420 
considerable changes are produced ; but it is quite as likely that 
they may be retarded or reversed. 
Tt is possible, on the other hand, that after the long period of 
quiescence which has elapsed there may be a new settlement of 
the ocean bed, accompanied with foldings of the crust, especially 
on the western side of the Atlantic, and possibly with renewed 
volcanic activity on its eastern margin. In either case a long 
time relatively to our limited human chronology may intervene 
before the occurrence of any marked change. On the whole, 
the experience of the past would lead us to expect movements 
and eruptive discharges in the Pacific rather than in the Atlantic 
area. It is therefore not unlikely that the Atlantic may remain 
undisturbed, unless secondarily and indirectly, until after the 
Pacific area shall have attained to a greater degree of quiescence 
than at present. But this subject is one too much involved in 
uncertainty to warrant us in following it further. 
In the meantime the Atlantic is to us a practically permanent 
ocean, varying only in its tides, its currents, and its winds, 
which science has already reduced to definite laws, so that we 
can use if we cannot regulate them. It is ours to take advan- 
tage of this precious time of quietude, and to extend the bless- 
ings of science and of our Christian civilisation from shore to 
shore until there shall be no more sea, not in the sense of that 
final drying-up of old Ocean to which some physicists look for- 
ward, but in the higher sense of its ceasing to be the emblem 
of unrest and disturbance, and the cause of isolation. 
I must now close this address with a short statement of the 
general objects which I have had in view in directing your at- 
tention to the geological development of the Atlantic. We 
cannot, I think, consider the topics to which I have referred 
without perceiving that the history of ocean and continent is an 
example of progressive design, quite as much as that of living 
beings. Nor can we fail to see that, while in some important 
directions we have penetrated the great secret of Nature, in 
reference to the general plan and structure of the earth and its 
waters, and the changes through which they have passed, we 
have still very much to learn, and perhaps quite as much to un- 
learn, and that the future holds out to us and to our successors 
higher, grander, and clearer conceptions than those to which we 
have yet attained. The vastness and the might of Ocean, and the 
manner in which it cherishes the feeble-t and most fragile beings, 
alike speak to us of Him who holds it in the hollow of His 
hand, and gave to it of old its boundaries and its laws; but its 
teaching ascends to a higher tone when we consider its origin 
and history, and the manner in which it has been made to build 
up continents and mountain-chains, and at the same time to 
nourish and sustain the teeming life of sea and land. 
SECTION A 
MATHEMATICAL AND PHYSICAL SCIENCE 
OPENING ADDRESS BY Prof. G. H. Darwin, M.A., LL.D., 
F.R.S., F.R.A.S., PRESIDENT OF THE SECTION 
A MERE catalogue of facts, however well arranged, has never 
led to any important scientific generalisation. For in any sub- 
ject the facts are so numerous and many sided that they only lead 
us to a conclusion when they are marshalled by the light of some 
leading idea. A theory is, then, a necessity for the advance of 
science, and we may regard it as the branch of a living tree, of 
which facts are the nourishment. In the struggle between com- 
peting branches to reach the light some perish, and others form 
vigorous limbs. And as in a tree the shape of the young shoot 
can give us but little idea of the ultimate form of the branch, so 
theories become largely transformed in the course of their exist- 
ence, and afford in their turn the parent stem for others. 
The success of a theory may be measured by the extent to 
which it is capable of assimilating facts, and by the smallness of 
the change which it must undergo in the process. Every theory 
which is based on a true perception of facts is to some extent 
fertile in affording a nucleus for the aggregation of new observa- 
tions. Anda theory, apparently abandoned, has often ultimately 
appeared to contain an element of truth, which receives acknow- 
ledgment by the light of later views. 
It will, I think, be useful to avail myself of the present occa- 
sion to direct your attention to a certain group of theories which 
are still in an undeveloped and somewhat discordant condition, 
but which must form the nucleus round which many observations 
have yet to be collected before these theories and their descend- 
NATURE 
[ Sept. 2, 1886 
ants can make a definitely accepted body of truth. If I am 
disposed to criticise some of them in their actual form, I shall 
not be understood as denying the great service which has been 
rendered to science by their formulation. 
