LIFE SUPERIMPOSED IN ZONES OF DEPTH. 455 



depends, first, upon the distribution in past time ; and, secondly, 

 upon the great physical changes which brought the known masses of 

 land into existence. Edward Forbes long since recognised the law 

 that life diffuses itself in the main under climatic conditions. 1 We 

 may say that life diffuses itself in the direction of least resistance, 

 which term may conveniently indicate comparatively uniform con- 

 ditions of life. The influence of climate is obvious, because it implies 

 similar physiological action for the parts of an organism and similar 

 conditions of nutrition, as well as similar food. And the low forms of 

 life may find similar climatic conditions in the deep waters of the 

 tropics, to those which are presented by the shallow waters of the 

 arctic seas ; so that all over the world the life of the deep ocean forms 

 as Sir Wyville Thomson observed, one natural history province so 

 far as its lower organisms are concerned. How far this uniformity 

 may be the consequence of the existence of deep-sea currents, diffus- 

 ing the cold water and the germs of life within it, may perhaps be 

 worth consideration. In like manner, above the surface of the earth 

 there is a zone of comparatively uniform climatic conditions, which 

 ascends from the poles higher and higher above the sea, following 

 the snow-line, until, at about 16,000 feet of elevation in the tropics, 

 the life is in the main the same in kind as that within the arctic 

 circle. These aspects of climate are complicated, however, by varia- 

 tions in pressure of air and water, and this difference of pressure has 

 a value in elaborating structures by inducing modified functions ; 

 so that the term alpine is not always synonymous with arctic, 

 just as the deep ocean has yielded many forms of life which are 

 unknown in the polar waters. The ordinary conception of climate, 

 however, as indicated by limits of latitude, is exhibited in the distri- 

 bution of many groups of organisms like reef-building corals. There 

 are probably no conditions of climate to which life may not adapt itself, 

 or be compelled to adapt itself, by physical circumstances. Perhaps 

 the influence of coast-lines is one of the most important influences 

 tending to modify the distribution of marine life in directions which 

 are different, because not only is the water aerated by tidal action near 

 to shore, but the conditions of existence are similar at similar depths. 

 Edward Forbes observed the superposition of distinct faunas on the 

 same shore. He defined by the term littoral all such animals as lived 

 between tide-marks ; laminarian zone was the home of the laminaria 

 and similar sea-weeds, in water reaching from low tide to fifteen 

 fathoms, where in our own seas the larger number of vegetable-feeding 

 mollusca, as well as the carnivorous mollusca like buccinum, nassa, 

 and natica, abound. Below this is the coralline zone, where the 

 ordinary sea-weeds give place to nullipores and horny zoophytes, where 

 the great scallop replaces the oyster of the laminarian zone, where 

 the common gasteropods are buccinum, fusus, pleurotoma, natica, 

 aporrhais, fissurella, emarginula, pileopsis, eulima, and chemnitzia ; 

 the bivalves are lima, area, nucula, astarte, venus, artemis, corbula, 

 thetis. But the muddy or sandy character of the sea-bed has an 

 1 See Keith Johnston's Physical Atlas. 



