily perish with the death of 

 its hosrt. Thus migration is one — and in fact the most impor- 

 tant — of the conditions of life for all Entoparasites. 



On the other hand we know that this rule, indispensable 

 to parasites, is not directly applicable to all other animals. 

 Many, on the contrary, continue to exist within very limited 

 and narrow regions ; most of the land moUusca of small islands, 

 for instance, have a very narrow range, and it would appear as 

 though their continuance as distinct species for very long periods 

 were secured by the maintenance of the balance between themselves 

 and the external conditions of life in the locality they inhabit. 

 Here migration, as a condition of existence, seems to be abso- 

 lutely excluded. But between this and the former extreme 

 there is every conceivable degree of transition, and we are there- 



