Part 15 (1/2)
Electricity too, in the form of galvanism, has never been idle. We have reason to think that it operates at this moment in acc.u.mulating metallic ores in veins; and this segregation appears to have operated in all ages, not only in filling veins, but also, probably, in giving a laminated character and jointed structure to mountains of slate, as well as a concretionary and prismatic form to others.
Last, though not least, we may reckon among the agents of geological change the forces of cohesion and affinity. When water and heat, gravity and galvanism, have brought the atoms of bodies into a proper state, these agents are always ready to change their form and const.i.tution; and they have ever been at hand to operate by the same laws, and we witness their effects in the oldest as well as the newest rocks found in the earth's crust. This point, however, has been sufficiently considered, when treating of the unvarying uniformity of the laws of chemistry and crystallography.
But though the nature of the agencies above considered has never changed, the intensity or amount of their action has varied; how much is a point not yet settled among geologists. Some regard that intensity, as it has existed during the present or alluvial period, as a standard for all preceding periods; that is, the intensity of these forces has never varied more during any period of the earth's history than it has since the alluvial period commenced. Most geologists, however, regard this as an extreme opinion, and think they see evidence in geology of a far greater intensity in these agencies in past periods than exists at present. They think they have proof that the world was once only a molten ma.s.s of matter, and some evidence that previously it was in a state of vapor. They believe that vast mountains, and even continents, have sometimes been thrown up from the ocean's bed by a single mighty paroxysmal effort; and such effects they know to be far greater than the causes of change now in operation can produce, without a vast increase of their intensity. But this question need neither be discussed nor decided for the sake of my present argument, since my object is to prove an ident.i.ty in the nature and laws, not in the intensity, of geological agencies.
_In the fourth place, the laws of zology and botany have always been the same on the globe._
An examination of the animals now living, amounting to some hundred thousand species, perhaps to one or two millions, shows that they may be arranged in four great cla.s.ses. The first cla.s.s embraces the vertebral animals, distinguished by having a vertebral column, or back-bone, a regular skeleton, and a regular nervous system. It comprehends all the quadrupeds and bipeds, with man at their head, and is much superior to all other cla.s.ses in complexity of organization and strength of the mental powers. The second cla.s.s embraces the mollusks, or animals inhabiting sh.e.l.ls. They are dest.i.tute of a spinal marrow, and for the most part their muscles are attached to the external covering, called the sh.e.l.l, although this sh.e.l.l is sometimes internal. The third cla.s.s are called articulated animals, having envelopes connected by annulated plates, or rings. It includes such animals as the lobster, bloodsucker, spider, and insects generally. The fourth cla.s.s have a radiated structure, and often resemble plants, or their habitation is a stony structure. Hence they are sometimes called zophytes, which means _animal plants_; or lithophytes, which means _stony plants_. They swarm in the ocean, and some of them build up those extensive stony structures called coral reefs.
Now, if we examine the descriptions of the organic remains in the rocks, we find that in all ages of the world these four great cla.s.ses of animals have existed. But in the earliest times, the three last cla.s.ses--the mollusks, the articulated, and the radiated tribes--vastly preponderated, while the vertebral cla.s.s had only a few representatives; and it is not till we rise as high as the new red sandstone, that we meet with any, except fishes, save a few batrachians in the old red sandstone, and the carboniferous group, detected alone by their tracks. Then the reptiles began to appear in abundance, with tortoises and enormous birds of a low organization, but no mammiferous animal is found, until we reach the olite; and scarcely any till we rise to the tertiary strata, when they became abundant; but not so numerous as at present, though for the most part of larger size. Thus we find that the more perfect animals have been developed gradually, becoming more and more complex as we rise on the scale of the rocks. But in the three other cla.s.ses, there does not appear to have been much advance upon the original types, although in numbers and variety there has been a great increase.
The plants now growing upon the globe, amounting probably to nearly one hundred thousand species, are divided into two great cla.s.ses, by a very decided character. Some of them have distinct flowers, and others are dest.i.tute of them. The former are called phenogamian, or flowering plants; and the latter cryptogamian, or flowerless plants.
