416 THE DISTRIBUTION OF ANIMALS AND PLANTS 



Now had a grove of Sequoias, however small, survived any- 

 where in Europe or Asia, and had we no knowledge of the 

 fossil forms, we might have been quite at a loss to account for 

 their peculiar distribution. The fossil remains of the Tertiary 

 rocks, both animal and vegetable, present us with many instances 

 of this kind. 



The discussion of the distribution of animals and plants, 

 when carried on in the light of geology, raises many interesting 

 questions as to time, which we have already glanced at, but 

 which deserve a little more attention. As to the vast duration 

 of geological time all geologists are agreed. It is, however, 

 now well understood that science sets certain limits to the time 

 at our disposal. Edward Forbes humorously defined a geolo- 

 gist to be " an amiable enthusiast who is content if allowed to 

 appropriate as much as he pleases of that which other men 

 value least, namely, past time " ; but now even the geologist 

 is obliged to be content with a limited quantity of this com- 

 modity. 



The well-known estimate of Lord Kelvin gave one hundred 

 millions of years as the probable time necessary for the change 

 of the earth from the condition of a molten mass to that which 

 we now see. On this estimate we might fairly have assumed 

 fifty millions of years as covering the time from the Laurentian 

 age to the modern period. The great physicist has, however, 

 after allowing us thus much credit in the bank of time, " sud- 

 denly put up the shutters and declared a dividend of less than 

 four shillings in the pound." 1 In other words, he has reduced 

 the time at our disposal to twenty millions of years. Other 

 physicists, reasoning on the constitution of the sun, agree 

 with this latter estimate, and affirm that " twenty millions of 

 years ago the earth was enveloped in the fiery atmosphere of 

 the sun." 2 Geology itself has attempted an independent cal- 



1 Bonney, Address before British Association, 1888. 

 2 Newcomb, Helmholtz, Tait, etc. 



