Marine Animal Forms in Fresh Water. 57 



the exclusive families. The common famihes form a greater por^ 

 tion of the total number of families represented in that zone. *^^ ■ 



B. But even the number of the exclusively freshwater families 

 increases in proportion to those which occur also in the sea, or 

 only in salt water, from the cold to the torrid zone; this is very 

 decidedly the case with the Fishes (1. 1 : oo ; II. 1 : I3i ; III. 

 1 : 9| ; IV. 1 : 4f ; V. 1 : 3y\), but also distinctly with the Mol- 

 lusca (I. 1 : 9; II. 1 : 6i; III. I ; 6-| ; IV. 1 : 7^) and with the 

 Crustacea (I. I : 20; III. 1 : 12; IV. I : 81; V. 1 : 6^). 



C. In the same way also the number of families occurring 

 in the fresh water generally, increases in proportion to that of 

 those occurring generally in the sea ; thus in 



I. 11. III. IV. V. 



Fishes 1:6 1 : 3f „ 1:2 1 : If 1 : li 



Crustacea ...1:5 1 : 3^ \ : 'd^^ \ -, 2\\1 1 : 3^V 



MoUusca 1:9 \ x ^ 1 : 6f 1 : 5f 1 : 3f 



Here again also the tropical freshwater Crustacea constitute 

 the sole exception, probably in consequence of deficient informa- 

 tion. This increase, like that under B, is the confirmation of 

 a general law, which has already been expressed as follows : — 

 Towards the poles, organic life retreats from the severe climate 

 of the land to the more temperate climate of the ocean : w^here, 

 as in Greenland *, the entire interior of the country is a perma- 

 nent mass of ice, and the alternation of thaw and frost only 

 occurs on the coasts and bays, the freshwater fauna will not be 

 very rich. 



D. Of the freshwater families those which are exclusively 

 peculiar to this medium, are in proportion to those common 

 to it and the sea in 



I. 11. III. IV. V. 



Fishes 1 : c» 1:4 1 : 2f 1 : If 1 : 1^ 



Crustacea ...1:3 (1:7) 1 : 2| 1 : 2^ 1:1? 

 Mollusca ...1:0 1:0 \ : \ 1 : ^ 1:1 



Here, therefore, there is a remarkable contrast between Fishes 

 and Mollusca; in the former the common families everywhere 

 predominate over the exclusive ones (although not in number 

 of species), but this preponderance diminishes constantly and 

 considerably from the cold zone, where it finds no balance, to 

 the equator ; in the Mollusca the common famihes never pre- 

 dominate over the exclusive ones, but their proportion to the 

 latter increases in the same direction from to equality; in 

 both classes, therefore, a progressive equalization takes place 

 towards the equator, but towards the poles a divergence in 



* See Rink, Gronland geographisk og statistisk beskrevet. Kjobnhavn, 

 1857. 



