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president’s address. 
are many others, must have reached their present habitat by 
means of the submarine outflow of subterranean watercourses, 
and in so doing they will have had the advantage of entering 
a water system not exposed to temperature fluctuations, though 
in other respects seemingly most unsuitable. 
The number of species of marine tpye living in fresh water 
is an exceedingly large one, and the examples given must 
suffice. They are found usually in the course of rivers, but 
may occur in lakes, and their presence therein is often adduced 
as evidence of recent geological changes. When found in 
lakes, these lakes are usually, though not always, deep, and 
consequently with a more or less equable temperature in their 
lower layers. 
We must now consider the actual routes which these species 
may be supposed to have taken in occupying their present 
habitats. 
Immigration into Fresh Water. 
There are three means by which a marine animal might 
become introduced to fresh water 
(1) Transportation. 
(2) By isolation of arms of the sea, the water of which 
becomes fresh, giving rise to “ Relict Lakes.” 
(3) By ascending rivers. 
1. Transportation. 
Since all experiments prove that a sudden transference from 
salt water to fresh, and vice versa, is immediately fatal to 
invertebrates, it is impossible to believe that marine animals 
could be succesfully introduced to fresh water by transportation 
through the air by the agency of animals or wind. 
Such methods of dispersal are certainly effective for the 
distribution of fresh water animals from lake to lake, or pond 
to pond ; firstly, by reason of the fact that many species have 
eggs, or resting stages, which are peculiarly resistant to drought, 
or specially adapted to adhere to the plumage of birds, and also 
because no contrast of water conditions is involved. For 
the same reason, in certain special cases, transportation from 
