x EARTH CHANGES & EVOLUTION 185 



formly, and with no serious break of continuity, during the 

 whole vast periods of geological time. These forces have 

 always been curiously balanced, and have been brought into 

 action alternately in opposite directions, so as to maintain, 

 ; over a large portion of the globe, land surfaces of infinitely 

 varied forms, which, though in a state of continuous flux, 

 yet never reached a stationary condition. Everywhere the 

 land is being lowered by denudation towards the sea-level, 

 and part by part is always sinking below it, yet ever being 

 renewed by elevatory forces, whose nature and amount we 

 can only partially determine. Yet these obscure forces have 

 always acted with so much regularity and certainty that the 

 long, ever-branching lines of plant and animal development 

 have never been completely severed. If, on the other hand, 

 the earth's surface had ever reached a condition of per- 

 manent stability, so that both degrading and elevating 

 forces had come to a standstill, then the world of life itself 

 would have reached its final stage, and, wanting the motive 

 power of environmental change, would have remained in a 

 state of long-continued uniformity, of which the geological 

 record affords us no indication whatever. 



Readers of my book on Man's Place in the Universe will 

 remember how, in chapters xi. to xiv., I described the long 

 series of mechanical, physical, and chemical adjustments of 

 the earth as a planet, which were absolutely essential to the 

 development of life upon its surface. The curious series 

 of geological changes briefly outlined in the present chapter 

 are truly supplementary to those traced out in my former 

 work. The conclusion I drew from those numerous cosmic 

 adaptations was that in no other planet of the solar system 

 were the conditions such as to render the development of 

 organic life possible upon them — not its existence merely, which 

 s a very different matter. That conclusion seemed to many 

 t>f my readers, including some astronomers, geologists, and 

 physicists, to be incontestable. The addition of the present 

 series of adaptations, whose continuance throughout the 

 vhole period of world-life history is necessary as furnishing 

 he motive power of organic development and adaptation, 

 lot only increases to an enormous extent the probability 

 igainst the development of a similar " world of life," cul- 



