which so eminent a geologist, who has seen the full effects of the 'Principles,' holds that work and its author.

Although Dr. Whewell, Mr. Conybeare, and others united in recognizing the importance and merit of the Principles' in a general way, most of the geologists of five-and-thirty years ago also joined with them in declaring that Sir Charles Lyell went too far; that the doctrine of uniformity does not hold good when applied to remote epochs; but that, for instance, the metamorphic rocks were altered by agencies of far greater intensity than any that prevail at the present day. They contended that although the forces which formerly produced changes on the surface of the globe were in bygone times the same in kind as they are now, they were different in degree. Sir Charles Lyell, on the other hand, has persistently maintained that we have no evidence to warrant us in assuming those forces to have possessed greater intensity than at present during any geological period, and that until such evidence is discovered we have no right to attempt to explain past events by reference to causes of greater intensity than now operate at and beneath the surface of the earth. In the first chapter of the 'Principles,' Sir Charles Lyell quotes Hutton for the purpose of showing that geology is not concerned "with questions as to the origin of things," and is entirely distinct from cosmogony and cosmogonic speculations.

The science of Geology is, indeed, like every other science, a knowledge of phenomena and their causes; and no period can therefore be considered geologic that is not represented by rockmasses on some part of the present surface of the globe. No doubt there were pre-Laurentian periods; but at present we know nothing of them, and they cannot yet be considered to come within the scope of geological inquiry. The objection which has frequently been made to the doctrine of uniformity, that it assumes the eternity of the globe, is therefore of no value, for geologists do not attempt to speculate on the causes of phenomena of which they have no kind of knowledge.

It is unnecessary to enter into any argument respecting the doctrine of uniformity, as it is now practically acknowledged on all hands; but we would observe, that an examination of geological literature will show that while the terms "convulsion," "catastrophe," and the like were in common use previous to the year 1830, since that time they have been used with a gradually decreasing frequency; and a careful study of the progress of geological thought will likewise show that this result is almost entirely attributable to the publication of Sir Charles Lyell's 'Principles of Geology.'

It has, however, been assumed by some geologists of the present day, that the only claim on our respect which it could be pretended

that the Principles' possesses is founded on the supposition that the Idea of Uniformity originated with Sir Charles Lyell, and that as this supposition is incorrect, no special degree of merit should be attributed to Sir Charles for that particular work. Our own opinion is diametrically opposed to this, for according to our conception of the case the value of the Principles' lies chiefly in the proof it contains of Hutton's Theory of the Earth' being supported by positive evidence, instead of being a mere unsupported effort of the imagination. Sir Charles Lyell brought together in the 'Principles' a great mass of facts bearing on every phase of the theory, and this not in a mere superficial manner. Before the publication of this work, Hutton's Theory was to the great body of geologists nothing but the dream of an enthusiast; for ever afterwards it became a reality, and the theory of a philosopher.

6

The vexed questions of originality and priority are frequently the stumbling-blocks to a correct estimate of the services of great men. It has often happened that a great discovery has been made by one man, the importance and value of which have not been recognized until made manifest by another. The familiar case of the accidental discovery of Voltaic Electricity by Galvani, and the recognition of its importance by Volta, is an extreme instance; and the one under discussion, though differing from it in some essential respects (especially as regards accident), falls into the same category in others. Hutton, no doubt, was too far in advance of his age for his theory to be accepted by men of his generation, and it was characterized as premature by Dr. Whewell, even in 1837. How much longer it would have remained neglected had not Sir Charles Lyell written the 'Principles,' may to some extent be inferred by a reference to dates. Hutton's theory was first made public in 1788; from that time to the publication of the 'Principles of Geology' in 1830, a period of forty-two years, the progress made in Geological Dynamics is insignificant compared with the advance made during the thirty-six years which have since elapsed. We therefore endorse the opinion expressed by eminent men long ago, that Sir Charles Lyell's work was the beginning of a new era of progress in our science-the commencement, in fact, of a Rational Geology.

After scrutinizing the effects of existing causes of known intensity, as exhibited at the present day, Sir Charles Lyell was naturally led to examine the Tertiary deposits of different parts of Europe; and it was in tracing backwards the more and more complete disappearance of recent forms from Tertiary faunas that he conceived the idea of determining the relative age of these strata by the ratio which the recent species of Mollusca in their respective faunas bore to the extinct. Hence he proposed the now worldrenowned classification of Tertiary deposits into Eocene, Miocene, and Pliocene; and enunciated the scheme for their determination

commonly known as the "percentage test." The Eocene strata were defined as having only about one-thirtieth part of their Mollusca identical with living species, the Miocene as having about one-fifth, the Older Pliocene from one-third to one-half, and the Newer Pliocine nine-tenths.

The terms Eocene, Miocene, and Pliocene were at once adopted, together with the principle of classifying the Tertiary strata by means of the percentage of recent shells which they contain. But of late years several geologists and palæontologists have raised objections to the percentage test, as did Mr. Charlesworth when it was first proposed. It may not be out of place, therefore, to discuss the advantages that have accrued from its adoption during the last thirty years, and the probability of its eventually becoming superseded by some other mode of classification.

