ANIMAL LIFE IN THE SEA 171 
are regions of great mortality among the pelagic 
organisms, for the stenothermal forms are killed by the 
sudden changes of temperature which occur. But there 
is something more than this. The delicate organisms 
which float in the surface waters of the sea are neces- 
sarily very sensitive to changes of density. They must 
be able to float without effort—that is, their specific 
gravity must be that of the water in which they float. 
Very many possess certain powers of adjustment—that 
is, they can vary their specific gravity in harmony with 
variations in the specific gravity of the water. But 
this power of adjustment has its limits. When warm, 
light water is floating on cold, dense water, it is appar- 
ently impossible for the more delicate forms at least to 
pass through the junction layer of the two, which thus 
forms what Dr. Hjort calls a ‘ false bottom ’, a region 
where living and dead plankton animals accumulate. 
By a series of very ingenious investigations he has shown 
that, in many parts of the ocean, not only does such a 
junction layer between the surface and lower layers of 
water occur, but that this layer is a region of great wealth 
of life. It isa false shore-line in mid-ocean, and Dr. Hjort 
has shown that many of the littoral fish at certain 
seasons leave the shore, and swim out into the open 
ocean, following this line of change of density, where 
food accumulates—where it is as abundant as it is 
near the land. There seems some reason to believe 
that such an accumulation of pelagic organisms in 
an area of sudden change of density occurs in the 
vicinity of all the great fishing regions of the world, 
and thus, as it were, prolongs the plenty of the shore 
out into the open, and helps to account for the abund- 
ance of fish. 
As to the actual temperature of ocean water, the 
