504 THE EVOLUTION OF LIFE. 



the sea: elevation producing a decrease of heat and conse- 

 quently an increase in the precipitation of water a precipit- 

 ation which takes the shape of snow where the elevation is 

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

 ings of clouds and descents of showers around mountain 

 tops, are familiar to every tourist. Inquiries in the neigh- 

 bouring 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 meteorological 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 different heights, in all the hollows 

 and ravines of each mountain side. 



As climatic variations thus geologically produced, are 

 compounded with those which result from slow astronomical 

 changes; and as no correspondence exists between 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 atmos- 

 pheric agencies, are ever passing into unparalleled combina- 

 tions; 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 humidity, causes a par- 

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

 unfavourable or favourable effect is wrought on all such 



