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the higher animals at least, by the subtler promptings of the intellect and reason. In the one case we deal with forms and forces that are extraneous; in the other, with those to which we ourselves belong, and hence the higher the interest excited, and the deeper the mysteries involved. What is life t Whence its origin 1 And how has it manifested itself during the long ages throughout which Geology has traced its presence in the rock formations of the globe 1 These are questions of the highest interest to science, and how feeble soever the indication towards a solution which human knowledge can offer, every tracing is of value so long as it is founded upon fact, and sketched by an honest hand. Such is the aim of the present Sketch—a deduction from the statements in the preceding chapters, a digest of the discoveries of pakeontology, an indication, if not of the nature and relations of life, at least of its order and succession as warranted by the truths of Geology.
Concerning the origin of life, Geology ventures no opinion. It may have started into being at the immediate fiat of the Creator, or it may have arisen through secondary causation, acting in obedience to Creative Law. It may have sprung from a single primordial germ, or it may have spread from several primordial sources. In its essence it may be a thing per se, or it may be merely a manifestation arising from the interactions of the subtler physical forces. On such points Geology offers no opinion. It deals with life as it finds it, and dates its commencement with the earliest traces yet discovered in the stratified formations. At one time this limit was found in the lower Silurians, more lately in the Cambrians, and now, as we have seen in Sketch No. 5, discovery has carried it back to the Laurentian system—a suite of strata of much higher antiquity. Whether these Laurentian rocks are the oldest or earliest in which traces of life can be detected, Geology does not aver; but as the forms of vitality become fewer and lower in kind with each successive remove in time, and as the Laurentian forms are both very lowly in organisation and scanty in number, inquirers are constrained to believe that they are nearly approaching, if indeed they have not already reached, the first beginnings of life on our planet. Be this as it may, the earliest known forms of vitality occur in the Laurentian system, and it seems something more than a mere coincidence, that these forms should belong at the same time to the very lowest orders that are known to Zoology. Such is all we know of the commencement of life on our globe; such is the ultimate limit to which geological science has yet been enabled to push her investigations. But while this is incontrovertible as matter of fact, we may believe—and all analogy seems to favour the belief—that life was contemporaneous with the laying down of the first-formed sediments. The external conditions (light, heat, cold, rains, rivers, and seas) that favour the one set of operations are usually those that accompany the other; and thus, wherever sedimentary strata occur, there also may we expect to find traces of vegetable or animal organisation. If the Laurentian be the earliest formed strata, we have already reached the goal; should others have existed before these, we can merely regard them as the provisional commencement of that long line of vital development which Geology is still labouring to reveal.
But though ignorant of the origin and commencement of life, we know something of its nature and functions. We perceive that minerals increase by the accretion or external addition of similar matter, but vegetables and animals grow by the internal assimilation of substances which they absorb and convert each into its own proper tissues. Once formed, and the course of the mineral is completed; once matured, and the plant or animal gives birth to similar plants and animals, and the course of reproduction may endure for ages. Wherever heat, light, and moisture are present, there life occurs, fitted partly for the air, partly for the land, partly for the waters, and partly also for a parasitic existence, on and within the tissues of other plants and animals. Unless under the extremes of heat and cold, life is everywhere present—restricted, no doubt, to a thin film of the globe measuring vertically, but spreading horizontally over every belt of latitude, and enjoying, each order according to its grade of organisation, the realisations of growth and reproduction. We can imagine a material world devoid of all manifestations of life, and such our planet may have been during ages of which we have no geological indication ; but, constituted as it now is, its harmonies would be incomplete without the presence both of vegetable and animal existences. Not only is the presence of the one necessary to the life of the other, but both are indispensable to the consumption and reproduction of those substances by which the structure and individuality of our globe are maintained. The crust of our earth is a thing of vegetable and animal as well as of mineral growth, and we may be assured, that from the beginning it was contemplated that each should perform its part in the harmonious maintenance of the whole. The mineral building up its chemically composite structure, the plant disintegrating and living upon these elements, the herbivorous animal feeding upon the plant, the carnivorous animal upon the herbivorous, the animal breathing the oxygen of the atmosphere and exhaling carbonic acid, and the vegetable imbibing carbonic acid and discharging in turn the oxygen, are but so many stages in a cosmical succession as harmonious in its adjustments as it seems interminable in its duration. Mineral, vegetable, and animal, are evident co-adaptations of the same great plan. We may never know how they originate, or why they exist, but we perceive the modes in which they operate, and can determine the results of their harmonious and incessant inter-actions.
At the present day we know that the great regulators of plants and animals are heat, light, and moisture. Some are adapted to the warmer regions of the globe, some to the temperate, and others, again, to the colder latitudes. Some are fitted for life on the dry land, some for life in the waters; while others, again, are fitted for both, or even to wing their way through the atmosphere. Some affect the marsh, while others cling to the thirsty upland; some rejoice in the shallow waters of the shore, while others find their fitting habitat only in the deeper ocean. Some are restricted to specific centres of limited extent, and present little variation in character, while others enjoy a wider range, and break into numerous and often widely divergent varieties. Such are the obvious conditions and distributions of life now, and such we may be certain were the nature of its conditions and dispersions during all previous periods. Again, some plants are rooted independently in the soil, while others find their subsistence on other plants, or even in animals. A vast number of animals live on vegetables, while others prey on the vegetable-feeders, and are fitted alone for this mode of existence. The tooth to tear is as necessary as the tooth to grind, the foot to seize as the foot to run, the hand to climb as the hand to hold, and the limb to fly as the limb to walk or the limb to swim. Such are the arrangements by which the balance and harmonies of life are now sustained, and such we may presume were the methods by which its harmonies were secured in former ages. Plant-feeder and flesh-feeder, life and death, reproduction and decay, are necessary concomitants in the great biological scheme of creation. We perceive them operating in full force now; palaeontology declares they were as universally energetic in bygone epochs.
Besides these obvious arrangements and inter-relations of life, there is also grade or degree of organisation—some orders being more simple in structure and fitted for lower functions, others being more complex and fitted for the higher offices of vitality. The lichen incrusting the rock or the sea-weed clustering the shelving reefs of the sea-shore, and devoid of stem and leaf, is a lowlier organism than the clubmoss or tree-fern; and these again are less highly organised than the true timber-tree, with its complex development of trunk, branches, leaves, flowers, and fruits. The sponge, rooted plant-like to the rock, is clearly a lower phase of animal life than the coral or star-fish; and these again are less highly organised than the crabs and shell-fishes. Animals like the star-fishes, crabs, and shell-fish, devoid of backbone and bony skeleton (the Invertebrata), are obviously lower in the scale of being than the fishes, reptiles, birds, and mammalia (the Vertebrata); and even among these vertebrates the cold-blooded water-breather is inferior to the warm-blooded air-breather—the fish on a lower stage than the reptile, the reptile than the bird, and the bird than the mammal. Still more, the marsupial or pouched mammal, bringing forth immature young and carrying them about for months, is less highly organised than the placental or true mammal, which gives birth to fully developed young; and among the true mammals themselves there are manifestly various grades of organisation—the whales are lower than the ruminants, the ruminants than the carnivora, the carnivora than the monkeys, and the monkeys than man. And whatever may have been the abundance or development of life .during the geologic epochs, we may presume that such grades and distinctions have ever pervaded the whole vital scheme—high and low, low and