Marine Animal Forms in Fresh Water. 59 



1 . The majority of the family-forms, both generally and in each 

 zone, belong exehisivcly to one of the two media (A). 



2. The inhabitants of the fresh water generally and in each 

 zone are more uniform (and less numerous) than the inhabitants 

 of the sea (C). 



3. The inhabitants of fresh water increase from the ])ole 

 towards the equator, not oidy absolutely, Ijut also relatively in 

 ])ro|)ortion to the inhabitants of the sea, in nudtiplicity of forms 

 (and in number) (C). 



4. This increase depends not only upon the development of 

 new peculiar forms, but also upon participation in the marine 

 forms (D). 



5. The similarity of the individual freshwater animals to 

 individual marine animals decreases from the pole towards the 

 equator (B). 



6. The similarity of the total freshwater fauna to the total 

 marine fauna increases from the pole towards the equator (A). 



The ap])arent contradiction of the two preceding propositions 

 is exi)lained by the fact that in the fifth the exclusively marine 

 families are not taken into consideration at all, but that in the 

 sixth they, as well as the exclusively freshwater forms, form the 

 negative factor. 



7. Numerous family-forms are exclusively marine in colder 

 regions ; in warmer regions (still principally marine, but) also 

 represented (by individual species) in the fresh water (E). 



Here especially belong those animals which led to the pre- 

 ])aration of the ])resent memoir. 



The above ])ropositions of course only apply so far as the 

 families adopted as the foundation for the calculation within 

 each class may be regarded as equivalent w'ith respect to the 

 similarity of their structure. Advances in systematic zoology, 

 therefore, as well as in the knowledge of faunas, which is still 

 so very deficient, especially for the tropical regions, may modify 

 them. A comparison of the three classes amongst themselves, 

 according to which the similarity between the Molluscous fauna 

 of the sea and of the fresh water in each zone is less than that of 

 the Crustacea, and this less than that of the Fishes, would also 

 at the same time presuppose the equivalence of the divisions 

 ado])ted in all the four classes, which however will remain a 

 matter of individual opinion. Thus, had I adopted as a founda- 

 tion for the Crustacea, the numerous subdivisions which Dana 

 calls families, the numbers for the freshwater species would 

 have proved but little greater, whilst those for the marine forms 

 would have been considerably higher, because amongst these sub- 

 divisions also the marine animals again predominate; within each 

 separate zone, therefore, the relative number of the inhabitants 



