142 THE INFLUENCE OF INANIMATE SUEEOUNDINQS. 



while other groups include very few fresh-water species ; to 

 these last belong the Sponges, of which only one genus, and the 

 Polyp i (Coelenterata), of which only two genera, inhabit fresh 

 water ; besides these, two families of Bryozoa and two of Annelida 

 are fresh-water forms. Other groxips, again, have as many repre- 

 sentatives in salt as in fresh water. On the other hand, only 

 one single class inhabits fresh water exclusively, the Amphibia. 

 If we consider the smaller groups, families, or genera, we cer- 

 tainly meet with several typical fresh-water forms ; to these 

 belong Melania, Neritina, the PlanorbidoB, Lymnceidce,Unio- 

 nidce, and Anodontidas, among the Mollusca ; the Astacidce and 

 AsellidcB among the Crustaceans ; PhylactolcBmata among the 

 Bryozoa ; the true leeches, Naididoe, Tubifex, and ChoBtogaater, 

 among the Annelida; the Cyprinoidce among the fishes, &c. But 

 their number is insignificant compared with the enormous number 

 of families typical of the sea and living in it exclusively. 



Thus the fact that a much greater abundance of different 

 forms is to be found in the sea than in fresh water is abso- 

 lutely beyond dispute. But it is doubtful whether this wealth 

 of forms does actually result, as has been supposed, from the 

 great quantity of salt contained in sea- water. The possibility 

 that it is so must of course be conceded, and I shall even 

 adduce certain facts which tend to prove the justness or the pro- 

 bability of this view ; but it must not be forgotten, in the first 

 place, that the sea covers three-fourths of the surface of the 

 globe, and so offers an infinitely wider surface for the develop- 

 ment of animal forms than is offered by fresh-water lakes and 

 streams. This circumstance alone would account for the greater 

 multiplicity of the marine forms. In the second place it must 

 not be forgotten that the animal life on our globe apparently 

 originated in the sea, and that therefore the oceanic world of 

 animal life has had a history of development of much longer 

 (geological) duration than the fresh-water favma ; thirdly, that 

 the infl.uence of natural selection in fresh water is much stronger 

 than in the sea, if only by reason of the abrupt variations of 

 temperature ; and finally — if we assume that animal life origi- 

 nally took its rise in the sea — that only such sea-creatures 

 could accustom themselves to living in fresh water as were 



