of the imagination leading us into error, inasmuch as we have a large number of well-ascertained truths, which we can confront with every speculation, no matter how plausible or ingenious it may appear.

On both these grounds, Leslie was, I apprehend, justified in taking the course which he did. At all events, it is certain, that, by following it, he came nearer than would otherwise have been possible, to the conceptions of the most advanced scientific thinkers of our day. He distinctly recognized that, in the material world, there is neither break nor pause; so that what we call the divisions of nature have no existence, except in our minds.164 He was even almost prepared to do away with that imaginary difference between the organic and inorganic world, which still troubles many of our physicists, and prevents them from comprehending the unity and uninterrupted march of affairs. They, with their old notions of inanimate matter, are unable to see that all matter is living, and that what we term death is a mere expression by which we signify a fresh form of life. Towards this conclusion, all our knowledge is now converging; and it is certainly no small merit in Leslie, that he, sixty years ago, when really comprehensive views, embracing the whole creation, were scarcely known among scientific men, should have strongly insisted that all forces are of the same kind, and that we have no right to distinguish between them, as if some were living, and others were dead.165

We owe much to him, by whom such views were advocated. But they were then, and, in a certain,

164 "We should recollect that, in all her productions, Nature exhibits a chain of perpetual gradation, and that the systematic divisions and limitations are entirely artificial, and designed merely to assist the memory and facilitate our conceptions." Leslie on Heat, p. 506.

165

"All forces are radically of the same kind, and the distinction of them into living and dead is not grounded on just principles." Leslie on Heat, p. 133. Compare p. 299: "We shall perhaps find, that this prejudice, like many others, has some semblance of truth; and that even dead or inorganic substances must, in their recondite arrangements, exert such varying energies, and so like sensation itself, as if fully unveiled to our eyes, could not fail to strike us with wonder and surprise."

though far smaller, degree, they are now, so out of the domain of physical experience, that Leslie never could have obtained them by generalizing in the way which the inductive philosophy enjoins. His great work on heat was executed, as well as conceived, on the opposite plan;166 and his prejudices on this point were so strong, that we are assured by his biographer, that he would allow no merit to Bacon, who organized the inductive method into a system, and to whose authority we in England pay a willing, and I had almost said a servile, homage.167

Another curious illustration of the skill with which the Scotch mind, when once possessed of a principle, worked from it deductively, appears in the geological speculations of Hutton, late in the eighteenth century. It is well known, that the two great powers which have altered the condition of our planet, and made it what it is, are fire and water. Each has played so considerable a part, that we can hardly measure their relative importance. Judging, however, from the present appearance of the crust of the earth, there is reason to believe, that the older rocks are chicfly the result of fusion, and that the younger are aqueous deposits. It is, therefore, not unlikely, that, in the order in which the energies of nature have unfolded themselves, fire preceded water, and was its necessary precursor. But, all that we are as yet jus

168

166 Mr. Napier, in his Life of Leslie, p. 17, says of it, very gravely, "It's hypotheses are not warranted by the sober maxims of inductive logic."

167

"Notwithstanding the contrary testimony, explicitly recorded by the founders of the English experimental school, he denied all merit and influence to the immortal delineator of the inductive logic." Napier's Life of Leslie, p. 42.

168 The supposition, that volcanic agencies were formerly more potent than they are now, is by no means inconsistent with the scientific doctrine of uniformity, though it is generally considered to be so. It is one thing to assert the uniformity of natural laws; it is quite another thing to assert the uniformity of natural causes. Heat may once have produced far greater effects than it can do at present, and yet the laws of nature be unchanged, and the order and sequence of events unbroken. What I would venture to suggest to geologists is, that they have not taken sufficiently into account the theory of the interchange of forces, which seems to offer a solution of at least part of the problem. For, by that theory, a large portion of the heat which formerly existed may have been metamorphosed into other forces,

tified in asserting is, that these two causes, the igneous and the aqueous, were in full operation long before man existed, and are still busily working. Perhaps they are preparing another change in our habitation, suitable to new forms of life, as superior to man, as man is superior to the beings who occupied the earth before his time. Be this as it may, fire and water are the two most important and most general principles with which geologists are concerned; and though, on a superficial view, each is extremely destructive, it is certain that they can really destroy nothing, but can only decompose and recompose; shifting the arrangements of nature, but leaving nature herself intact. Whether one of these elements will ever again get the upper hand of its opponent, is a speculation of extreme interest. For, there is reason to suspect, that, at one period, fire was more active than water, and that, at another period, water was more active than fire. That they are engaged in incessant warfare, is a fact with which geologists are perfectly familiar, though, in this, as in many other cases, the poets were the first to discern the truth. To the eye of the geolo

such as light, chemical affinity, and gravitation. The increase of these forces consequent on the diminution of heat, would have facilitated the consolidation of matter; and until such forces possessed a certain energy, water, which afterwards became so prominent, could not have been formed. If the power of chemical affinity, for instance, were much weaker than it is, water would assuredly resolve itself into its component gases. Without wishing to lay too much stress on this speculation, I submit it to the consideration of competent judges, because I am convinced that any hypothesis, not absolutely inconsistent with the known laws of nature, is preferable to that dogma of interference, which what may be called the miraculous school of geologists wish to foist upon us, in utter ignorance of its incompatibility with the conclusions of the most advanced minds in other departments of thought.