Great as have been the advances of geology during the present 
century, we have no precise knowledge of one of its fundamental 
units. The scale of time on which we must suppose geological 
history to be drawn is important not only for geology itself, but 
it has an intimate relation with some of the profoundest questions 
of biology, physics, and cosmogony. 
We can hardly hope to obtain an accurate measure of time 
from pure geology, for the extent to which the events chronicled 
in strata were contemporaneous is not written in the strata them- 
selves, and there are long intervals of time of which no record 
has been preserved. 
An important step has been taken by Alfred Tylor, Croll, and 
others, towards the determination of the rate of action of geo- 
logical agents (Geikie, ‘‘ Text-book of Geology,” 1882, p. 442). 
From estimates of the amount of sediment carried down by 
rivers, it appears that it takes from 1000 to 6000 years to remove 
one foot of rock from the general surface of a river basin. 
From a consideration of the denuding power of rivers, and a 
measurement of the thickness of stratified rock, Phillips has made 
an estimate of ihe period of time comprised in geological history, 
and finds that, from stratigraphical evidence alone, we may re- 
gard the antiquity of life on the earth as being possibly between 
38 and 96 millions of years (‘‘ Life on the Earth,” Rede Lecture, 
1860, p. 119). 
Now while we should perhaps be wrong to pay much attention to 
these figures, yet at least we gain some insight into the order of 
magnitude of the periods with which we have to deal, and we 
may feel confident that a million years is not an infinitesimal 
fraction of the whole of geological time. 
It is hardly to be hoped, however, that we shall ever attain to 
any very accurate knowledge of the geological time scale from 
this kind of argument. 
But there is another theory which is precise in its estimate, 
and which, if acceptable from other points of view, will furnish 
exactly what is requisite. Mr. Croll claims to prove that great 
changes of climate must be brought about by astronomical events 
of which the dates are known or ascertainable (‘‘ Climate and 
Time”). The perturbation of the planets causes a secular vari- 
ability in the eccentricity of the earth’s orbit, and we are able 
confidently to compute the eccentricity of many thousands of 
years forward and backward from to-day, al hough it appears that, 
in the opinion of Newcom» and Adams, no great reliance can 
be placed on the values deduced from the formule at dates so 
remote as those of which Mr. Croll speaks. According to Mr. 
Croll, when the eccentricity of the earth’s orbit is at its maxi- 
mum, that hemisphere which has its winter in aphelion would 
undergo a glacial period. Now, as the date of great eccentricity 
is ascertainable, this would explain the great Ice Age and give us 
it; date. 
The theory has met with a cordial acceptance on many sides, 
probably to a great extent from the charm of the complete answer 
it affords to one of the great riddles of geology. 
Adequate criticism of Mr. Croll’s views is a matter of great 
difficulty on account of the diversity of causes which are said to 
co-operate in the glaciation. In the case of an effect arising 
from a number of causes, each of which contributes its share, 
it is obvious that if the amount of each cause and of each effect 
is largely conjectural the uncertainty of the total result is by no — 
means to be measured by the uncertainty of each item, but is 
enormously augmented. Without going far into details it may 
be said that these various concurrent causes result in one funda- 
mental proposition with regard to climate, which must be regarded 
as the keystone of the whole argument. That proposition amounts 
to this—that climate is unstable. 
Mr. Croll holds that the various causes of change of climate 
operate zmfer sein such a way as to augment their several effi- 
ciencies. Thus the trade-winds are driven by the difference of 
temperature between the frigid and torrid zones, and if from the 
astronomical cause the northern hemisphere becomes cooler the 
trade-winds onthat hemisphere encroach onthose of the other, and 
the part of the warm oceanic current which formerly flowed into 
the cold north zone, will be diverted into the southern hemisphere. 
Thus the cold of the northern hemisphere is augmented, and this 
in its turn displaces the trade-winds further, and this again acts 
on the ocean currents, and so on ; and this is neither more nor 
less than instability. 
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