who knows that he can recognise strata in remote countries by groups of plants, shells, and animals, is not likely to be disturbed by this epigram, or by the many similar sententious sayings which accompany it. The association of the members of such groups, given partly as a constant fact of observation, partly as a circumstance traceable to the condition of the earth at the time of their existence, is so far from being an identical or trivial proposition, that it is one of the most startling and weighty, as it is one of the most certain and universal, of geological data. But if we are tempted to smile when we are told that such facts are insufficient for the identification of strata, the smile becomes quite irrepressible when the belief which is thus denied to a most complex and peculiar combination of evidence, is claimed for the single circumstance of the rocks containing beds of muriate of soda or sulphate of lime.

We venture, therefore, to retain our belief that we shall best discharge the duty of exhibiting the most recent advances towards a knowledge of the earth's past history, by following those who have endeavoured to trace it by the aid of organic fossils. But before we proceed to give an account of this train of speculations, we must notice the course of discovery which has led the geologist to subjects, as we have already observed, apparently so foreign to his original aim.

. It being ascertained that strata can be identified over a wide extent of country, by means of their materials and contents, two very remarkable general facts are found to offer themselves in the phenomena of these masses. In the first place, the strata are in a great variety of positions with regard to the earth's surface, and to each other: some are highly inclined, some horizontal; some mutually parallel, others placed upon the edges or against the slopes of the subjacent beds; some continuous and of uniform inclination, others contorted and disturbed, broken and separated. The arrangement of the beds irresistibly suggests the belief that each was deposited at first horizontally, and that then, by the action of mechanical violence, the masses were variously shattered and disturbed. In the next place, the species of organised beings which are contained in each formation, or main division of beds, are, for the most part, different. We trace a succession of several conditions of the animal and vegetable world which had little or nothing in common. Each of many periods appears to have had its own Flora and Fauna, and none of these seem to have included the animals and plants with which we are now surrounded. A geological theory should obviously include these two capital classes of facts. That such a theory is at present attainable, may, we think, well be doubted. But though we should not be so sanguine as to

look

look to known causes for an explanation of such appearances, or to define the mode in which the unknown have acted, it may still be interesting and instructive to follow out the most promising of the analogies which present themselves.

The differences of position and the mutations of organic forms which have taken place in the pre-existing earth, appear, at first sight, to have but a dim and remote resemblance to anything which is at present occurring. There seems to be little chance of identifying what is now going on with an era when the Andes were raised from the bottom of the ocean, or with the state of our earth when that flying dragon, the pterodactyl, succeeded the trilobite.) But the theorist is not so easily daunted. In matters of change, as all know, a beginning is everything. If he can once shake the stability of the existing order, it is difficult to say what revolutions he may not produce. The adventure is, at least, worth a trial.

Now, it appears that, in the present order of things, certain changes do go on, both in the position of portions of the earth, and in the forms of certain organised beings. The volcano and the earthquake are seldom long idle; the ocean is an unremitting assailant of the solid earth; the countless host of streams and showers second his attack, on the other side, by efforts formidable from their multitude and perseverance. The coast yields; the crest of the mountain descends; large tracts of the earth tremble and change their elevation: the volcanic island lifts its head above the waves. Here, at least, are some elements of mutations in the form of the earth and of the bed of the sea. The more we examine such causes, the more constant, the more extensive, the more powerful, does their operation appear. In the course of progressive ages, what effects may they not produce? And what limit are we to place to the time during which their work has proceeded? We know that this past period must be long; we know not how long. Who shall prove to us that the forces which we ourselves witness are too weak, or unfit, to produce all the facts of position which the earth's crust exhibits? Such is the reasoning of the advocate of the geological adequacy of the existing dynamical laws of the world.

But changes also take place in the organic creation by cultivation and domestication, by climate and food, by mixture of and perpetuation of peculiarities, plants, brutes, and man undergo extensive changes. The various breeds of domestic animals, the results of chance or care, are monuments

races

We restrict the word dynamical here to its usual scientific sense,-that which relates to forces producing motion.

of

of this mutability of the forms of species. Man, when he has long inhabited a strange land, acquires new characteristics, which enable the most casual observer to recognise his adopted country. Nature provides, by a new covering, for the warmth of animals in cold climates. Fruits and flowers accept innumerable modifications from the hand of the gardener. Where does this capacity of change terminate? Is it impossible that it may reach so far as to transmute the organised forms of one geological period into those of another? Here, as in the former case, we shall not lack time. We have no occasion to embarrass ourselves for want of thousands, or, if it be necessary, of millions of years. With this liberty, need we despair of passing from one set of shells, of reptiles, of mammiferous quadrupeds, to another? May we not thus, through natural causes, obtain a transition from the plesiosaur of the lias to the crocodile of the Nile; from the mastodon of the Paris basin to the elephant of Asia? Ought we not at least to try the possibility of thus identifying the demonstrated changes of the past with the known mutability of the present? And such an identity is maintained by the assertor of the geological adequacy of the existing organic laws of the world.

