SEPTEMBER 21, 1899] 
NATURE 
499 
tinuity of life—must be limited within some such period of past 
time as 100,000,000 years” (Rep. Brit. Assoc., 1886, p. 517)- 
More recently Prof. 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 Prof. Darwin, he dismisses as fallacious. In regard 
to the argument from the secular cooling of the earth, he con- 
tends that it is perfectly allowable to assume a much higher 
conductivity for the interior of the globe, and that this assump- 
tion 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 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 pub- 
lished 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. I have shown that we have reasons 
for believing that the age, from all these, may be very consider- 
ably under-estimated. It is to be observed that if we exclude 
everything but the arguments from mere physics, the prodadie 
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” (NATURE, vol. li. p. 585, April 18, 1895). 
This remarkable admission from a recognised authority on the 
physical side re-echoes and emphasises the warning pronounced 
by Prof. 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 
hose Sue now appear allowable” (Rep. Brit. Assoc., 1886, 
p- 518). 
This ‘‘ wrong,”’ which Prof. Darwin so seriously deprecated, 
has been committed, not once, but again and again in the 
history of this discussion. Lord Kelvin has never taken any 
notice of the strong body of evidence adduced by geologists and 
palzontologists in favour of a much longer antiquity than he is 
now disposed to allow for the age of the earth. His own three 
physical arguments have been successively re-stated, with such 
corrections and modifications 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 geological history, 
his latest pronouncement being that ‘‘it was more than twenty 
and less than forty million years, and probably much nearer 
twenty than forty.”4 But in none of his papers is there an 
admission that geology and palzontology, though they have 
again and again raised their voices in protest, have anything to 
say in the matter that is worthy of consideration. 
It is difficult satisfactorily to carry on a discussion in which 
your opponent entirely ignores your arguments, while you have 
given the fullest attention to his. In the present instance, 
geologists have most carefully listened to all that has been 
brought forward from the physical side. Impressed by the force 
of the physical reasoning, they no longer believe that they can 
make any demands they may please on past time. They have 
been willing to accept Lord Kelvin’s original estimate of 100 
millions of years as the period within which the history of life 
upon the planet must be comprised, while some of them have 
even sought in various ways to reduce that sum nearer to his 
lower limit. Yet there is undoubtedly a prevalent misgiving, 
whether in thus seeking to reconcile their requirements with the 
demands of the physicist they are not tying themselves down 
within limits of time which on any theory of evolution would 
have been insufficient for the development of the animal and 
vegetable kingdoms. 
It is unnecessary to recapitulate before this Section of the 
British Association, even in briefest outline, the reasoning of 
geologists and paleontologists which leads them to conclude that 
the history recorded in the crust of the earth must have re- 
quired for its transaction a much vaster period of time than that 
1‘ The Age of the Earth, ’ Presidential Address to the Victoria Institute 
for 1897, p- 10; alsoin PAz/. Mag., January 1899. 
NO. 1560, VOL. 60] 
to which the physicists would now restrict it.1 Let me merely 
remark that the reasoning is essentially based on observations of 
the present rate of geological and biological changes upon the 
earth’s surface. It is not, of course, maintained that this rate 
has never varied in the past. But it is the only rate with which 
we are familiar, which we can watch and in some degree 
measure, and which, therefore, we can take as a guide towards 
the comprehension and interpretation of the past history of our 
planet. 
It may be, and has often been, said that the present scale of 
geological and biological processes cannot be accepted as a 
trustworthy measure for the past. Starting from the postulate, 
which no one will dispute, that the total sum of terrestrial energy 
was once greater than it is now and has been steadily declining, 
the physicists have boldly asserted that all kinds of geological 
action must have been more vigorous and rapid during bygone 
ages than they are to-day ; that volcanoes were more gigantic, 
earthquakes more frequent and destructive, mountain-upthrows 
more stupendous, tides and waves more powerful, and com- 
motions of the atmosphere more violent, with more ruinous 
tempests and heavier rainfall. Assertions of this kind are 
temptingly plausible and are easily made. But it is noi 
enough that they should be made; they ought to be 
supported by some kind of evidence to show that they are 
founded on actual fact and not on mere theoretical possibility. 
Such evidence, if it existed, could surely be produced. The 
chronicle of the earth’s history, from a very early period down 
to the present time, has been legibly written within the sedi- 
mentary formations of the terrestrial crust. Let the appeal 
be made to that register. Does it lend any support to the 
affirmation that the geological processes are now feebler and 
slower than they used to be? If it does, the physicists, we 
might suppose, would gladly bring forward its evidence as 
irrefragable confirmation of the soundness of their contention- 
But the geologists have found no such confirmation. On the 
contrary, they have been unable to discover any indication 
that the rate of geological causation has ever, on the whole, 
greatly varied during the time which has elapsed since the 
deposition of the oldest stratified rocks. They do not assert 
that there has been no variation, that there have been no 
periods of greater activity, both hypogene and epigene. But 
they maintain that the demonstration of the existence of such 
periods has yet to be made. They most confidently affirm that 
whatever may have happened in the earliest ages, in the whole 
vast succession of sedimentary strata nothing has yet been 
detected which necessarily demands that more violent and rapid 
action which the physicists suppose to have been the order of 
nature during the past. 
So far as the potent effects of prolonged denudation permit us 
to judge, the latest mountain-upheavals were at least as 
stupendous as any of older date whereof the basal relics can 
yet be detected. They seem, indeed, to have been still more 
gigantic than those. It may be doubted, for example, whether 
among the vestiges that remain of Mesozoic or Palaeozoic moun- 
tain-chains any instance can be found so colossal as those of 
Tertiary times, such as the Alps. No volcanic eruptions of the 
older geological periods can compare in extent or volume with 
those of Tertiary and recent date. The plication and dislocation 
of the terrestrial crust are proportionately as conspicuously dis- 
played among the younger as among the older formations, 
though the latter, from their greater antiquity, have suffered 
during a longer time from the renewed disturbances of successive 
periods. 
As regards evidence of greater violence in the surrounding 
envelopes of atmosphere and ocean, we seek for it in vain 
among the stratified rocks. Among the very oldest formations 
of these islands, the Torridon sandstone of North-west Scotland 
presents us with a picture of long-continued sedimentation, 
such as may be seen in progress now round the shores of many 
a mountain-girdled lake. In that venerable deposit, the enclosed 
pebbles are not mere angular blocks and chips, swept by a 
sudden flood or destructive tide from off the surface of the land, 
and huddled together in confused heaps over the floor of the 
sea. They have been rounded and polished by the quiet 
operation of running water, as stones are rounded and polished 
1 The geological arguments are briefly given in my Presidential 
Address to the British Association at the Edinburgh Meeting of 1892. 
The biological arguments were well stated, and in some detail, by Prof- 
Poulton, in his Address to the Zoological Section of the Association at the 
Liverpool Meeting of 1896. 
