TRANSACTIONS OF SECTION C. 798 
‘sorest troubles.’ In another, he pronounced the physicist himself to be ‘ an odious 
spectre.’ 
The same self-confidence of assertion on the part of some, at least, of the dispu- 
tants on the physical side has continued all through the controversy. Yet when we 
examine the three great physical arguments in themselves, we find them to rest on 
assumptions which, though certified as‘ probable’ or ‘very sure,’ are nevertheless 
admittedly assumptions. The conclusions to which these assumptions lead must 
depend for their validity on the degree of approximation to the truth in the 
premisses which are postulated. 
Now it is interesting to observe that neither the assumptions nor the conclusions 
drawn from them have commanded universal assent even among physicists them- 
selves. If they were as self-evident as they have been claimed to be, they should 
at least receive the loyal support of all those whose function it is to pursue and 
extend the applications of physics. It will be remembered, however, that thirteen 
years ago Professor George Darwin, who has so often shown his inherited sympathy 
in geological investigation, devoted his presidential address before the Mathe- 
matical Section of this Association to a review of the three famous physical 
arguments respecting the age of the earth. He summed up his judgment of them 
in the following words: ‘In considering these three arguments I have adduced 
some reasons against the validity of the first [tidal friction] ; and have endeavoured 
to show that there are elements of uncertainty surrounding the second [secular 
cooling of the earth]; nevertheless they undoubtedly constitute a contribution of 
the first importance to physical geolory. Whilst, then, we may protest against 
the precision with which Professor Tait seeks to deduce results from them, we are 
fully justified in following Sir William Thomson, who says that “the existing 
state of things on the earth, life on the earth—all geological history showing con- 
tinuity of life, must be limited within some such period of past time as 100,000,000 
years.” ’? 
More recently Professor Perry has entered the lists, from the physical side, to 
challenge the validity of the conclusions so confidently put forward in limitation 
of the age of the earth. He has boldly impugned each of the three physical argu- 
ments. That which is based on tidal retardation, following Mr. Maxwell Close 
and Professor Darwin, he dismisses as fallacious. In regard to the argument from 
the secular cooling of the earth, he contends that it is perfectly allowable to assume 
a much higher conductivity for the interior of the globe, and that this assumption 
would vastly increase our estimate of the age of the planet, As to the conclusions 
drawn from the history of the sun, he maintains that, on the one hand, the sun 
may have been repeatedly fed by infalling meteorites, and that on the other the 
earth, during former ages, may have had its heat retained by a dense atmospheric 
enyelope. He thinks that ‘almost anything is possible as to the present internal 
state of the earth,’ and he concludes in these words: ‘To sum up, we can find no 
published record of any lower maximum age of life on the earth, as calculated by 
hysicists, than 400 millions of years. From the three physical arguments, Lord 
elvin’s higher limits are 1,000, 400, and 500 million years. I have shown that 
we have reasons for believing that the age, from all these, may be very considerably 
underestimated. It is to be observed that if we exclude everything but the argu- 
ments from mere physics, the probable age of life on the earth is much less than 
any of the above estimates; but if the paleontologists have good reasons for 
demanding much greater times, I see nothing from the physicist’s point of view 
which denies them four times the greatest of these estimates.’ ® 
This remarkable admission from a recognised authority on the physical side 
re-echoes and emphasises the warning pronounced by Professor Darwin in the 
address already quoted: ‘ At present our knowledge of a definite limit to geolo- 
gical time has so little precision that we should do wrong to summarily reject any 
1 Darwin's Life and Letters, vol. iii. pp. 115, 146. 
® Rep. Brit. Assoc., 1886, p. 517. 
% Nature, vol. li. p. 585, April 18, 1895. 
