398 Causes and Course of Organic Evolution 



But many of the Ostracoda and Copepoda are of like ha])it 

 now. It seems therefore superfluous to argue that the Crus- 

 tacea i)rimitively had a marine origin. Ratlier we should 

 view all or most of the marine types of the two latter groups 

 as derivatives from the originally lacustrine and even it may 

 have been cooler hot-spring areas, in which as now an abundant 

 saline constituent was dissolved. The appearance of the 

 purely marine group of the Cirripedia in Cambrian rocks, how- 

 ever, would indicate that by that time at least littoral forms 

 had developed, probably by derivation from some ostracoid 

 type like Cypris that had migrated to the shore, and there 

 undergone modification. 



Amongst the Malacostraca numerous species are fresh- 

 water, while a good many of them are so nearly related to 

 purely marine types that their origin from marine forms which 

 more anciently were lacustrine seems likely. But even here 

 it is well to keep in mind the w^ide geographic range of not a 

 few of these lacustrine or land forms, their unquestioned ancient 

 land distribution, and their occurrence in lakes at high eleva- 

 tion. Now fossil remains of Decapoda only begin to appear 

 in earlier mesozoic strata, and so even here the possibility 

 exists of lines of evolutionary divergence having taken place 

 amongst these, from lacustrine or brackish areas to fresh- 

 water or even land areas on the one hand, and to littoral or 

 in time deep sea situations on the other. Even amongst the 

 divisions of Macrura, Brachyura, Cumacea, Isopoda, and 

 Amphipoda, species, genera, or even whole families are either 

 terrestrial, fresh-water, amphibious, or marine at all levels 

 from the upper shore limit to miles of ocean depths, so we 

 begin more fully to realize that the entire group is one that 

 may have wavered since early cambrian times down to the 

 present as a fresh-water series that has in one case given off 

 lines of descent that have reached high on mountains, and 

 others that have descended to dej^ths of ocean, while many 

 have kept along the sea margin. 



So, while we estimate that 213 genera of living Crustacea 

 are land, lacustrine, or brackish-water, in their en\'ironment, 



