332 Life Changes on the Globe. 



Palaeozoic, Mesozoic, and Cainozoic faunae and florae, taken 

 together, bear somewhat; the same proportion to the whole 

 series of living beings which have occupied this globe, as the 

 existing faunas and floras do to them." 



Serious difficulties are experienced when it is attempted to 

 convert geological into historical time ; but the periods re- 

 quired for even minor changes in the earth's surface must 

 greatly exceed those with which the records of human civiliza- 

 tion have to deal, and no geologist of repute would contend 

 that a small number of ages would suffice for the deposition 

 of any considerable thickness of sedimentary rock, or for 

 the denudation of an extensive area by aqueous and atmo- 

 spheric action. When, therefore, we contemplate an epoch in 

 the physical history of the globe, we must beware of assigning 

 limits upon imaginary grounds ; and if on the one hand we 

 accept Professor Phillips's caution, to avoid random speaking 

 about " millions on millions of ages," we must at any rate be 

 equally on our guard lest we condemn ourselves to an erroneous 

 mode of interpretation by refraining from the perception of the 

 fact that the operations which geology traces could not possibly 

 have taken place within any duration that our faculties enable 

 us to appreciate. In astronomical distances our understanding 

 is soon brought to a condition in which the multiplication of 

 figures fails to produce a corresponding enlargement in our 

 ideas, and we must not expect more success in the attempt to 

 grasp the ages of the earth. Taking the whole group of sta- 

 tin" ed rocks in the British Isles at 100, Professor Phillips 

 assigns 79 to the Palaeozoic, 18 to the Mesozoic, and 3 to the 

 Cainozoic ; and if the thicker formations absorbed a propor- 

 tionably greater length of time, it would not be easy to imagine 

 calculations that would transcend the truth. Life changes on the 

 earth can only be imperfectly understood from the incomplete 

 series of remains that have been discovered, but from them the 

 authority last cited computes the rate of progressive change 

 at —■ for the Palaeozoic -y$ for the Mesozoic, and -g- for the 

 Cainozoic time, and he adds, " The slowness of early changes 

 has been ascribed to a greater uniformity of terrestrial tempe- 

 rature than is now experienced." 



How and in what manner climates have altered is too large 

 a question to be treated incidentally ; but, in connection with 

 the present inquiry, we may quote some remarks of Mr. Hop- 

 kins, which cut off the recourse to central fires and rapid 

 refrigeration as causes which produced noticeable effects in a 

 few thousands or a few millions of years. " The part of the 

 superficial temperature due to primitive heat is very small, 

 amounting to about one-twentieth of a degree of Fahrenheit. 

 It must have been constantly diminishing for an immense 



