500 W. H. DALL 
almost wholly absent from them. However, where the granitic 
masses which form the core of some of the islands (and in other 
places stand alone, domelike in the sea) are within reach of the waves, 
we find a special flora of the more bright-colored algae and a special 
fauna dependent upon them. No matter how isolated the patch of 
granite, the characteristic animals recur, and in many cases reproduce 
in their own tints the rosy hue of the plants upon which they depend 
for food. 
In the abysses where the absence of sunlight excludes plant life 
the animals are almost exclusively carnivorous and largely subsist 
on the abundant rain of dead organisms which slowly descends from 
the surface layers of the sea. 
It has been customary to regard the 1oo-fathom line as constituting 
a sort of boundary between the fauna of the shores and of the deeps. 
This has a certain foundation in the fact that at greater depths no 
living algae can exist for want of sunlight. A more or less constant 
migration, casual or accidental, is constantly taking place between 
the littoral region and the deeps, but it is so slow, and the process of 
adaptation to the new conditions so gradual, that we may safely 
regard the abyssal fauna as even geologically old. I have called 
attention to certain features of the eastern Pacific and Antillean abyssal 
faunas which illustrate these remarks in the introduction to a recent 
monograph.’ 
Freshwater and terrestrial invertebrates are subject not infre- 
quently to one set of influences which is rarely noticed in the open 
sea. This is, in the case of the limnophilous species, a change in 
the mineral content of the water in which they live. ‘This is usually 
gradual and when injurious chiefly due to the concentration of salts 
(which exist in all freshwaters arising from drainage) by evaporation. 
In the case of many large Pleistocene lakes, of which the prehistoric 
Lake Bonneville may be taken as an example, this process has been 
carried on until the saline content of the water became so excessive 
that all molluscan life became extinct, as in the Great Salt Lake of 
Utah. A careful study of the beds of shell-marl deposited by the 
shrinking lake shows that the effect of the gradually increasing 
salinity of the water on the freshwater mollusks contained in it was 
« Bull. Mus. Comp. Zoélogy, Vol. XLIII, No. 6, October, 1908, pp. 205-12. 
