found in any of the intermediate deposits. Just so it is with the mammals. In the lower beds of the middle part of the series we have a few but distinct traces of their presence; these, however, are totally lost in the newer beds, and even in the Weald, a fresh-water deposit, we are still without a single fragment that can be supposed to have belonged to any quadruped, the companion on land of the gigantic dinosaurians.

The Invertebrata illustrate the same fact yet more decisively. A vast development of the highest type, the Cephalopoda, seems to have been throughout characteristic of the period, and one genus alone, that of ammonites, was represented then by many more species that now form the whole group. The belemnites and other free-swimming animals of the same kind (cuttle-fish) were no less abundant, and their remains are found every where, distributed it is true locally, owing to the conditions of deposit being only occasionally favourable for their preservation. Other univalve shells are also met with in great abundance, proving clearly how similar the conditions of existence must have been to those which still obtain in various parts of the world. The abundance of Terebratulæ in particular localities, the presence of other bivalve shells of almost all the different existing families, the development of Foraminifera and minute infusorial animalcules in deep sea deposits, and, indeed, almost every natural history fact presented to us, offers the strongest analogy to similar facts now observable. These facts all prove how greatly the study of existing nature may aid us in working out the problems of the past

history of the world, and how nearly the past stands in relation to the present.

But though there are thus very near and important resemblances, neither can it be questioned that there are wide and startling differences. Where now, or during more modern geological periods, we should find remains of elephants, rhinoceroses, and hippopotamuses, we have multitudes of the bones of gigantic reptiles, assuming their form, occupying their place, and substituted for them in the islands and continents of the ancient world. The whale tribe, an exceedingly important and widely extended group of animals, now inhabiting the seas from the poles to the equator, represented by many species of various proportions and dimensions, and manifestly occupying a very important place amongst the inhabitants of the deep, is, in like manner, totally absent. It must be remembered, too, that in this case the argument arising from the absence of the group is far stronger than in the case of land animals, for the remains of these creatures, both of teeth and bones, were just as likely to be preserved as those of other marine animals, and in more modern deposits they have been so preserved. But there is also a good reason for their absence. Their task was performed by others; they were not needed, because their place was supplied by the gigantic marine reptiles, which have abounded during the whole period, and which

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This is the case at least with the large-toothed carnivorous Cetacea. It is worth while to observe that we do not at present know of any fossil remains in secondary rocks of large herbivorous or animalculefeeding animals, whether reptilian or otherwise, corresponding to the dugong, &c., and the gigantic true whales (Balana).

have left undoubted marks of their existence and wide distribution.

This method of substitution, by which, at different times and in various places, animals and vegetables of very different organization have replaced one another, is found by geological observations to have been universally adopted in nature. Large groups may be observed performing their appointed task at one period or in one place, and these are represented by other species at another time or at some distant spot, the representative species exhibiting some analogous peculiarity, either of structure or habit, which fits it for that similar function.

This important fact is, however, not made known to us exclusively by geological researches, but also by investigations into existing nature; and these latter, when combined with the former, become more distinctly recognised, and by their manifest relation to long periods of time as well as extensive tracts in space, exhibit their true importance with reference to all nature.

Viewed as one epoch, the secondary rocks present to us a well-marked and very distinct group of animals and vegetables, from the careful study of which we may arrive at some probable conclusion with regard to the condition of the earth and seas during the period of deposit. We thus learn to consider the secondary period as indicating a condition very different from that of the earlier epoch, and as cut off from it absolutely by the introduction of new forms of organic existence. Its great characteristic undoubtedly appears to be the introduction of reptiles, or rather the presence of reptiles in such vast variety of form, and

in such relative numerical abundance, as to render it probable that the apparent absence of animals of higher organization is not accidental, or a conclusion based on imperfect examination, but that it was real and absolute.

The corresponding characteristic of the former and earlier period we found to be the great development of fishes, which occupied then the position afterwards filled by reptiles. Now it is this apparent substitution or representation that gives confidence and justifies the conclusions drawn. By the help of comparative views of this kind, we perceive the meaning, the symmetry, and the perfectness of each group, and the adaptation of each to its own end. By carefully carrying out the method of comparison, we gradually attain more distinct, less exaggerated, and more rational views of the differences of organic being at various periods; and there is no doubt that the views thus attained are infinitely more real than those derived from the examination of a few isolated facts of marvellous character, which attract the imagination, but which for that very reason are likely to mislead the judgment.

If called upon to give a general account of the physical geography of the secondary period, in the vicinity of our own island, I might venture to suggest,

1. That during this period there were many inconsiderable disturbances of the bed of an ocean which then covered our island, but which was partially bounded in several directions by islands extending to the west and north, the lines of coast being more or less extensive, and generally at no great distance. 2. That the effect of these disturbances was for a

long time gradually to depress the bed of the ocean, while the deposits, chiefly of calcareous matter, were sufficiently rapid and abundant, at least near shore, to keep pace with the sinking. 3. That after a time partial elevation took place, converting into dry land a large tract, in consequence of the wide extent of a moderately shallow sea, arising from the condition just mentioned. 4. That there was an exception to this conversion into dry land in the southeast of England, and some other spots; but that here the waters were chiefly fresh, indicating the presence of an extensive river emptying itself not far off into the sea. 5. That while this elevation took place in England, depression was going on in the south of Europe, accompanied by disturbances, and producing a deep sea, 6. That such deep sea was afterwards enlarged by a wider movement of depression, until it extended from Scandinavia to Spain, and from England to the Caucasus, being throughout that wide tract the recipient of deposits of chalky mud; and, finally, that this sea was probably deeper and was soonest deepened in the southern part of the district, but that just in that part the sea-bottom afterwards again underwent elevation on a grand scale, and formed either continuous land or a chain of islands extending from England to Asia Minor, and connected with the elevation of the Alps, the Pyrenees, and the Caucasus.

This last disturbance was accompanied, however, by a total destruction of almost the whole series of animated beings in these parts of the world; it was accompanied also by the outburst of a considerable quantity of lava (seen in the north of Ireland), and