2000 of its volume of carbonic acid gas. That apparently trifling quantity taken away, in an instant the whole surface of the earth would become a desolate waste, without the possibility of vegetable life.

As physical geology advanced, the Coal period was perThe Coal ceived to be the chief epoch in the history of our period. planet. Through a slow decline of temperature, a possibility had gradually been attained, so far as the condition of heat was concerned, for a luxuriant vegetable growth. All that prodigious mass of carbon now found in the earth in the various forms of coal existed as carbonic acid in the atmosphere. The proportion of free oxygen was less than at present by a volume equal to the excess of carbonic acid. A change in the constitution of this primæval atmosphere was occasioned by the action of the Effects of light light; for, under the influence of the sun-rays, on the atmo- plants decompose carbonic acid, appropriating sphere, its carbon, and, for the most part, setting the oxygen free. The quantity of carbon which can thus be condensed for the use of a plant, and, indeed, every such decomposing action by light, is directly proportionate to the quantity of light consumed, as experiments which I have personally made have proved. For the production of so great a weight of combustible matter a very long period of time was necessarily required, that the sun might supply the necessary luminous influence.

Age after age the sunbeams continued their work, changing the mechanical relations and composition of the atmosphere, the constitution of the sea, and the appearance of the surface of the earth. There was a prodigious growth of ferns, lepidodendra, equisetaceæ, coniferæ. The percentage of oxygen in the air continually increased, that of carbonic acid continually declined; the pressure of the air correspondingly diminished, partly because of the replacement of a heavy gas by a lighter one, and partly because of the general decline of temperature slowly taking place, which diminished the absolute volume of vapour. The sea, in its deepest abysses, was likewise affected by the sunlight; not directly, but in an indirect way; for, as the removal of carbonic acid from the atmosphere went on, portions of that gas were per

and also on the sea.

petually surrendered by the ocean in order to maintain a diffusion-equilibrium between its dissolved gas and the free gas of the air. And now no longer could be held in transparent solution by the water those great quantities of carbonate of lime which had once been concealed in it, the deposit of a given weight of coal in the earth being inevitably followed by the deposit of an equivalent weight of carbonate of lime in the sea. This might have taken, place as an amorphous precipitate; but the probabilities were that it would occur, as in fact it did, under forms of organization in the great limestone strata coeval with and posterior to the coal. The air and the ocean were thus suffering an invisible change through the disturbing agency of the sun, and the surface of the solid earth was likewise undergoing a more manifest, and, it may be said, more glorious alteration. Plants, in wild luxuriance, were developing themselves in the hot and dank climate, and the possibility was now approaching for the appearance of animal types very much higher than any that had yet existed. In the old heavy atmosphere, full of a noxious gas, none but slowly-respiring cold-blooded Cold-blooded animals could maintain themselves; but after animals sucthe great change in the constitution of the air ceeded by hot. had been accomplished, the quickly-respiring and hotblooded forms might exist. Hitherto the highest advancement that animal life could reach was in batrachian and lizard-like organisms; yet even these were destined to participate in the change, increasing in magnitude and vital capacity. The pterodactyl of the chalk, a flying lizard, measures nearly seventeen feet from tip to tip of its wings. The air had now become suitable for mammals, both placental and implacental, and for birds. One after another, in their due order, appeared the highest vertebrates: marine, as the cetacean; aerial, as the bat; and in the terrestrial, reaching, in the Eocene, quadrumanous animals, but not, until after the Pliocene, man.

Although the advance of geology may here- The date of after lead to a correction of some of the con- organisms may change, clusions thus attained to respecting the first but the order dates of different organic forms, and carry them not. back to more ancient times, it is scarcely likely that any

