294 THE FHKSH-WATER LOCHS OF SCOTLAND 
winter cooling begins. Smaller lochs, which are affected to the bottom 
by the heat of summer, enjoy a similar equable climate during the 
greater part of the year, and the temperature of the bottom water is 
only raised a few degrees in summer. Onlv in very shallow lochs is 
there a greater range of temperature. Even in the deeper lakes a gale 
of exceptional duration may temporarily change the temperature of 
the abyssal region, but it will quickly return to the normal. The 
temperature of the bottom water of the deeper lakes is in the 
neighbourhood of 42' "0 Fahr. 
Such a uniform and comparatively mild climate might be supposed 
favourable to many forms of animal life. Though climates offering 
extremes of temperature may produce the more robust forms of life, 
it is often foiuid that regions where equable conditions prevail are 
occupied by many forms which desire to take life easy. Yet the 
abvssal region of the lakes is but sparsely peopled ; only about a 
dozen species live there. 
What determines the restriction of the abyssal population ? 
Those animals which have become naturalised in the deep muds 
appear to be all indifferent to changes of temperature, light, etc., as 
they are found in all situations. An animal with good eyes, like 
Cyclops viridis^ may deliberately choose the darkness of the muds. 
It appears to be of a timid disposition, and to be always trying to 
hide itself If the abyssal region is mainly recruited, as there is reason 
to believe, by animals which have accidentally fallen or slid down the 
steep slopes of the lakes, then most of the littoral species might be 
introduced in this way. If so, why do so few survive ? Are some of 
the abyssal conditions inimical to most species? Is the unfavourable 
condition the scarcity of oxygen in available form ? It was formerly 
supposed that life at great depths, even in the sea, was impossible 
from this cause. 
It is now known that, owing to currents set up by wind, and 
perhaps still more to the " internal seiche which may originate in 
such currents, the water at the bottom of the lakes is less stagnant 
than was supposed. Matter in suspension in the water may carry 
down air to the bottom. Still, from all causes the renewal of the 
oxygen in the abyssal region must be very slow. Many of the abyssal 
animals are able to exist in very impure water, from which air is 
excluded. When an abyssal collection is tightly corked up, the 
different species exhibit different degrees of power to exist in these 
conditions. The Molluscs are most sensitive, and die at once ; the 
Worms are most tenacious of life. 
Animals having fair swimming powers, and eyes sufficiently sensitive 
to light, may be able to save themselves from the catastrophe of 
falling into the abyss. Some of the plankton Cladocera appear by 
