420 
established and reduced to laws—laws, indeed, of phenomena at 
present, but gradually acquiring the character of laws of causa- 
tion. 
Among the important discoveries by which our knowledge of 
the earth’s structure and history has been greatly enlarged 
within forty years, place must be given to the results of the 
labours of Sedgwick and Murchison, who established the 
Cambro-Silurian systems, and thus penetrated into ancient time- 
relics very far toward the shadowy limit of palzeontological 
research. Stimulated by this success, the early strata of the 
globe have been explored with unremitting industry in every 
corner of the earth ; and thus the classification and the nomencla- 
ture which were suggested in Wales and Cumberland are found 
to be applicable in Russia and India, America and Australia, so 
as to serve as a basis for the general scale of geological time, 
founded on organic remains of the successive ages. 
This great principle, the gift of William Smith, is also em- 
ployed with success ina fuller study of the deposits which stand 
among the latest in our history and inyolve a vast variety of 
phenomena, touching a long succession of life on the land, 
changes of depth in the sea, and alterations of climate. Among 
these evidences of physical revolution, which, if modern as 
geological events, are very ancient if estimated in centuries, the 
earliest monuments of men find place—not buildings, not in- 
habited caves or dwellings in dry earth-pits, not pottery or 
fabricated metal, but mere stones shaped in rude fashion to 
constitute apparently the one tool and one weapon with which, 
according to Prestwich, and Evans, and Lubbock, the poor 
inhabitant of northern climes had to’sustain and defend his life. 
Nothing in my day has had such a decided influence on the 
public mind in favour of geological research, nothing has so 
clearly brought out the purpose and scope of our science, as 
these two great lines of inquiry, one directed to the beginning, 
the other to the end of the accessible scale of earthly time ; for 
thus has it been made clear that our purpose can be nothing less 
than to discover the history of the land, sea, and air, and the 
long sequence of life, and to marshal the results in a settled 
chronology—not, indeed, a scale of years to be measured by the 
rotations or revolutions of planets, but a series of ages slowly 
succeeding one another through an immensity of time. 
There is no question of the truth of this history. The facts 
observed are found in variable combinations from time to time, 
and the interpretations of these facts are modified in different 
directions ; but the facts are all natural phenomena and the inter- 
pretations are all derived from real laws of these phenomena—some 
certified by mathematical and mechanical research, others based 
on chemical discovery, others due to the scalpel of the anatomist, 
or the microscopic scrutiny of the botanist. The grandest of 
early geological phenomena have their representatives, however 
feeble, in the changes which are now happening around us ; the 
forms of ancient life most surprising by their magnitude or singular 
adaptations can be explained by analogous though often rare and 
abnormal productions of to-day. Biology is the contemporary 
index of Paleontology, just as the events of the nineteenth 
century furnish explanations of the course of human history in 
the older times... . : 
During the long course of geological time the climates of the 
earth have changed. In many regions evidence of such change 
is furnished by the forms of contemporary life. Warm climates 
have had their influence on the land, and favoured the growth 
of abundant vegetations as far north as within the arctic circle ; 
the sea has nourished reef-making corals in northern Europe 
during Palzeozoic and Mesozoic ages ; crocodiles and turtles were 
swimming round the coasts of Britain, among islands clothed 
with Zamize and haunted by marsupial quadrupeds. Tow have 
we lost this primzeval warmth? Does the earth contribute less 
heat from its interior stores? does the atmosphere obstruct more 
of the solar rays or permit more free radiation from the land and 
sea? has the sun lost through immensity of time a sensible portion - 
of its beneficent influence? or, finally, is it only a question of the 
elevation of mountains, the oceanic currents, and the distribu- 
tion of land and sea? 
