ff 



V. ON TUB ORIGIN OF FRESHWATER FAUNAS : A JSTUDY IN EVOLU- 

 TION. By W. J. SOLLAS, M. A., Dublin ; D. Sc., Cambridge ; Fellp^ of fit. John's 

 College, Cambridge; Professor of Geology and Mineralogy in the Tfiil versify of 

 Dublin. 



[Read, May 19, 1884] . 



" I what an endlesse worke have I in hand, 



To count the seas abundant progeny, 

 H'hotefruitfuU itede Jarrt patseth thoie on land, 



And also those which wonne in th' azure sky : 

 For much more eath to tell the starres on hy, 



Albe they endlesse seeme in estimation, 

 Then to recount the Seas posterity : 



So fertile be the flouds in generation, 



So huge their numbers, and so numberlesse their nation." 



Faerie Queene, Canto xii. Bk. iv. 



THE amazing fertility of the sea never fails to excite the wonder of the naturalist, 

 and has often impressed the imagination of the poet. The comparative poverty 

 of our freshwater fauna, though no less astonishing, is seldom remarked upon, 

 and has not been made the subject of systematic investigation. A suggestive 

 Paper* by Von Martens, which appeared in 1857, and a chapter in Semper' s 

 thoughtful work, " The Natural Conditions of Existence as they affect Animal 

 Life," are the chief references bearing directly on the subject which I am able 

 to cite.f 



One reason for the barrenness of our literature on this subject is to be found in 

 the too readily made assumption that the difference in composition of fresh and 

 salt water is sufficient to account for all the facts. The susceptibility of marine 

 animals to the influence of fresh water is supposed to be so great, thut only under 



* " On the Occurrence of Marine Animal Forms in Fresh Water," by Dr. E. von Martens, Ann. 

 and Mag. Nat. JTut., 1858, ser. in., vol. i. p. 60 (translated by W. 8. Dallas, F. L. 8.). 



t See incidental remarks on the origin of some freshwater species in Dr. P. M. Duncan's Presi- 

 dential Address to the Geological Society of London, Quart. Jovarn. Grol. Nc., 1878, and a I'hajiter 

 on distribution of the freshwater Crayfish, Prof. Huxley (The Crayfish, /<<rn///.. > t-'t 



TRANS. OT. DCB. SOC., X. 8. TOI . III. 



