Hill. — Denudation as a Factor of Geological Time. 669 



sions of the earth were much greater than now, and the pres- 

 sure of the atmosphere on the surface was therefore less. We 

 know as a physical fact that there is a relationship between 

 the pressure of the air and the change of water into vapour, 

 and if the pressure of the air, as suggested, was less in the 

 earlier history of the earth and long after the surface-tempera- 

 ture was below 100° C. the conditions then existing were 

 quite unlike what we are acquainted with in these times. Then 

 the water would have been heated by convection from below 

 rather than by conduction, as now, from above. There would 

 certainly have been more evaporation, and the slightest tem- 

 perature-changes in the atmosphere such as there would 

 necessarily be must have brought about heavy mists and a, 

 moist heated climate over the entire surface of the earth. 

 Conditions such as these would produce abundance of life 

 \vithout great differentiations, for differentiations in the organic 

 world are the direct outcome of temperature contrasts. When 

 the earth was in process of solidifying it was with its atmo- 

 sphere in direct contact with its external surroundings. The 

 temperature of outer space — that is, of the space outside the 

 limits of the atmosphere — is said to be 250° or more below the 

 zero of Centigrade. An incandescent earth with a temperature 

 outside its enclosing envelope approaching the zero of cold is 

 placing two opposites in direct contrast to one another. At 

 such a time there were no contrasts within the earth itself, 

 and it was only when the earth began to part with its heat into 

 outer space at an unequal rate, as it does now, that earth-con- 

 trasts became possible, and that differentiation in the animal 

 and vegetable world began. But these differentiations for a 

 long time were necessarily small, seeing that they varied in 

 proportion to the conductivities and radiating capacities of the 

 surface-rocks at that time. As pointed out above, the waters 

 were heated by convection rather than by conduction, and it 

 was by this means that the climatic conditions of the earth 

 were similar, or laearly so, throughout each zone. 



It will appear from these considerations that when life first 

 made its appearance on the earth the types were the simplest 

 possible ; for high types, both in the animal and vegetable king- 

 dom, imply great dil3^erentiations and wide contrasts. Thus 

 the types have continued to increase in number and complexity 

 in direct proportion to the changes which have been brought 

 about on the earth's surface by the operation of those laws 

 which depend upon the inequality of temperatures for their 

 efiective action. From this it follows that life is dependent 

 on environment. To suppose that animals or plants could 

 flourish under conditions not adapted or suited to them is to 

 assume an impossibility. Plants and animals in a state of 

 nature are best adapted to their environment, because were 



