saurians which began to appear in the new red sandstone, have now multiplied, and play on the shores of the oolitic land; insects inhabit the luxuriant forests which cover that land; and that most extraordinary of all created beings, the pterodactylus, or flying lizard, executes the functions for which it was designed. The sea has acquired new inhabitants: not only monstrous reptiles, but new forms of fishes-zoophyta, mollusca, and articulosa; but still we find neither birds nor mammalia. The plants of this series afford evidence sufficient of a tropical climate, but the saurian animals furnish proof yet more conclusive. You may remember the cause of animal heat, as I stated it to you in a former lecture. It is a combustion of certain unazotised ingredients of food by means of the oxygen of the air; and as a product of the combustion, carbonic acid and water are formed. The heat occasioned by the conversion of the carbon and hydrogen into carbonic acid and water in the interior of the body, must be as great as if the elements were burned in the open air

We mentioned that it was quite possible that marine animals may have existed when terrestrial animals could not. Of course the grand object of respiration is the same in both classes of animals, viz. transformation of the food, and of particular constituents of the blood, by means of the oxygen of the air. Hence aquatic respiration differs from aerial only in that the water becomes the medium of conveying the air to the respiratory organs. In the lower classes of marine animals, the respiration is entirely cutaneous, the air not being supplied through distinct channels, but by a transudation through membranes permeable to it. As we go higher in the scale, we find a bronchial respiration, or respiration by means of lungs. In these the water holding oxygen in solution meets with a net work of veins, and aerates the blood circulating within them. The cause of animal heat being a combustion of carbon and hydrogen by means of oxygen, it is obvious that in the cold-blooded animals, the quantity of oxygen required will be much less than in the warm-blooded. Accordingly we find this to be the case. A tench lives for some time in water containing only 5000 its bulk of oxygen, whilst river water generally contains from to 1 per cent. of this gas. Unfortunately I am not aware of any experiments which show how much carbonic acid may be in water without being detrimental to the life of marine animals; but certain it is, that the luxuriant vegetations which analogy leads us to believe must have existed in the sea at this period, would extract carbonic acid from, and furnish oxygen to the surrounding water.

The amphibians require very little oxygen for the support of their vital functions. Frogs will live for four or six hours in an atmosphere of pure hydrogen or nitrogen, which does not contain a particle of oxygen. And although the amphibians require less oxygen than the terrestrial saurians, still we find that the economy of the latter requires much less oxygen than that of higher animals. Their lungs are,

therefore, comparatively imperfect, and the two systems of circulation incomplete; for their arteries circulate a mixture of venous and arterial blood. I do not know if there are any experiments on record, of the powers of any particular saurian to live in an atmosphere deficient in oxygen and surcharged with carbonic acid, such as we suppose existed during "the age of reptiles." I hoped to have been able to show you some experiments of their power to do so; but from their general torpidity at present I have not been able to meet with any lizards to secure for this purpose. In a few weeks, however, they will be coming out of their holes, and I shall introduce them into an atmosphere of the olden times, such as their progenitors revelled in when they were masters of the world, and multiplied themselves to such an amazing extent.

All reptiles are distinguished by the small amount of food which they consume, and by their tenacity of life under very trying circumstances. The food which they do take is not of a nature to be transformed into carbonic acid; nor are their organs of respiration suited for this transformation, even it were. Hence it is, that they do not require much oxygen for the support of their respiratory functions, and that they expire so little carbonic acid. Hence it is also, that they depend upon the warmth of the air to keep up the temperature of their bodies, having no means of generating heat within. Considering then these facts, and the supposed nature of the atmosphere during this period, we discover a sufficient cause why this should have been (as Mr. Mantell aptly denominates it) "an age of reptiles ;"-they did not require much oxygen, which the air could not have afforded. Their respiratory functions were not retarded by an excess of carbonic acid, which would have proved fatal to animals of a higher organization; nor did they come in collision with the plans of the intelligent Creator to remove from the atmosphere by means of organic life the excess of carbonic acid.

