We learn that, during the states of progress which geology, looking into the arcana of time, has made us acquainted with, a great variety of animal forms were brought into existence. They lived their periods. The conditions of the surface of the earth, the sea, or the atmosphere, were altered; and, no longer fitted for the enjoyments of the new life, these races passed away, and others occupied their places, which, in turn, went through all the stages of growth, maturity, and decay; until at length, the earth being constituted for the abode of the highest order of animals, they were called into existence; and man, the intellectual monarch of the world, was placed supreme amongst them all. Types of nearly all those forms of life which are found in the fossil state are now in existence; and if we examine the geographical distribution of animals—the zones of elevation over the surface of the earth, and the zones of depth in the ocean,—we shall find, now existing, animal creations strikingly analogous to the primitive forms and conditions of the earth’s inhabitants. From the depths of the ocean we may even now study—as that most indefatigable naturalist, Professor Edward Forbes, has done—the varying states of organization under the circumstances of imperfect light and varying temperature.[261]
The gradual advance of animal life in the ascending strata has led to many speculations, ingenious and refined, on the progressive development of animals. That the changes of the inorganic world have impressed new conditions on the organic structures of animals, to meet the necessities of their being, must be admitted. Comparative anatomy has demonstrated that such supposed differences really existed between the creatures of secondary formations—those of the tertiary and the present periods. It has been imagined, but upon debatable foundations, that the atmosphere, during the secondary periods, was highly charged with carbonic acid; and, consequently, that though beneficial to the growth of plants, and peculiarly fitted for the conditions required by those which the fossil flora makes us acquainted with, it was not adapted to support any animals above the slow-breathing, cold-blooded fishes and reptiles. Under the action of the super-luxuriant vegetation of these periods, this carbonic acid is supposed to have been removed, an addition of oxygen furnished; and thus, consequently, the earth gradually fitted for the abode of warm-blooded and quick-breathing creatures. We do, indeed, find a very marked line between the fossil remains of the lias formations which enclose the saurians, and the wealden, in which birds make their appearance more numerously than in any previous formation.
Founded upon these facts, speculations have been put forth on the gradual development of animals from the lowest up to the highest orders. Between the polype and man a continuous series has been imagined, every link of the chain being traced into connection with the one immediately succeeding it; and, through all the divisions, zoophytes, fishes, amphibia, reptiles, birds, and mammalia are seen, according to this hypothesis, to be derived by gradual advancement from the preceding orders. The first having given rise to amphibia,—the amphibion gives birth to the reptile,—the reptile advances to the bird,—and from this class is developed the mammal. A slight investigation will convince us that this view has no foundation. Although a certain relationship may be found between some of the members of one class, and those of the other immediately joining it, yet this is equally discovered to exist towards classes more remote from each other; and in no one instance can we detect anything like the passage of an animal of one class into an animal of another. Until this is done, we cannot but regard the forms of animal life as distinct creations, each one fitted for its state of being, springing from the command of the great First Cause.[262]
But it is time to quit these speculative questions, and proceed to the examination of the general conditions of animal life at the present time.
Lowest in the scale of animals, and scarcely distinguishable from a vegetable, we find the sponge, attached to and passing its life upon a rock, exhibiting, indeed, less signs of feeling than many of the vegetable tribes. The chemical differences between vegetables and sponges are, however, very decided; and we find in the tissues of the sponge a large quantity of nitrogen, a true animal element, which exists, but in smaller quantities, in vegetables.
These creations, standing between vegetable and animal life, possess the singular power of decomposing carbonic acid, as plants do; and the water in which they live always contains an excess of free oxygen.
The polypes are a remarkably curious class. “Fixed in large arborescent masses to the rocks of tropical seas, or in our own climate attached to shells or other submarine substances, they throw but their ramifications in a thousand beautiful and plant-like forms; or, incrusting the rocks at the bottom of the ocean with calcareous earth, separated from the water which bathes them, they silently build up reefs and shoals, justly dreaded by the navigator; and sometimes giving origin, as they rise to the surface of the sea, to islands, which the lapse of ages clothes with luxuriant verdure, and peoples with appropriate inhabitants.”[263]
Most of the polypes are fixed and stationary; but the hydra and some others have the power of changing their positions, which they do in search of the light of the sun. They do not appear to have organs of sight requiring light; but still they delight in the solar influences. The most extraordinary fact connected with the hydra is its being multiplied by division. If an incision be made in the side of a hydra, a young polype soon developes itself; and if one of these creatures be divided, it quickly restores the lost portion of its structure. The varieties of the polypes are exceedingly numerous, and many of them are in the highest degree curious, and often very beautiful. The actiniæ, like flowers, appear to grow from the rocks, unfolding their tentacula to the light; and, in the excitement due to their eagerness for prey, they exhibit a beautiful play of colours and most interesting forms. Microscopic zoophytes of the most curious shapes are found,—all of which attest, under examination, the perfection of all created things.
Infusoria and animalcula,—animals, many of them, appearing under the microscope as little more than a transparent jelly,—must be recognized as the most simple of the forms of life. They exist in all waters in uncountable myriads; and, minute creatures as they are, it has been demonstrated that many of the great limestone hills are composed entirely of their remains.
The acalephæ, or the phosphorescent animals of the ocean, are no less curious. From creatures of the most minute size, they extend to a considerable magnitude, yet they appear to be little more than animated masses of sea-water. If any one of these sea-jellies, or jelly-fishes as they are often called (even the largest varieties of them), is cast upon the shore, it is soon, by the influence of the sun, converted into a mere fibre no thicker than a cobweb: an animal weighing seven or eight pounds is very soon reduced to as many grains. There are numerous kinds of these singular creatures, most of which are remarkable for the powerful phosphorescent light they emit. The beroes and the pulmonigrade shine with an intense white light many feet below the surface, whilst the Cestum Veneris, or girdle of Venus, gliding rapidly along, presents, on the edge of the wave, an undulating riband of flame of considerable length. There can be no doubt that this arises from the emission of phosphorescent matter of an unknown kind from the bodies of these animals.