$14 THE EVOLUTION OF LIFE. 



fereiit aspects. Extrusions of trap, wherever they take 

 place, revolutionize the localities ; both over the areas covered, 

 and over the areas on which their detritus is left. 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 previously deposited. 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 re-ar- 

 rangement of 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 

 ^dth ever-increasing complexity. As already shown (First 

 Principles, 118), it follows from the law of the multiplication 

 of effects, that during long periods, each tract of the Earth's 

 Burface increases in heterogeneity of both form and substance. 

 Hence plants and animals of all kinds, are, in the course of 

 generations, subject by these alterations in the crust of the 



