SOLLAS On the Origin of Freshwater Faunas. *'.' 



tresh water, nnd even die after an exposure of one minute to their ancestral 

 clcnicnt, assuredly we can see no reason why any animal in earth or sea may not 

 in time heroine fitted to change its element" (Nature, June 24, 1880). 



Another cause which has been relied upon in explanation of the poverty of our 



-hwater faunas, and which no doubt is partially operative, lies in the greater 

 severity of a freshwater climate as compared with a marine one. Von Martens, in 

 his now well-known I'aper already referred to, concludes his argument in the 

 following words: "The great richness of the sea is explained not only by its 



iter extent, but also by its more uniform temperature. The fresh waters 



.ul in the same relation to it as a continental to an insular climate :_) their 



alternation of temperature is the principal hindrance to their becoming populous; 



and this attains its maximum by freezing in the colder zones; with the increase 



inperature the populousness of the fresh waters increases, but is still limited 

 in the sub-tropical zone by partial dessication. In the tropical zone the conditions 

 of temperature of the fresh waters approach most nearly to those of the^sea, and 

 with them their populousness." 



In support of his thesis Von Martens enumerates several families which, else- 

 where exclusively marine, exhibit a mixed or entirely freshwater habit in tropical 

 >ns. He mentions Area scaphula, Benson, as living in the Jumna, near.Humer- 

 poor, 1000 miles distant from the sea, and Pholas rivicola, Sow., which is found 

 in floating timber on the river Pantai, twelve miles about its" mouth. He also 

 calls attention to the freshwater prawn of Jamaica, Palcemon* jamaicensis, and to 

 the Thclphusiadae, a heterogeneous family of freshwater crabs, which occur in 

 tub-tropical regions. 



Aei\ a this cause, brought to light by Von Martens, may be, it furnishes by 

 no means a complete solution : the exceptional cases quoted by Von Martens are 

 not numerous enough, and still leave the overwhelming preponderance of marine 

 forms unexplained. If the Unionidae and other freshwater molluscs have learnt to 

 adapt themselves to a freshwater climate, one sees no good reason why other forms 

 which endure the rigours of our winter along the coast, such as Patella and Litto- 

 i ina, should not have done so too. 



Considering the merciless struggle for existence which the superabundance of 



marine life involves, sufficient, according to some writers, to drive the. less suc- 



-ful competitors into the desolate depths of the abyssal sea, where the only 



remaining comfort lies in an unchanging uniformity of temperature of about 32 F. 



considering the effects of this struggle, one would have expected to find numerous 



rine animals enterprisingly working their way along the shores of the abun- 

 dant streams which open all along the coast, and every river characterized by a 

 modified marine fauna derived from the neighbourhood of its mouth. 



* The genuine Palwmon is now recognized as a freshwater genus. 



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