xii INTRODUCTION 



some of them are less well marked than in the almost 

 islandless Atlantic ; the Indian, representing, as it 

 were, the southern half of the other two, shows them 

 more clearly during the south-west monsoon than 

 during that from the north-east ; but they are recog- 

 nisable in all. The diagram in Fig. 5 gives a rough 

 idea of these resemblances between the three great 

 oceanic areas, in air pressures, in winds, and in cur- 

 rents ; the latitudes of corresponding regions, however, 

 vary somewhat in the different areas. 



In these phenomena of pressure, wind, and current, 

 the sun is the source of energy. It may be said, in the 

 most general terms, that the distribution of marine 

 animals and plants depends mainly on water tempera- 

 tures, the water temperatures almost entirely on 

 oceanic currents, the currents chiefly on prevailing 

 winds, the winds on barometric pressures, and the 

 pressures on the effects of the sun's rays upon Earth, 

 Air, and Water. It is easy to make a general state- 

 ment of this kind : it is not so easy to prove every link 

 in the chain ; and, until we have accumulated far more 

 knowledge than at present we possess, some parts of 

 it must be of the nature of Articles of Faith ; and, like 

 most such Articles, they have their sceptics. Still less 

 do we know of the details of the inner working of those 

 stupendous phenomena which affect five-sevenths of the 

 Earth's surface. 



At the other end of the scale — so far as mere size is 

 concerned — we do not at present really understand 

 the life of a single rock-pool, the delicate adjustment 

 which exists between the myriads of organisms, its 

 inhabitants. We recognize dimly the outlines of the 

 main processes — the unending cycle in which food 

 material passes from plant to animal, and back to plant 

 again ; the struggle for existence — as keen amongst 



