502 THE EVOLUTION OF LIFE. 



being produced in its several parts, by their exposures to 

 different aspects. Extrusions of trap, wherever they take 

 place, revolutionize the localities ; both over the areas covered 

 and over the areas on to which their detritus is carried. And 

 where volcanoes are formed, the ashes they occasionally send 

 out modify the character of the soil throughout large sur- 

 rounding tracts. 



In like manner alterations in the Earth's crust cause the 

 ocean to be ever subjecting the organisms it contains to new 

 combinations of conditions. Here the water is being deep- 

 ened by subsidence, and there shallowed by upheaval. While 

 the falling upon it of sediment brought down by neighbour- 

 ing large rivers, is raising the sea-bottom in one place, in 

 another the habitual rush of the tide is carrying away the 

 sediment deposited in past times. The mineral character of 

 the submerged surface on which sea-weeds grow and molluscs 

 crawl, is everywhere occasionally changed; now by the 

 bringing away from an adjacent shore some previously un- 

 touched strata; and now by the accumulation of organic 

 remains, such as the shells of pteropods or of foraminifera. 

 A further series of alterations in the circumstances of marine 

 organisms, is entailed by changes in the movements of the 

 water. Each modification in the outlines of neighbouring 

 shores makes the tidal streams vary their directions or 

 velocities or both. And the local temperature is from time 

 to time raised or lowered, because some far-distant change 

 of form in the Earth's crust has wrought a divergence in those 

 circulating currents of warm and cold water which pervade 

 the ocean. 



These geologically-caused changes in the physical charac- 

 ters of each environment, occur in ever-new combinations, 

 and with ever-increasing complexity. As already shown 

 (First Principles, § 158), it follows from the law of the mul- 

 tiplication of effects, that during long periods each tract of 

 the Earth's surface increases in heterogeneity of both form 

 and substance. So that plants and animals of all kinds are. 



