Sortas—On the Origin of Freshwater Faunas. 89 
fresh water, and even die after an exposure of one minute to their ancestral 
element, assuredly we can see no reason why any animal in earth or sea may not 
in time become fitted to change its element ” (ature, June 24, 1880). 
Another cause which has been relied upon in explanation of the poverty of our 
freshwater 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 Paper already referred to, concludes his argument in the 
following words:—‘‘ The great richness of the sea is explained not only by its 
greater extent, but also by its more uniform temperature. The fresh waters 
stand 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 
of temperature 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 
regions. He mentions Arca 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, Palemon* jamaicensis, and to 
the Thelphusiade, a heterogeneous family of freshwater crabs, which occur in 
sub-tropical regions. 
Active as 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 Unionide 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- 
rina, should not have done so too. 
Considering the merciless struggle for existence which the superabundance oi 
marine life involyes, sufficient, according to some writers, to drive theless suc- 
cessful 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 
marine 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 Palemon is now recognized as a freshwater genus. 
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