xlviii PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



The accusation that we have been running counter to ih.Q principles 

 of natural philosophy, therefore, is devoid of foundation. The only 

 question which can arise is whether we have, or have not, been 

 tacitly making assumptions which are in opposition to certain con- 

 clusions which may be drawn from those principles. And this ques- 

 tion subdivides itself into two : — the first, are we really contra- 

 vening such conclusions ? the second, if we are, are those conclu- 

 sions so firmly based that we may not contravene them ? I reply 

 in the negative to both these questions, and I wiU give you my rea- 

 sons for so doing. Sir William Thomson believes that he is able to 

 prove by physical reasonings, " that the existing state of things on 

 the earth, life on the earth — all geological history showing conti- 

 nuity of life — must be limited within some such period of time as 

 one hundred million years " (loc. cit. p. 25). 



The first inquiry which arises plainly is, has it ever been denied 

 that this period may be enough for the purposes of geology ? 



The discussion of this question is greatly embarrassed by the vague- 

 ness with which the assumed limit is, I will not say defined, but 

 indicated, — '* some such period of past time as one hundred million 

 years." Now does this mean that it may have been two, or three, 

 or four hundred million years ? Because this really makes all the 

 difference*. 



I presume that 100,000 feet may be taken as a full allowance for 

 the total thickness of stratified rocks containing traces of life; 

 100,000 divided by 100,000,000=0-001. Consequently, the deposit 

 of 100,000 feet of stratified rock in 100,000,000 years means that 

 the deposit has taken place at the rate of y^q^ of a foot, or, say, y-L 

 of an inch, per annum. 



Well, I do not know that any one is prepared to maintain that, 

 even making all needful allowances, the stratified rocks may not 

 have been formed, on the average, at the rate of -^ of an inch per 

 annum. I suppose that if such could be shown to be the limit of 

 world-growth, we could put up with the allowance without feeling 

 that our speculations had undergone any revolution. And perhaps, 

 after all, the qualifying phrase " some such period " may not neces- 

 sitate the assumption of more than j-Lg., or -g-i-g-, or -^^ of an inch 

 of deposit per year, which, of course, would give us stiU more ease and 

 comfort. 



But it may be said that it is biology, and not geology, which asks 

 for so much time — that the succession of life demands vast intervals ; 

 but this appears to me to be reasoning in a circle. Biology takes 

 her time from geology. The only reason we have for believing in the 

 slow rate of the change in living forms is the fact that they persist 

 through a series of deposits which geology informs us have taken a long 

 while to make. If the geological clock is wrong, all the naturalist 

 will have to do is to modify his notions of the rapidity of change ac- 

 cordingly. And I venture to point out that, when we are told that the 



* Sir William Thomson implies {loc. cit. p. 16) that the precise time is of no 

 consequence, *'the principle is the same "; but as the principle is admitted, the 

 whole discussion turns on its practical results. 



