ON SHALLOW WATER FAUNAS. 39 



The relation of the shallow water to the terrestrial fauna 

 is an interesting one, but cannot be considered here in detail. 

 It appears to have been of a give and take nature, for while 

 on the one hand certain marine animals, such as crabs, have 

 left the sea and become adapted more or less completely 

 to a terrestrial habitat ; on the other, Oncliidium^ a slug 

 found on the shores of the Pacific and Indian oceans, and 

 which by its whole organisation shows its descent from land 

 slugs, is certainly no solitary example of a terrestrial form 

 that has become more or less completely marine. 



The mutual relations of the shallow water and fresh water 

 faunas are of much greater importance. Geologically con- 

 sidered, the land is far less constant than the sea, and there 

 are probably few spots on the earth that cannot be proved to 

 have been under water at least once. It hence follows 

 that the terrestrial and fresh water faunas are most probably 

 derived from the more constant marine fauna, and in most 

 cases presumably from the shallow water fauna, as being that 

 immediately adjacent to the land. 



In the case of the fresh water hydroid, Cordylophora, 

 this migration from a marine habitat can actually be traced 

 historically, and in the case of those fish, as the salmon and 

 lamprey, which are partly marine and partly fresh water, 

 such a derivation may be regarded as proved. 



The general characters of the fresh water fauna entirely 

 bear out this view. Of the great groups of animals the 

 Echinoderms are absolutely unrepresented, and the Sponges 

 and Coelenterates have not half-a-dozen fresh water genera 

 between them. The other large groups are all present, but 

 many important divisions of them, as the Cephalopoda and 

 Tunicata, are completely absent. Here, very much as in 

 the case of the deep sea fauna, certain members of most of 

 the groups seem to have worked their way from the sea up 

 the rivers, the determining causes being the same in the 



