PALÆONTOLOGY.[F]
87. Fossils.—In our description of rock-masses, and again in our account of geological agencies, we referred to the fact that certain rocks are composed in large measure, or exclusively, of animal or vegetable organisms, or of both together; and we saw that analogous organic formations were being accumulated at the present time. But we have deferred to this place any special account of the organic remains which are entombed in rocks. Fossils, as these are called, consist generally of the harder and more durable parts of animals and plants, such as bones, shells, teeth, seeds, bark, and ligneous tissues, &c. But it is usual to extend the term fossil to even the casts or impressions of such remains, and to foot-marks and tracks, whether of vertebrates, molluscs, crustaceans, or annelids. The organic remains met with in the rocks have usually undergone some chemical change. They have become petrified wholly or in part. The gelatine which originally gave flexibility to some of them has disappeared, and even the carbonate and phosphate of lime of the harder parts have frequently been replaced by other mineral matter, by flint, pyrites, or the like. So perfect is the petrifaction in many cases, that the most minute structures have been entirely preserved—the original matter having been replaced atom by atom. As a rule, fossils occur most abundantly and in the best state in clay-rocks, like shale; while in porous rocks, like sandstone, they are generally poorly preserved, and not of so frequent occurrence. One reason for this is, that clay-rocks are much less pervious than sandstone, and their imbedded fossils have consequently escaped in greater measure the solvent powers of percolating water. But there are other reasons for the comparative paucity of fossils in arenaceous strata, as we shall see presently.
88. Proofs of varied Physical Conditions.—Organic remains are either of terrestrial, fresh-water, or marine origin, and they are therefore of the utmost value to the geologist in deciphering the history of those great changes which have culminated in the present. But we can go a step further than this. We know that at the present day the distribution of animal and vegetable life is due to a variety of causes—to climatic and physical conditions. The creatures inhabiting arctic and temperate regions contrast strongly with those that tenant the tropics. So also we observe a change in animal and vegetable forms as we ascend from the low grounds of a country to its mountain heights. Similar changes take place in the sea. The animals and plants of littoral regions differ from those whose habitat is in deeper water. Now, the fossiliferous strata of our globe afford similar proofs of varying climatic and physical conditions. There are littoral deposits and deep-sea accumulations: the former are generally coarse-grained (conglomerates, grit, and sandstone); the latter are for the most part finer-grained (clay, shale, limestone, chalk, &c.); and both inshore and deep-water formations have each their peculiar organic remains. Again, we know that some parts of the sea-bottom are not so prolific in life as others—where, for example, any considerable deposit of sand is taking place, or where sediment is being constantly washed to and fro upon the bottom, shells and other creatures do not appear in such numbers as where there is less commotion, and a finer and more equable deposit is taking place. It is partly for the same reason that certain rocks are more barren of organic remains than others.
89. Fossil Genera and Species frequently extinct.—It might perhaps at first be supposed that similar rocks would contain similar fossils. For example, we might expect that formations resembling in their origin those which are now forming in our coral seas would also, like the latter, contain corals in abundance, with some commingling of shells, crustaceans, fish, &c., such as are peculiar to the warm seas in which corals flourish. And this in some measure holds good. But when we examined carefully the fossils in certain of the limestones of our own country, we should find that while the same great orders and classes were actually present, yet the genera and species were frequently entirely different; and not only so, but that often none of these were now living on the earth. Moreover, if we extended our research, we should soon discover that similar wide differences actually obtained between many of the limestones themselves and other fossiliferous strata of our country.
90. Fossiliferous Strata of Different Ages.—Another fact would also gradually dawn upon us—this, namely, that in certain rocks the fossils depart much more widely from analogous living forms, than the organic remains in certain other rocks do. The cause of this lies in the fact that the fossiliferous strata are of different ages; they have not all been formed at approximately the same time. On the contrary, they have been slowly amassed, as we have seen, during a long succession of eras. While they have been accumulating, great vicissitudes in the distribution of land and sea have taken place, climates have frequently altered, and the whole organic life of the globe has slowly changed again and again—successive races of plants and animals flourishing each for its allotted period, and then becoming extinct for ever.[G] Thus, strata formed at approximately the same time contain generally the same fossils; while, on the other hand, sedimentary deposits accumulated at different periods are charged with different fossils. Fossils in this way become invaluable to the geologist. They enable him to identify formations in separate districts, and to assign to them their relative antiquity.[H] If, for example, we have a series of formations, A, B, C, piled one on the top of the other, A being the lowest, and C the highest, and each charged with its own peculiar fossils, we may compare the fossils met with in other sets of strata with the organic remains found in A, B, C. Should the former be found to correspond with the fossil contents of B, we conclude that the rocks in which they occur are approximately of contemporaneous origin with B, even although the equivalents of the formations A and C should be entirely wanting. Further, we soon learn that the order of the series A, B, C, is never inverted. If A be the lowest, and C the highest stratum in one place, it is quite certain that the same order of succession will obtain wherever the equivalents of these strata happen to occur together. But the succession of strata is not invariably the same all the world over; in some countries, we may have dozens of separate formations piled one on the top of the other; in other countries, many members of the series are absent; in brief, blanks in the succession are of constant occurrence. But by dovetailing, as it were, all the formations known to us, we are enabled to form a more or less complete series of rocks arranged in the order of their age. A little reflection will serve to shew that the partial mode in which the strata are distributed over the globe arises chiefly from two causes. We have to remember, first, that the deposits themselves were laid down only here and there in irregular spreads and patches—opposite the mouths of rivers, at various points along the ancient coast-lines, and over certain areas in the deeper abysses of the ocean—the coarser accumulations being of much less extent than those formed of finer materials. And, second, we must not forget the intense denudation which they have experienced, so that miles and miles of strata which once existed have been swept away, and their materials built up into new formations.
