Animals possessed of the power of flight might also similarly find their way into aqueous deposits, but, when it is remembered that many birds and mammals habitually spent a great part of their time in the water, it is not to be wondered at that they should present themselves as fossils in sedimentary rocks. Even plants, such as have undoubtedly grown upon land, do not prove that the bed in which they are found was formed on land, for many of their remains are extraneous to the bed in which they now occur, having reached their present site by falling into lakes or rivers, or by being carried out to sea by floods or gales of winds. Still, there are many cases which obviously show that plants have grown on the very spot where we now find them. The great coal-fields of the Carboniferous Age, it is now generally conceded, are the result of the growth _in situ_ of the plants which compose coal, as well as that they grew on vast marshy or partially submerged tracts of level alluvial land. While fossils enable us in many cases to arrive at important conclusions as to the climate of the period in which they lived, yet it is only in the case of marine fossils, which constitute the majority of such remains, that we acquire such knowledge, but it is mostly the temperature of the sea which can thus be determined. However, let it be remembered that, owing to the existence of heated currents, the marine climate of a designated area does not necessarily imply a correspondingly warm climate in the adjoining land, for land-climates can only be determined by the relics of land-animals or land-plants, and these are comparatively rare as fossils. But all conclusions on this head are really based upon the existing distribution of vegetable and animal life upon the globe, and are therefore liable to be vitiated by the considerations that no certainty exists that the habits and requirements of an extinct animal were exactly similar to those of its nearest living relative; that far back in time groups of organisms, so unlike anything we know at the present day, are met with, which render all conjectures of climate based upon their supposed habits more or less uncertain and unsafe; that in the case of marine animals we are as yet very far from knowing the precise limits of distribution of many species within our present seas as to render conclusions drawn from living forms in relation to extinct species unsatisfactory and, probably, incorrect; and, finally, that the distribution of animals to-day, is certainly dependent on other conditions than climate alone, the causes limiting the range of given animals being assuredly such as belong to the existing order of things, and are different from what they were in former times, not necessarily because the climate has changed, but because of the alteration of other conditions that are essential to the life of the species or conducive to its extension. But notwithstanding the difficulties in the way, we are able in many cases