As the animal species which inhabit bodies of fresh water are always different from those found in the sea, their remains constitute the best means of determining whether a formation is of fresh water or marine origin. In order to decide this point, it may, in some cases, be necessary to be acquainted with the habits of particular species. In most cases, however, it will be sufficient to remember that in fresh-water formations, first, there are no sponges, corals, or chambered shells; second, the univalves all have entire mouths ([Fig. 44]). Third, the bivalves are all bimuscular ([Fig. 47]). If, therefore, a formation is found to contain sponge, coral, a chambered shell, a univalve with a deeply notched mouth ([Fig. 45]), or a unimuscular bivalve ([Fig. 46]), it must be considered a marine formation.
| Fig. 46. | Fig. 45. | Fig. 47. |
We have seen that the same formation, as exhibited in different places, differs in its thickness, composition and degree of solidification. If we could trace the strata through all the intermediate space, we might be certain of their being the same formation, notwithstanding the change in lithological characters. But this can seldom be done, even for a few miles in extent. Sections of the strata are obtained only occasionally, where rivers have cut through them, or where, over limited areas, the soil has been removed from the outcropping edges. It is also frequently the case that the strata are so much disturbed that their position will furnish no aid in determining their age. When folded axes occur (as here represented), the older strata are often the uppermost. There is an instance in the Alps in which strata of vast thickness have been inverted during the process of upheaval, and now rest on a bed of rock formed from the debris which they had supplied.
And yet it is important to determine what formations are of the same age, notwithstanding their displacements, difference in lithological characters, and separation by great distances and by mountains or oceans. This determination can be made only by a comparison of the imbedded fossils. It is found that every formation, and every important member of a formation, contains an assemblage of fossils peculiar to itself. When very widely separated, the species of fossils may not be identical, but so very similar that they are regarded as equivalent species. The identification of formations consists in the identification of fossils. It is for this purpose mainly that fossils are regarded as of so great importance.
6. If each formation is characterized by the presence of new species, it follows that the work of creation was a progressive one, continued through long periods of time. The latest creation of which we have any geological evidence is that of man. And if the leading design of the existence of this earth was as a theatre for the development of moral character, it is to be presumed that the work of creation ceased when a species possessing moral capacities had been introduced.
It follows also, from what has been said, that there has been a constant disappearance, a death, of species. It would seem that each species has a life assigned to it, which is to be completed and surrendered. Though its continuance is many times longer than the life of any individual of the species, yet it is the course of nature that species should disappear.
There may be something in the constitution of each species by which its continuance is limited, making an old age and death necessary, as it is in individuals. But there are other causes by which the duration of species may often be terminated. The subsidence of New Holland would cause the destruction of a large number of species. The preservation of the human species was at one time effected only by a special and miraculous interference. Slowly operating causes are now at work, by which many species, such as the elephant, wolf and tiger, will at length become extinct. Their existence in a natural state cannot long be continued in a civilized country. The forest, their natural abode, disappears, and some are intentionally destroyed, because they render life and property unsafe. Under the operation of these causes, the Irish elk (cervus giganteus) has become extinct, probably within the human era. The Dodo, a gallinaceous bird, found living when maritime communication between Europe and the East Indies was first established, is now extinct. The Apteryx, a bird belonging to New Zealand, has probably become extinct since the commencement of the present century.