91. Gradual Extinction of Species.—When a sufficient number of fossils has been diligently compared, we discover that those in the younger strata approach most nearly to the present living forms, and that the older the strata are, the more widely do their organic remains depart from existing types of animals and plants. We may notice also, that when a series of beds graduate up into each other, so that no strongly marked line separates the overlying from the underlying strata, there is also a similar gradation amongst the fossils. The fossils in the highest beds may differ entirely from those in the lowest; but in the middle beds there is an intermingling of forms. In short, it is evident that the creatures gradually became extinct, and were just as gradually replaced by new forms, until a time came when all the species that were living while the lowest beds were being amassed, at last died out, and a complete change was effected.
92. Proofs of Cosmical Changes of Climate.—From the preceding remarks it will be also apparent that fossils teach us much regarding the climatology of past ages. They tell us how the area of the British Islands has experienced many vicissitudes of climate, sometimes rejoicing in a warm or almost tropical temperature, at other times visited with a climate as severe as is now experienced in arctic and antarctic regions. Not only so, but we learn from fossils that Greenland once supported myrtles and other plants which are now only found growing under mild and genial climatic conditions; while, on the other hand, remains of arctic mammals are met with in the south of France. Such great changes of climate are due, according to Mr Croll, to variations in the eccentricity of the earth's orbit combined with the precession of the equinox. It is well known that the orbit of our earth becomes much more elliptical at certain irregularly recurring periods than it is at present. During a period of extreme ellipticity, the earth is, of course, much further away from the sun in aphelion[I] than it is at a time of moderate ellipticity, while, in perihelion,[J] it is considerably nearer. Now, let us suppose that, at a time when the ellipticity is great, the movement known as the precession of the equinox has changed the incidence of our seasons, so that our summer happens in perihelion and not in aphelion, while that of the southern hemisphere occurs in aphelion, and not, as at present, in perihelion. Under such conditions, the climate of the globe would experience a complete change. In the northern hemisphere, so long and intensely cold would the winter be, that all the moisture that fell would fall as rain, and although the summer would be very warm, it would nevertheless be very short, and the heat then received would be insufficient to melt the snow and ice which had accumulated during the winter. Thus gradually snow and ice would cover all the lands down to temperate latitudes. In the southern hemisphere, the reverse of all this would obtain. The winter there would be short and mild, and the summer, although cool, would be very long. But such changes would bring into action a whole series of physical agencies, every one of which would tend still further to increase the difference between the climates of the two hemispheres. Owing to the vast accumulation of snow and ice in the northern hemisphere, the difference of temperature between equatorial and temperate and polar regions would be greater in that hemisphere than in the southern. Hence the winds blowing from the north would be more powerful than those coming from the southern and warmer hemisphere, and consequently the warm water of the tropics would necessarily be impelled into the southern ocean. This would tend still further to lower the temperature of our hemisphere, while, at the same time, it would raise correspondingly the temperature at our antipodes. The general result would be, that in our hemisphere ice and snow would cover the ground down to low temperate latitudes—the British Islands being completely smothered under a great sea of confluent glaciers. In the southern hemisphere, on the contrary, a kind of perennial summer would reign even up to the pole. Such conditions would last for some ten or twelve thousand years, and then, owing to the precession of the equinox, a complete change would come about—the ice-cap would disappear from the north, and be replaced by continuous summer, while at the same time an excessively severe or glacial climate would characterise the south; and such great changes would occur several times during each prolonged epoch of great eccentricity. This, in few words, is an outline of Mr Croll's theory. That theory is at present sub judice, but there can be no doubt that it gives a reasonable explanation of many geological facts which have hitherto been inexplicable. Of course, it is not maintained that all changes of climate are due directly or indirectly to astronomical causes. Local changes of climate—changes affecting limited regions—may be induced by mutations of land and sea, resulting in the partial deflection of ocean currents, which are the chief secondary means employed by nature for the distribution of heat over the globe's surface.
From what has been stated in the foregoing paragraphs, it is clear that in our endeavours to decipher the geological history of our planet, palæontological must go hand in hand with stratigraphical evidence. We may indeed learn much from the mode of arrangement of the rocks themselves. But the test of superposition does not always avail us. It is often hard, and sometimes quite impossible, to tell from stratigraphical evidence which are the older rocks of a district. In the absence of fossils we must frequently be in doubt. But physical evidence alone will often afford us much and varied information. It will shew us what was land and what sea at some former period; it will indicate to us the sites of ancient igneous action; it will tell us of rivers, and lakes, and seas which have long since passed away. Nay, in some cases, it will even convince us that certain great climatic changes have taken place, by pointing out to us the markings, and débris, and wandered blocks which are the sure traces of ice action, whether of glaciers or icebergs. The results obtained by combining physical and palæontological evidence form what is termed Historical Geology.