The remarks in Sir Roderick Murchison's great work (Siluria, London, 1854, pp. 475, 476) on the "grander intensity of former causation," and on the difficulty this opposes to the "uniformitarians," apply merely to those who take for granted that each force has always been equally powerful: they do not affect those who suppose that it is only the aggregate of force which remains unimpaired. Though the distribution of forces may be altered, their gross amount is not susceptible of change, so far as the highest conceptions of our actual science extend. Consequently, there is no need for us to believe that, in different periods, the intensity of causation varies; though we may believe that some one agent, such as heat, had at one time more energy than it has ever had since.

VOL. II.

L L

gist, water is constantly labouring to reduce all the inequalities of the earth to a single level; while fire, with its volcanic action, is equally busy in restoring those inequalities, by throwing up matter to the surface, and in various ways disturbing the crust of the globe.169 And as the beauty of the material world mainly depends on that irregularity of aspect, without which scenery would have presented no variety of form, and but little variety of colour, we shall, I think, not be guilty of too refined a subtlety, if we say that fire, by saving us from the monotony to which water would have condemned us, has been the remote cause of that development of the imagination which has given us our poetry, our painting, and our sculpture, and has thereby not only wonderfully increased the pleasures of life, but has imparted to the human mind a completeness of function, to which, in the absence of such a stimulus, it could not have attained.

When geologists began to study the laws according to which fire and water had altered the structure of the earth, two different courses were open to them, namely, the inductive and the deductive. The deductive plan was to compute the probable consequences of fire and water, by reasoning from the sciences of thermotics and hydrodynamics; tracking each element by an independent line of argument, and afterwards coördinating into a single scheme the results which had been separately obtained. It would then only remain to inquire, how far this imaginary scheme harmonized with the actual state of things; and if the discrepancy between the ideal and

169 "The great agents of change in the inorganic world may be divided into two principal classes, the aqueous and the igneous. To the aqueous belong rain, rivers, torrents, springs, currents, and tides; to the igneous, volcanos and earthquakes. Both these classes are instruments of decay as well as of reproduction; but they may also be regarded as antagonist forces. For the aqueous agents are incessantly labouring to reduce the inequalities of the earth's surface to a level; while the igneous are equally active in restoring the unevenness of the external crust, partly by heaping up new matter in certain localities, and partly by depressing one portion, and forcing out another, of the earth's envelope." Lyell's Principles of Geology, 9th edit., London, 1853, p. 198.

the actual were not greater than might fairly be expected from the perturbations produced by other causes, the ratiocination would be complete, and geology would, in its inorganic department, become a deductive science. That our knowledge is ripe for such a process, I am far, indeed, from supposing; but this is the path which a deductive mind would take, so far as it was able. On the other hand, an inductive mind, instead of beginning with fire and water, would begin with the effects which fire and water had produced, and would first study these two agents, not in their own separate sciences, but in their united action as exhibited on the crust of the earth. An inquirer of this sort would assume, that the best way of arriving at truth would be to proceed from effects to causes, observing what had actually happened, and rising from the complex results up to a knowledge of the simple agents, by whose power the results had been brought about.

If the reader has followed the train of thought which I have endeavoured to establish in this chapter, and in part of the preceding volume, he will be prepared to expect that when, in the latter half of the eighteenth century, geology was first seriously studied, the inductive plan of proceeding from effects to causes, became the favourite one in England; while the deductive plan of proceeding from causes to effects, was adopted in Scotland and in Germany. And such was really the case. It is generally admitted, that, in England, scientific geology owes its origin to William Smith, whose mind was singularly averse to system, and who, believing that the best way of understanding former causes was to study present effects, occupied himself, between the years 1790 and 1815, in a laborious examination of different strata.170

170 Dr. Whewell, comparing him with his great German contemporary, Werner, says, "In the German, considering him as a geologist, the ideal element predominated." "Of a very different temper and character was William Smith. No literary cultivation of his youth awoke in him the speculative love of symmetry and system; but a singular clearness and precision of the classifying power, which he possessed as a native talent, was exercised and developed by exactly those geological facts among which his