From these facts we see that reptiles could have had no difficulty in living in an atmosphere containing less oxygen than at present; but we find that in the oolitic period, a particular kind of quadruped existed. This seems to have been a marsupial animal allied to the didelphys. It was evidently an insectivorous animal, from the conformation of its jaw bones found in the Stonesfields oolitic beds, where the elytra of land beetles are found accompanying them. Now can we find nothing in the respiratory system of the marsupials which would lead us to believe that they might have been in an atmosphere such as we have supposed to exist? The marsupials of course breathe, like other mammals, like man himself, by the lungs alone; and if any peculiarity of the respiratory system existed in the primeval didelphys, we could scarcely expect to find anything but mere traces of it in its modern congeners, changed as they must have been to suit the varied conditions of the atmosphere; but let us try to discover whether such traces may not have been preserved. Now in saurians, chelonians, and fishes, two canals are observed

to issue one on each side of the anus into the peritoneum, that is to the external surfaces of the viscera. Their use seems to be to carry on a partial aquatic respiration, or, in other words, to supply aerated water to the blood circulating in particular vessels. It would also appear that more highly oxygenated blood is required for those vessels which supply the brain. This adjunctive respiratory system is supposed to subserve this purpose. Its presence evidently indicates a want of oxygenation of the blood. Now it is very remarkable that traces of these canals exist in the marsupials. Mere traces, however, of any structure, do not subserve functions; they may be considered in two lights, either as remains of what have been, or as general indications of some of the phases through which all the parts of organized bodies pass during developement. Before drawing conclusions regarding the full developement of these traces in former types, we must also remember, that no class, order, family, or genus of animals or plants, ever did or ever can pass, beyond the bounds of the type after which they are formed. Again, traces do not necessarily indicate that a full developement once existed in the family; it only proves, that such a family was or is made after the type of some division or other of the animal or vegetable kingdom; hence the traces may never have been fully developed at all,-they may be consequences of a law of developement, and not active organs, or parts in the organism in which they are found. Thus in the present state of physiology, it would be rash to draw the conclusion that the traces of the canals to which we referred as now existing in marsupials, are certainly but the remains of what were fully developed in their primeval types. We incline to the idea, and suggest it to the consideration of those more conversant with such subjects; but at the same time allow, that it is not a necessary consequence of their existence, although it may be a probable one. So far, however, is certain, that it forms a kind of connecting link between the respiratory systems of marsupials and reptiles. But, in considering a matter such as this, we must not confine our views to the respiratory system alone, but must take into consideration the whole organization of the animal, and the peculiar connection of its various systems, with their modes of reaction. Viewing it in this light we find the animals in question occupying only a low position in the scale of creation; it is, therefore, highly probable that they could live in an atmosphere considerably worse than they now enjoy. Here, again, I must apologise for want of a few experiments which would have at once decided the question; but the short time which has elapsed since I was requested to prepare this lecture for you, has prevented me from procuring an opossum to treat to such an atmosphere. Any of you who may have such an animal may easily satisfy yourselves by a few experiments.

But whilst wandering through these ancient lands, how comes it that we have not met any of the winged tribes ? Forests are there to form a habitation-insects abound to afford them food; the genial

climate or the smiling face of nature invites them to sing its praises ; but in vain we penetrate the deep recesses of those ancient forests to discover traces of the feathered songsters of former days. We search the shores and the rivers to find aquatic birds feasting on the fishes which so abound, but we are scared away by the saurians which line their banks. We enter the forests, and meet nought but the monstrous pterodactylus sailing along on its filmy wings. If birds existed, where are their remains? Though rare, still we find them scattered throughout the tertiary and modern lacustrine deposits. Why is there not a single evidence of their existence in all the secondary strata? Simply because (if our view of the state of the atmosphere be correct) they could not have existed. Birds require a very large supply of oxygen for the support of their vital functions, and are peculiarly susceptible to the effects of an excess of carbonic acid; neither of these conditions being yet favourable, nature was not fitted to receive them.

