158 THE LIFE OF CRUSTACEA 



surprise at the similarity of the fresh-water insects, 

 shells, etc., and at the dissimilarity of the surround- 

 ing terrestrial beings, compared with those of 

 Britain." This uniformity is well illustrated by 

 many of the smaller Crustacea. In a gathering of 

 Cladocera, Copepoda, and Ostracoda, from Central 

 Africa or from Australia, we find that most of the 

 genera, and even some of the species, are identical 

 with those found in similar situations in this country. 

 It is by no means the case that all the species and 

 genera are thus universally distributed, for there are 

 many, especially among the larger forms, which 

 have a very restricted range ; but this does not 

 render less striking the general uniformity of the 

 fauna over very wide areas. 



When we consider the physical environment of 

 fresh-water animals, it seems at first sight as if this 

 wide distribution were the reverse of what might 

 have been expected, for the area occupied by them 

 is far more discontinuous than in the case of terres- 

 trial or marine animals. The inhabitants of a pond 

 or lake are to a great extent isolated ; and although 

 they may spread to other ponds and lakes by way of 

 communicating streams or rivers, where these are 

 not too swiftly flowing and are not interrupted by 

 falls, yet direct passage from one river system to 

 another is rarely possible. Further, since practically 

 the whole of the fresh water on the surface of the 

 globe is constantly flowing, more or less rapidly, 



