OcToOBER 13, 1899.1] 
of the first (tidal friction) ; and have en- 
deavored to show that there are elements of 
uncertainty surrounding the second (secu- 
lar cooling of the earth) ; nevertheless they 
undoubtedly constitute a contribution of 
the first importance to physical geology. 
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 continuity 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 arguments. 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 allow- 
able to assume a much higher conductivity 
for the interior of the globe, and that this 
assumption would vastly increase our esti- 
mate 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 
envelope. 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 physicists, than 
400 millions of years. From the three 
physical arguments, Lord Kelvin’s higher 
limits are 1000, 400, and 500 million years. 
_* Rep. Brit. Assoc., 1886, p. 517. 
SCIENCE. 
519 
I have shown that we have reasons for be- 
lieving that the age, from all these, may be 
very considerably underestimated. It is to 
be observed that if we exclude everything 
but the arguments 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 de- 
manding 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 recog- 
nized authority on the physical side re- 
echoes and emphasizes the warning pro- 
nounced by Professor Darwin in the address 
already quoted—‘‘ at present our knowledge 
of a definite limit to geological time has so 
little precision that we should do wrong to 
summarily reject any theories which appear 
to demand longer periods of time than those 
which now appear allowable.” + 
This ‘ wrong,’ which Professor Darwin so 
seriously deprecated, has been committed 
not once, but again and again, in the his- 
tory of this discussion. Lord Kelvin has 
never taken any notice of the strong body 
of evidence adduced by geologists and pale- 
ontologists in favor of a much longer antiq- 
uity than he is now disposed to allow for 
the age of the earth. His own three phys- 
ical arguments have been successively re- 
stated, with such corrections and modifica- 
tions as he has found to be necessary, and 
no doubt further: alterations are in store for 
them. He has cut off slice after slice from 
the allowance of time which at first he was 
prepared to grant for the evolution of geo- 
logical history, his latest pronouncement 
being that ‘it was more than twenty and 
less than forty million years, and probably 
much nearer twenty than forty.’{ But 
* Nature, Vol. LI., p. 585, April 18, 1895. 
+ Rep. Brit. Assoc., 1886, p. 518. 
{‘The Age of the Earth,’ Presidential Address to 
the Victoria Institute for 1897, p. 10, Phil. Mag., 1899. 
