416 THE EVOLUTION OF LIFE. 



quently an increase in the precipitation of water — a precipit- 

 ation that takes the shape of snow where the elevation is 

 very great, and of rain where it is not so great. The gather- 

 ing of clouds and descent of showers around mountain tops, 

 are familiar to every tourist. Inquiries in the neighbouiing 

 valleys, prove that within distances of a mile or two the 

 recurring storms differ in their frequency and violence. 

 Nay, even a few yards off, the meteorologic conditions vary in 

 such regions : as witness the way in which the condensing 

 vapour keeps eddying round on one side of some high crag, 

 while the other side is clear ; or the way in which the snow- 

 line runs irregularly to many different heights, in all the minor 

 valleys and ravines and hollows of each mountain side. 



Climatic variations that are thus geologically produced, 

 being compounded with those which result from the slow 

 astronomical changes; and no correspondence existing be- 

 tween the geologic and the astronomic rhythms ; it results 

 that the same plexus of actions never recurs. Hence the 

 incident forces to which the organisms of every locality are 

 exposed by atmospheric agencies, are ever passing into un- 

 paralleled combinations ; and these are on the average ever 

 becoming more complex. 



§ 151. Besides changes in the incidence of inorganic 

 forces, there are equally continuous, and still more involved, 

 changes in the incidence of forces which organisms exercise 

 on one another. As before pointed out (§ 105), the plants 

 and animals inhabiting each locality, are held together in so 

 entangled a web of relations, that any considerable modifica- 

 tion which one species undergoes, acts indirectly on many 

 other species ; and eventually changes, in some degree, the 

 circumstances of nearly all the rest. If an increase of heat, 

 or modification of soil, or decrease of himiidity, causes a par- 

 ticular kind of plant either to thrive or to dwindle ; an 

 unfa vo arable or favourable effect is wrought on all such 

 competing kinds of plants, as are not immediately influenced 