material modification of their order of occurrence will ever be made. Birds, mammals, reptiles, fishes, and invertebrates may each be detected in earlier strata; even in some of those formations now regarded as non-fossiliferous, organisms may be found; but it is not at all probable that the preponderance of reptiles will ever cease to be the essential characteristic of the Secondary rocks, or that of mammals of the Tertiary, or that a preceding period of vast duration, in which the type of life had been the invertebrate, will ever be doubted. Nothing, probably, will ever be discovered to invalidate the physical conclusion that, while there was an excess of carbonic acid in the air, the Flora would tend to be Cryptogamic and Gymnospermic, and that there would be a scarcity of monocotyledons and dicotyledonous angiosperms in the coal; nothing to disprove the fact that the animals were slow-breathing and cold-blooded; and that it was not until after the oxygen of the air had increased and the mean temperature had declined that birds made their appearance. Though both placental and marsupial animals may hereafter be found earlier than in the Stonesfield slate; though wood and herb-eating beetles, grasshoppers, dragon-flies, and May-flies may be found beneath the lias, and scorpions and cockroaches beneath the coal, though, also beneath the coal, salamanders and Sauroid batrachians, of which the archegosaurus is an example, may occur; though reptiles, as the telerpeton, may be found deeper than the old red sandstone; yet the connexion, between aerial constitution and form of life will never be shaken. Still will remain the facts that the geographical distribution of types was anterior to the appearance of existing species; that organisms first appeared in a liquid medium, primitively marine, then fluviatile, and at last terrestrial; that Radiates, Molluscs, Articulates, Vertebrates, were all at first aquatic, and that the Radiates have ever remained so; that the plane of greatest vital activity has ever been the sea-level, where the earth and air touch each other; that the order of individual development is the order of mundane development. Still will remain the important conclusions that the mammalian Fauna has diverged more rapidly than the testaceous; that hot-blooded animals have not had that longevity of species

which has been displayed by the cold, just as we observe in the individual the possibility of muscular contraction by a given galvanic force lasts much longer in the latter than in the former; that if the hot-blooded tribes have thus a briefer duration, they enjoy a compensation in the greater energy of their life-perhaps this being the cause and that the effect; that, notwithstanding the countless forms exhibited by species, their duration is so great that they outlive vast changes in the topographical configuration of countries-the Fauna of some countries having been in existence before those countries themselves; that the plan of individual development has ever been as it is now, and that sameness of external influence produces similarity of organization.

formity.

In its early history theoretical geology presented two schools--one insisting on a doctrine of catas- The doctrine trophes, one on a doctrine of uniformity. The of catastroformer regarded those changes which have mani- phes and unifestly taken place in the history of our planet as having occurred at epochs abruptly. To this doctrine the prevailing impression that there had been providential interventions lent much force. The other school, reposing on the great principle of the invariability of the laws of Nature, insisted that affairs had always gone on at the same rate and in the same way as they do now. Hence it maintained an opposition to the catastrophists, and in this, it may be said, was actually not true to its own principles. Any doctrine of uniformity, rightly considered from its most general point of view, includes an admission of catastrophes. Numerous illustrations of this truth spontaneously suggest themselves. A tower, the foundations of which are slowly yielding, may incline more and more for many centuries, but the day must come in which it will fall at last. In the uniformity of the disturbance a catastrophe was eventually involved. And thus, in what has been said respecting geological events, though they are spoken of as proceeding quietly and with uniformity, it may be understood that sudden crises are also contemplated. Moreover, those who adopt the doctrine of uniformity in an absolute sense must pay a due regard to the variations in intensity of physical acts which their

own principles imply. The uniform cooling of a hot body actually means a cooling at first fast, and then slower and slower ; and invariability of chemical change actually implies more violent and summary modifications at a high temperature than at one which is low.

But, though it may at first sight have appeared that an admission of the doctrine of catastrophes is in harmony with a providential government of the world, and that the emergence of different organic forms in successive ages is a manifestation of creative intervention, of which it was admitted that as many as from twelve to twenty, if no more, successive instances might be recognized, we may well congratulate ourselves that those important doctrines rest upon a far more substantial basis. Rightly considered, the facts lead to a very different conclusion. Physiological investigations have proved that all animals, even man,

Successive forms assumed by

man.

in

during the process of development, pass succession through a definite cycle of forms. Starting from a simple cell, form after form, in a definite order is assumed. In this long line of advance the steps are ever, in all individuals, the same. But no one would surely suppose that the changed aspect at any moment presented is due to a providential interposition. On the contrary, it is the inevitable rigidly deter- result of what has been taking place under the mined by law. law of development, and the sure precursor of what is about to follow. In the organic world, the successive orders, and genera, and species are the counterparts of these temporary embryonic forms of the individual. Indeed, we may say of those successive geological beings that they are mere embryos of the latest-embryos that had gained a power of reproduction. How shall we separate the history of the individual from the history of the whole? Do not the fortunes and way of progress of the one follow the fortunes and way of progress of the other? If, in a transitory manner, these forms are assumed by the individual, equally in a transitory manner are they assumed by the race. Nor would it be philosophical to suppose that the management in the one instance differs from the management in the other. If the one is demonstrably the issue of a law in action, so must