326 OUTLINES OF EVOLUTIONARY BIOLOGY 



and other fish which go down to the sea periodically, but for the 

 most part the inhabitants of fresh water are largely dependent 

 upon external agencies for their dispersal. Accordingly we find 

 two groups of such animals, widely contrasted with one another 

 as regards their distribution. Those which do not go down to 

 the sea and which are not likely to be carried about by external 

 agencies, such as most of the fishes, have usually restricted areas 

 of specific distribution, and individual mountain lakes sometimes 

 contain peculiar species of fish which are found nowhere else in 



FIG. 165. 



FIG. 166. 



FIG. 166. Gemmule or Statoblast of a Fresh Water Polyzoon, Cristateila 



mucedo, X 40. (From Sollas.) 



FIG. 166. Two Gemmules of a Fresh Water Sponge, Ephydatia (Spongilla)- 

 fluviatilis, X 60. (From a photograph. ) 



gem., gemmules ; sp., spicules of the parent sponge. 



the world. Galaxias nigothoruk, for example, is a small fish 

 which occurs abundantly in lake Nigothoruk in Victoria 

 (Australia). This lake is in a very isolated position in a 

 mountainous region and the only outlet is by percolation under- 

 ground. There appears to be no natural means by which the 

 fish could be transferred to any other locality at the present 

 time, and it is not known to occur elsewhere. 



On the other hand many fresh water invertebrates, such as 

 the Polyzoa, hydras and sponges, and above all the microscopic 

 Protozoa, are remarkable for their wide distribution. Identical 

 genera if not identical species of tliese groups occur almost all 

 over the world, and the reason for this is not far to seek, for all 