Such are the two very remarkable questions which at present offer themselves as the prominent points for the attention of the geological theorist; and, in the discussion of these two doctrines, Mr. Lyell's work is mainly employed. We do not intend, by any means, to place upon the same footing, with regard to their philosophical character or their evidence, the two theories which we have noticed. It is very conceivable that the first may be true, though the second is false; that the dynamical processes which form part of the present course of the world, may have produced all the effects of that kind which the state of the earth exhibits, while the transmutation of species into other species, may turn out, on examination, to be a visionary and unauthorised speculation. That this is the state of the case, is Mr. Lyell's opinion. His first volume contained a very masterly exposition of the present mode of action and the intensity of the moving forces of the earth, with a defence of the sufficiency of these to explain all the geological phenomena which belong to that part of the subject. The present volume is occupied with various discussions on the laws and limits of the variability of organisation, and is an estimate of the nature and amount of the alterations which causes, belonging to the animal and vegetable world, are now producing. The author's conclusion is, that the changes of this kind at present going on, are highly important towards the explanation of many of the facts of geology: but that the appearance of new species, at successive

epochs,

epochs, which we learn from irresistible geological evidence to have repeatedly occurred, is a fact not belonging to the operation of that tendency to change in organised beings, which we see still brought into play.

We consider this to be a very important point of doctrine. The opposite opinion, indeed, is perhaps not likely to make many converts in this country, yet it has been embraced, we believe, by no small number of continental geologists; and is one of those conjectures easily suggested to the spirit of wide and venturous speculation which these studies almost irresistibly call into action. The question itself, too, and the evidence, are eminently curious, physiologically considered. We shall, therefore, follow Mr. Lyell a little while in the discussion of it. The book abounds with remarkable facts showing the modifications which various influences and conditions can effect among animals and plants. Thus

'Some of our countrymen, engaged of late in conducting the principal mining association in Mexico, carried out with them some English greyhounds of the best breed, to hunt the hares which abound in that country. The great platform which is the scene of sport is at an elevation of about nine thousand feet above the level of the sea, and the mercury in the barometer stands habitually at the height of about nineteen inches. It was found that the greyhounds could not support the fatigues of a long chase in this attenuated atmosphere, and before they could come up with their prey, they lay down gasping for breath; but these same animals have produced whelps which have grown up, and are not in the least degree incommoded by the want of density in the air, but run down the hares with as much ease as the fleetest of their race in this country.'-p. 40.

Into the questions concerning mule animals and plants, considered in chap. iv., interesting as they are, we have not space to enter. The general facts of such cases are sufficiently familiar to enable the reader to follow us in judging whether the existing laws of life can have led to such changes of the species inhabiting the globe, as those with which geology presents us.

We may begin by observing that it belongs to a very loose and headlong style of speculation to maintain, that because existing laws may lead to some change, they may lead to any change. Because the course of domestication and breeding may possibly extract, in the progress of generations, a lap-dog from a mastiff,may reduce the legs of a sheep to the minutest size,-and alter the outline of a pig or a bull,-it is quite gratuitous to assert that such causes can, therefore, turn a bull into a buffalo, or a pig into an elephant. But where, says the theorist, do you place your limit of possible alteration? We answer, that our not knowing the precise place of the boundary line, affords absolutely no presumption that there is no boundary. What is the limit of a

man's

man's stature ?-of his life?—of the change that one generation may receive by the influence of circumstances? If the theorist will tell us this, we may then join him in calculating what change a thousand or a million generations may produce. But this calculation will not be rightly performed by repeating the aimount of the first change for a given number of times, any more than a similar process will enable us to deduce a man's growth in fifty years from a child's in one year. And till we have it shown that the deviations of species are, from their nature, unlimited, we have at least the force of analogy in our favour, when we suppose that as the strength, and size, and powers of each species are confined within limits narrow in themselves though indefinite to us, so also is the capacity of such deviation, that certain modifications may be produced by external causes, but that there is an invisible line which the waves of these fluctuations cannot pass, that the mutability of the species is finite, as all its other properties are finite,—and that, though we may alter some of the attributes of an animal, as the size and proportion of its parts, by suitable agency, there are other changes, less considerable perhaps to the eye, but more attended to by nature, which neither man, nor the elements, nor time itself can ever bring about.

This, we say, is what we should expect from analogy: how completely this view is confirmed by an appeal to facts, we are now enabled most satisfactorily to show. Yet the gainsayers of the fixity of species have begun by going directly in opposition to this philosophical presumption. They have reasoned as if change, once shown to exist in the constitution of organic beings, could have neither limit nor selection ;-could not possibly be affected by the remoteness of the succeeding steps from the starting point of the change, or by the clashing of the conditions which this progression may bring into play. And in this way, all metamorphoses having become to them equally probable, they have given us a history of the gradations by which nature has ascended from the lowest step of organic life to the production of man, which it is not easy to repeat with a grave face, but of which we will say a word shortly.

It will, however, be more instructive to proceed to something more definite. The assertion of the Transmutationists which is most deserving of consideration is, that by the modifying influence of external circumstances, or of mixed generation, new species may be produced, capable of permanently existing and supporting themselves in a state of nature. This is a proposition susceptible of being tried and decided by an appeal to facts. Here, at least, the advocates of gradual change have an application of their doctrine, where the period which the process requires is not so vast,

nor