But we have now nearly approached the termination of the secondary period, for the paucity of terrestrial plants and animals in the cretaceous series and greensand does not tempt us to linger long amidst their oceanic deposits. But to the chemist it seems strange from whence came those vast deposits of carbonate of lime in the cretaceous beds. The plants are not numerous, and almost entirely marine; yet from the quantity of pyrites which occurs, it is apparent vast quantities of organic matter must have been in decay.

The perishable nature of marine plants prevents them being accumulated in very large quantity. Hence we might still conceive that the ocean was covered with marine plants at this period, which by their decay, might so furnish carbonic acid as to decompose the sulphate of lime existing in the sea water, and thus occasion the deposition of carbonate of lime; thus the atmosphere would be robbed of large quantities of carbonic acid. But the magnitude of such deposits astonish us, and would compel us to relinquish this even as a partial cause, did we not, on the other hand, consider the great extent of geological epochs.

Now came the close of the secondary period. We have glanced at the nature of its organic remains, from the silurian rocks to the greensand; we have been struck with the developement of organic life; we found it partly owing to the physical conditions and position of land and sea-and with these we had nothing to do ;but we found it also dependent on the chemical constitution of the atmosphere; and that when vegetation gradually purified the air from its noxious ingredients, other forms of animal life sprung into existence. When traversing these lands of former times, it is difficult for us to conceive that we are examining our own world. The striking progression of organic life must have been due to some cause. Why, may I ask the followers of Lyell, who believe that organic beings may have existed long before the primary periods, although their remains have been destroyed by heat, why this pro

gression of animal life? And, wherefore, may I ask the followers of Daubeny, did land animals not sport in the forests of the carboniferous land? No mighty operations of nature then acted as antagonists to their existence; the climate must have been congenial; the war of the elements not greater, if so great as now. And wherefore, when in the course of ages, those forests became entombed, did land animals then start into being? Can these, or many similar questions be replied to, without admitting some changes in the states of the medium in which both plants and animals exist, or can the uniform progression of animal life be attributed to entirely local or adventitious causes? Let us not contort the face of nature to give countenance to our theories, but let us frame those theories on nature as it is.

Following, therefore, the train of reasoning which we have adopted, we are struck with amazement in stepping over the boundary which separates the secondary from the tertiary periods. We no longer encounter those monstrous reptiles which haunted the shores and rivers of former lands; we do not now wonder at the vast difference of the former state of things, but feel a difficulty in believing we are not looking upon a world similar to our own. We have seen that the forms of animal life either gradually run into one another in the various strata, or that these were separated by a distinct line of demarcation, and characterised their own peculiar class of fossils. With respect to land animals, we have also seen that when any great addition of forms was effected, evidences exist of causes having been in operation to remove carbonic acid from the atmosphere. Now whence do we derive the evidences of this withdrawal in the era between the secondary and the tertiary periods? Nothing can be more striking than the difference of organic remains between the infra, and supracretaceous beds. The supracretaceous deposits are characterised by the wonderful similitude of organic remains to types now existing-the infracretaceous, by their utter dissimilarity. On this account we are disposed to lay more weight to our former explanation of the manner in which the cretaceous beds may have been formed; viz., by a decomposition of the sulphate of lime existing in sea water by means of carbonates formed through the agency of decaying plants. Such, at all events, may have been a partial cause, and would account, in a great measure, for the small coherence and chalky nature of the limestone; a character of a precipitate.

The

There are many circumstances which countenance this idea. recent experiments of Kuhlman on the preparation of artificial stones by means of silicate of potash, are powerful advocates of its truth. There cannot be the slightest doubt that silicate of potash formed an important ingredient in the sea of former times; every rock of marine origin proves this. Now as the carbonic acid was evolved by the decay of the marine vegetation, it would decompose this silicate of potash, forming carbonate of potash, and depositing