the earth from the surface to the centre.1 "All that lies under this," says Humboldt, " is even as much unknown to us as is the interior of other planets belonging to our system. . . . Where all knowledge of the chemical and mineralogical natural constitution of the interior of the earth fails us," he adds, "we are again thrown upon conjecture, just as we are with reference to the farthest bodies which revolve round the sun." "Who shall guarantee us," he says in another place, "that the entire number of the vital forces efficient in the universe has been fathomed?" 2

The book of nature then is still in great measure closed to man, and although many leaves now lie open before us, which were unread half a century ago, no one will deny that there is still much which we cannot understand; and that although the progress of inquiry may give us data which are now unknown to us, much will probably always remain unfathomed by us on earth; natural science in this age cannot even claim to know its own province thoroughly, and an absolute completeness of observation must, humanly speaking, remain for ever an unattainable ideal. "The sciences of experiment," says Humboldt, "are never complete; the realm of the impressions of sense is not to be exhausted; no generation of men will ever have it in their power to boast that they have surveyed the whole of the world of phenomena." 3

But the branch of natural science with which we shall be principally concerned, geology, does not con

1 Elements of Geology, i. 2.

2 Kosmos, i. 166, 167, 31; Eng. tr. 170, 33.

3 Kosmos, i. 65; Eng. tr. 67.

fine itself to the scientific examination of the earth in

its present condition. It endeavours by help of the knowledge of its present condition, and of the forces and laws of nature now at work, to ascertain what were its earlier stages, and to discover what changes have taken place since the beginning. Science has undoubtedly made surprising advances in this direction also in this century. The theories which were formerly propounded in the name of geology, or geogony, were in many cases only arbitrary speculations and pictures of the imagination, which could have no claim to scientific truth, because it was unhesitatingly assumed that forces and laws had been at work of whose existence there is no proof. But this has now been entirely given up. Burmeister says, "We must explain the signs of past change entirely from the condition in which we find the earth at the present time. For all scientific experience proves that the same forces are still at work in the earth which have developed and altered its surface since it first existed in space as a distinct body. A close study of its present condition must therefore be the foundation of all knowledge of its history, and armed with the results of these inquiries, we may endeavour to explain and represent the earlier periods." 1

"1

Now if, as we have seen, our information concerning the present condition of the earth is so imperfect, it is impossible that geology, whose knowledge is entirely based upon this information, should fulfil its task. But further, in geology the question is not what are the facts, but what are the conclusions from these facts.

1 Geschichte der Schöpfung, p. 2.

"Those statements," continues Burmeister, "which we call hypotheses, must always play a great part in our history of creation: the further the time we are considering is from the present, and the less an event can be investigated and understood by the help of present facts, the more we must have recourse to hypotheses.

Now no doubt many scientific hypotheses are based on facts which are so completely proved, and on conclusions which are so undeniable, that their probability borders on certainty,' and they may therefore be the foundation of a scientific theory. But, on the other hand, there are many points in which such probability has up to this time not been attained, and in the course of our inquiries we shall find more than one case in which a hypothesis has been universally acknowledged as scientifically certain, and yet has been proved later to be erroneous. Finally, from the differences of opinion which prevail among the most eminent savants, we may infer that on many points of geology there are no certain conclusions.

"The most confident men of science," says Deutinger with truth, "will not deny that in natural science error is even now in many cases not only possible, but up to a certain point even inevitable."" Really thorough inquirers are very modest in their estimate of what geology and natural science can declare to be perfectly assured results, and they are very strict in their criticism of hypotheses about the earlier periods of the earth's history. After Huxley has enumerated

those parts of the earth which have been thoroughly examined by geologists, as I have quoted above, he goes on to say, "Therefore it is with justice that the most thoughtful of those who are concerned in these inquiries insist continually upon the imperfection of the geological record; for I repeat it is absolutely necessary from the nature of things that that record should be of the most fragmentary and imperfect character. Unfortunately this circumstance has been constantly forgotten. Men of science, like young colts in a fresh pasture, are apt to be exhilarated on being turned into a new field of inquiry, and to go off at a hand gallop in total disregard of hedges or ditches, losing sight of the real limitation of their inquiries, and to forget the extreme imperfection of what is really known. Geologists have imagined that they could tell us what was going on at all parts of the earth's surface during a given epoch: they have constructed a universal history of the globe, as full of wonders and portents as any other story of antiquity."1

"True geognosy," says Humboldt, "describes the exterior crust of our globe as it exists at present. This science has no less certainty than the physical descriptive science in general; on the other hand, whatever relates to the ancient state of our planet. . is as uncertain as the formation of the atmosphere of the planets; yet the time is still not very remote when geologists were occupied from choice in the solution of problems whose solution is almost impossible, and with this fabulous period of the physical history 'Huxley, Op. cit. p. 38.

of the globe."1 J. Bischof says, "Geology in its essential parts will always remain hypothetical." 2

A French geologist, A. Brongniart, concludes a book on the mountain ranges of the earth with these words, "If there are any who claim to possess sufficient knowledge of geological phenomena, and a spirit bold and penetrating enough to be able to deduce the mode of the earth's creation from the few materials we possess, we will willingly resign to them this noble enterprise; we feel that we have neither means nor power sufficient for so bold but perhaps so transitory a construction." " The English savant Whewell expresses himself in a like manner, "We have accumulated a vast store of facts of observation, and have laboured with intense curiosity, but hitherto with very imperfect success, to extract from these facts a clear and connected knowledge of the history of the earth's change.

"4

And to return to Germany, Quenstedt says, "It is true that the natural sciences may boast that they know with certainty some few superficial facts, but even this knowledge is only attained through a system of errors. For if one generation announces as an undoubted fact what by the previous generation was declared to be superstition, the ordinary observer cannot fail to be persuaded that it is all a question of human convictions, which appear under different aspects, as soon as the further progress of science has opened up new

1 Essai geognostique sur le gisement des roches, p. 5.

2 Lehrb. der chem. u. phys. Geol. (1st ed.) i. 2.

3 Tableau des terrains qui composent l'écorce du globe. Alex. Brongniart. Paris 1829. 8.

J. Trimmer, Practical Geology and Mineralogy, p. 478. 5 Sonst und Zetzt, p. 281.