76 GEOGRAPHICAL DISTRIBUTION OF ANIMALS. 



two or three allies, a medusa (!) (Limnocodiuin\ many " worms " 

 \e.g., Planarians, Annelids like Nais and Tubifex, Rotifers 

 etc.), many small Crustaceans and the large crayfish, 

 some Insects, Bivalves like Anodon, Unto, Cyclas ; snails 

 like Lymnceus, Paludina, Planorbis ; many Fishes ; Amphi- 

 bians, in their youth at least ; some Reptiles, Birds, and 

 Mammals. 



In lakes there are not many different kinds of animals 

 except near the shore. Neither in the depths nor at the 

 surface is the animal population diverse. Small Crustaceans, 

 Rotifers, and Infusorians are commonest, but there are not 

 many different genera, though Prof. Zacharias and other 

 enthusiasts are rapidly increasing our appreciation of this 

 fauna. 



A few fresh-water animals have come from the land, 

 others have gradually spread from the sea up estuaries and 

 rivers, probably most were originally marine, for many lakes 

 are the residues of inland seas. The young stages of many 

 have interesting adaptations, which obviate the risks of being 

 swept away by currents. 



(5.) The terrestrial fauna probably arose by gradual migra- 

 tions from the shore landwards. It includes some Protozoa 

 which live in damp earth ; leeches and earthworms ; a few 

 Crustaceans (land-crabs and wood-lice) ; many Myriopods, 

 Insects, and Spiders ; snails and slugs ; adult Amphibians ; 

 besides most Reptiles, Birds, and Mammals. 



But some terrestrial animals have become more or less 

 aerial, notably Insects, Birds, and Bats. 



Moreover, not a few animals from the shore, or from 

 fresh water, or from land have found a very different kind of 

 habitat as parasites on or in other organisms. 



Geographical Areas. — The terrestrial fauna varies in dif- 

 ferent countries, and has often varied in the same country, 

 as one kind of climate has succeeded another through the 

 long past. 



From their original homes animals have spread mostly by 

 active migration, but also with the help of wind and water- 

 currents and other dispersing powers. As they have spread, 

 they have varied, according to their nature and the force of 

 changed circumstances, and the degree of their isolation. 



The geographical position of the original birth-places, the 



