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 is a false shore-hne 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 aU 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 



