THE DISTRIBUTION OF ANIMALS AND PLANTS 417 



culation based on the wearing down of our continents, which 

 appears to proceed at the rate of about a foot in four or five 

 thousand years, and on the time required to deposit the sedi- 

 ments of the several geological formations, estimated at about 

 70,000 feet in thickness. These calculations would give us, 

 say, eighty-six millions of years since the earth began to have 

 a solid crust, which would, like Lord Kelvin's earlier estimate, 

 give us nearly fifty millions of years for the geological time 

 since the introduction of life. The details of the several esti- 

 mates made it would be tedious and unprofitable to enter into, 

 but I may state as my own conclusion, that the modern rates of 

 denudation and deposit must be taken as far below the average, 

 and that perhaps the estimate stated by Wallace on data sup- 

 plied by Houghton, namely, twenty-eight millions, may be not 

 far from the truth, though perhaps admitting of considerable 

 abatement. 



This reduced estimate of geological time would still give 

 scope enough for the distribution of animals and plants, but 

 it will scarcely give that required by certain prevalent theories 

 of evolution. When Darwin says, " If the theory (of natural 

 selection) be true, it is indisputable that before the lowest Cam- 

 brian stratum was deposited long periods elapsed, as long as, 

 or probably far longer than, the whole interval from the Cam- 

 brian to the present day," he makes a demand which geology 

 cannot supply ; for independently of our ignorance of any 

 formations or fossils, except those included in the Archaean, 

 to represent this vast succession of life, the time required 

 would push us back into a molten state of the planet. This 

 difficulty is akin to that which meets us with reference to the 

 introduction of many and highly specialized mammals in the 

 Eocene, or of the forests of modern type in the Cretaceous. 

 To account for the origin of these by slow and gradual evolu- 

 tion requires us to push these forms of life so far back into 

 formations which afford no trace of them, but, on the contrary, 