Sir Charles Lyell's classification depends on two principles; first, that the Mollusca are typical of the rest of the animal kingdom, and are, at the same time, the most convenient for the purpose; and secondly, that the percentage of recent species in a fauna varies inversely with its age. In the first place, it is certain that the fossils which are most generally and completely preserved belong to the class Mollusca. The shells of these animals are so durable, so easily recognized, and, generally speaking, so characteristic of the species to which they belong, that they form a much better medium of comparison than the remains of land-animals or of plants, which generally occur in a fragmentary condition, and the preservation of which is usually the result of some fortuitous circumstance. Again, other classes of marine organisms are either not so abundant in species and individuals, or they are not likely to be preserved in the fossil state. It is difficult to say whether the Mollusca are, or are not, typical, in their duration, of the rest of the animal kingdom. They are not so prone to change as higher organisms; but, on the other hand, species of shells do not, as a rule, exist through several geological periods, like species of Foraminifera. We should imagine, however, that while the scale furnished by each class of animals is true, each scale has a value of its own, which has a certain ratio to that of each of the rest. The scale furnished by the Mollusca being neither too large nor too small, is on this account preferable to several others; therefore, from every point of view the Mollusca seem more convenient for the purpose than any other group of organisms.

The proposition that the greater the age of a Tertiary fauna the smaller is the proportion of recent species that it contains, is extremely difficult either to prove or to disprove; but even if it can now be shown to be untrue, it was at the time the percentage test was proposed a most convenient fiction. Sir Charles Lyell urged that the recent faunas formed a common point of departure in all

countries, and that in the event of Tertiary deposits being discovered in any region they could be referred, by means of the percentage of recent forms amongst their fossils, to their place in the Tertiary series. For many years this test has been applied with useful, even if but temporary, results; but it must also be admitted that in several cases the application has not been successful. We have Eocene and Miocene deposits in India, for instance; but the determination of the former depends almost entirely upon the fact of their containing a large number of species of Nummulites, and not upon any percentage calculation; while the reference of the latter to the Miocene division is wholly based on its Mammalian fauna. In Australia a very varied series of Tertiary deposits has been known for many years; but even the percentage test has not yet enabled Australian geologists to come to any agreement as to their Eocene, Miocene, or Pliocene date. One amateur geologist, indeed, appears to have been for years in a state of perpetual oscillation between the three.

In a series of papers Mr. Charlesworth stated thirty years ago some of the objections which he then saw to the use of the percentage test; but although he alluded to other sources of error, he more especially dwelt on the disagreement existing between naturalists as to the amount of divergence necessary to constitute a species. To render this nugatory, he suggested an attempt to classify Tertiary strata by means of "the totality of the characters which each series exhibits," on the principle that there is a "uniform approximation to existing species, shown by the fossils of different deposits, corresponding to their respective antiquity."† But it is to be regretted that he did not himself construct the "table of degrees" which he proposed, nor illustrate his suggestion by making the attempt to classify Tertiary strata by means of it. The principle is no doubt correct, and has been used with signal success in the classification of plants into Natural Orders; it is also the one commonly used in classifying the older rocks, and ought not to be difficult of application to the Tertiary. The misfortune is that while many men possess a destructive" faculty in an eminent degree, there are so few who, like Sir Charles Lyell, are gifted with a "constructive" genius. The former class of men do not benefit science, although they show that a scheme which works well is nevertheless faulty; but the latter are entitled to our gratitude for a system which, faulty though it may be, is infinitely better than none.‡

Mag. Nat. Hist.,' vol. ix., p. 537. Phil. Mag., 3rd ser., vol. vii., p. 81; vol. viii., p. 529; vol. x., p. 1.

Phil. Mag.,' 3rd ser., vol. x., p. 8.

"A maxim which it may be useful to recollect is this,-that hypotheses may often be of service to science, when they involve a certain portion of incompleteness, and even of error."—WHEWELL's Philosophy of the Inductive Sciences, vol. ii., p. 223.

After all, changes in physical geography and in climate, in any given region, are the chief causes of uncertainty in the application of the percentage test. The climate of Europe, for instance, was much warmer during the Eocene and Miocene periods than it is now. As the climate became colder no doubt the animals and plants which inhabited Europe migrated to warmer regions. In Europe there are many deposits of the age of these warmer periods, and it does not seem unreasonable to believe that formations in more southern latitudes, containing fossilized members of the same fauna, would be more recent in date than the apparently contemporaneous strata in Europe. Indeed, if some members of a species become modified during such a struggle for existence, as takes place in a country whose climate is becoming unsuitable for its inhabitants, while stronger individuals retain their specific characters; and if the modified form does not survive, as a species, the one from which it descended, it is easy to see that a formation containing a larger proportion of extinct species may be more recent than one containing a smaller proportion, in a different latitude, or possessing in past times a different climate.

When the percentage test was proposed, the scientific world was not ripe for the consideration of matters so calculated to disturb the principles of geological chronology, and therefore Sir Charles Lyell's scheme passed almost unchallenged. That its adoption has been attended with beneficial results is quite certain, and until some better and equally simple scheme is proposed, it will no doubt continue to be the one most generally adopted. But it behoves every philosophical geologist to remember that increase of knowledge has rendered faulty that which at one time appeared to be perfect, "inasmuch as it had the appearance of possessing arithmetical accuracy."

As science advances we are rather apt to forget that what to us are mere elementary, and apparently self-evident truths, were at one time original and great discoveries. So the services of our predecessors are not unfrequently too much underrated, and the truth of the old maxim that "familiarity breeds contempt" is proved in a new way. It seems, therefore, a good thing now and then to consider how large a debt we really do owe to those who have gone before us; often men who with imperfect aids have indicated the clue to some of nature's mysteries, which a more perfect knowledge of natural laws now enables us firmly to grasp. And if it should, as no doubt it frequently does, eventually become manifest that old ideas, interpretations, and theories are erroneous, there is not the less credit due to their authors; for have not their readings of nature for years answered all the requirements of a more perfect interpretation, and materially assisted science thereby? * Quart. Journ. Geol. Soc,' vol. xxii., p. 230.