The problems thus suggested are not of easy solution, though 
in each branch of the subject some real progress is made. The 
globe is slowly changing its dimensions by cooling ; thus in- 
equalities and movements of magnitude have arisen and are still 
in progress on its surface: the effect of internal pressure, when 
not resulting in mass-movement, is expressed in the molecular 
action of heat which Mallet applies to the theory of volcanoes. | 
The sun has no recuperative auxiliary known to Thomson for 
NATURE 
replacing his decaying radiation ; the earth, under his influence, 
as was shown by Herschel and Adhemar, is subject to periods © 
of greater and less warmth, alternately in the two hemispheres — 
and generally over the whole surface; and finally, as Hopkins 
has shown, by change of local physical conditions the climate of 
northern zones might be greatly cooled in some regions and — 
greatly warmed in others. 5 
One is almost frozen to silence fn presence of the vast sheets 
of ice which some of my friends (followers of Agassiz) believe 
themselves to have traced over the mountains and vales of a great 
part of the United Kingdom, as well as over the kindred regions 
of Scandinavia. One shudders at the thought of the innumerable 
icebergs with their loads of rock, which floated in the once 
deeper North Sea, and above the hills of the three Ridings of 
Yorkshire, and lifted countless blocks of Silurian stone from 
lower levels, to rest on the precipitous limestones round the 
sources of the Ribble. bog 
Those who, with Professor Ramsay, adopt the glacial hypo- 
thesis in its full extent, and are familiar with the descent of ice 
in Alpine valleys where it grinds and polishes the hardest rocks, 
and winds like a slow river round projecting cliffs, are easily 
conducted to the further thought that such valleys have been 
excavated by such ice-rubbers, and that even great lakes on the 
course of the rivers have been dug out by ancient glaciers which 
once extended far beyond their actual limits. That they did so 
extend is in several instances well ascertained and proved ; that 
they did in the manner suggested plough out the valleys and 
lakes is a proposition which cannot be accepted until we possess 
more knowledge than has yet been attained regarding the resistance 
offered by ice to a crushing force, its tensile strength, the measure 
of its resistance to shearing, and other data required for a just 
estimate of the problem, At present it would appear that under 
a column of its own substance 1000 ft. high, ice would not-retain — 
its solidity ; if so, it could not propagate a greater pressure in — 
any direction, This question of the excavating effect of glaciers 
is distinctly a mechanical problem, requiring a knowledge of 
certain data ; and till these are supplied, calculations and con- 
jectures are equally vain. 
A distinguishing feature of modern geology is the greater 
development of the doctrine that the earth contains in its burial- 
vaults, in chronological order, forms of life characteristic of the 
several successive periods when stratified rocks were deposited in 
the sea. This idea has been so thoroughly worked upon in all 
countries, that we are warranted to believe in something like one 
universal order of appearance in time, not only of large groups 
but even of many genera and species. ‘The Trilobitic ages, the 
Ammionitic, Megalosaurian, and Paleotherian periods are 
familiar to every geologist. What closed the career of the 
several races of plants and animals on the land and in the sea, is _ 
a question easily answered for particular parts of the earth’s 
surface by reference to “physical change ;” for this is a main 
cause of the presence or absence, and in general of the unequal 
distribution of life. But what brought the succession of different 
races in something like a constant order, not in one tract only, 
but one may say generally in oceanic areas, over a large portion 
of the globe? ° 
Life unfolds itself in every living thing, from an obscure, often” 
undistinguishable cell germ, in which resides a potential of both 
physical and organic change—a change which, whether continual 
or interrupted, gradual or critical, culminates in the production — 
of similar germs, capable under favourable conditions of assum= 
ing the energy of life. a 
How true to their prototypes are all the forms with which we 
are familiar, how correctly they follow the family pattern for 
centuries, and even thousands of years, is known to all students” 
of ancient art and explorers of ancient catacombs. But much — 
more than this is known. Very small differences separate the 
elephant of India from the mammoth of Yorkshire, the Wald- 
Aeimia of the Australian shore from the Zerebratula of the 
Cotswold oolite, the dragon-fly of our rivers from the Zibellula 
of the Lias, and even the RAynchonelle and Lingule of the - 
modern sea from the old species which swarm in the Palzeozoic © 
rocks. ; 
But concurrently with this apparent perpetuity of similar forms — 
and ways of life, another general idea comes into notice. No 
two plants are more than alike ; no two men have more than the 
family resemblance ; the offspring is not in all respects an exact 
copy of the parent. A general reference to some earlier type, 
accompanied by special diversity in every case (‘‘descent with — 
modification”), is recognised in the case of every living being. 