At present, the flowering plants very much predominate in the flora of every country. But in the earliest periods of organic existence, the reverse was the case. We find, indeed but very few flowering plants, and these of a character somewhat intermediate between flowering and flowerless; such as the conifer and cycade, including the pine tribe. A few palms appeared almost as early, and some other monocotyledons. But most of the dicotyledons did not appear till the tertiary period, where more than two hundred species have been found. Of the three hundred species found in and beneath the carboniferous group, two thirds are tree ferns, or gigantic equisetace. More than one third of the entire flora of the secondary formation consists of cycade; whereas, this family of plants forms not more than the two thousandth part of the existing flora.
In short, we find the more perfect plants as well as animals to be few in the earliest periods, and to have been gradually introduced up to the present time. But as to the flowerless plants, most of them seem to have been as perfect at first as they now are.
These facts teach us conclusively that the outlines of organic life on the globe have always been the same; that the great cla.s.ses of animals and plants have always had their representatives, and that the variations which have been introduced, have been merely adaptations to the varying condition of the earth's surface. The higher and more complex natures, both of animals and plants, were not introduced at first, because the surface was not adapted to their existence; and they were brought in only as circ.u.mstances, favorable to their development, prepared the way.
There is another fact of great interest on this subject. Even a cursory examination of the animals and plants now on the globe, shows such a gradation of their characters that they form a sort of chain, extending from the most to the least perfect species. But we see at once that the links of this chain are of very unequal length; or, rather, that there are in some instances wide intervals between the nearest species, as if one or more links had dropped out. How remarkable that some of these lost links should be found among the fossil species! I will refer to a few examples.
Among existing animals no genera or tribes are more widely separated than those with thick skins, denominated pachydermata; such as the rhinoceros and the elephant. But among the fossil animals of the tertiary strata, this tribe of animals was much more common; and many of them fill up the blanks in the existing families, and thus render more perfect and uniform the great chain of being which binds together into one great system the present and past periods of organic life.
A similar case occurs among fossil plants. In tropical climates we find a few species--not much over twenty--of a singular family of plants, the cycade connecting the great families of conifer, or dicotyledons, with the palms, which are monocotyledonous, and the ferns, which are acotyledonous. The chasm, however, between those great and dissimilar cla.s.ses of plants is but imperfectly filled by the few living species of cycade. But of the fossil species. .h.i.therto found above the coal formation, almost one half are cycade; so that here, too, the lost links of the chain are supplied.
”Facts like these,” says Dr. Buckland, ”are inestimably precious to the natural theologian, for they identify, as it were, the Artificer, by details of manipulation throughout his works. They appeal to the physiologist, in language more commanding than human eloquence; the voice of very stocks and stones, that have been buried for countless ages in the deep recesses of the earth, proclaiming the universal agency of one all-directing, all-sustaining Creator, in whose will and power these harmonious systems originated, and by whose universal providence they are, and have at all times been, maintained.”--_Bridgewater Treatise_, vol. i.
p. 502.
One other fact, showing the ident.i.ty of former zological laws with those which now prevail, must not be omitted. I refer to the existence on the globe in all past periods of organic life of the two great cla.s.ses of carnivorous and herbivorous animals; and they have always existed, too, in about the same proportion. To the harmony and happiness of the present system, we know that the existence and proper relative number of these different cla.s.ses are indispensable. For in order that the greatest possible number of animals that live on vegetable food should exist, they must possess the power of rapid multiplication, so that there should be born a much larger number than is necessary to people the earth. But if there existed no carnivorous races to keep in check this redundancy of population, the world would soon become so filled with the herbivorous races that famine would be the consequence, and thus a much greater amount of suffering result than the sudden death inflicted by carnivorous races now produces. To preserve, then, a proper balance between the different species is, doubtless, the object of the creation of the carnivorous. This system has been aptly denominated ”the police of nature.” And we find it to have always existed. The earliest vertebral animals--the sauroid fishes and sharks--were of this description. The sharks have always lived, but the sauroid fishes became less numerous when other marine saurians were created; and when they both nearly disappeared, during the tertiary period, other predaceous families were introduced, more like those now in existence.
The history of the mollusks, or animals inhabiting sh.e.l.ls, furnishes us with an example still more striking. These animals, as they now exist, are divisible into the two great cla.s.ses of carnivorous and herbivorous species, being distinguished by their anatomical structure; and so has it ever been. In the fossiliferous rocks below the tertiary, we find immense numbers of nautili, ammonites, and other kindred genera of polythalamous sh.e.l.ls, called cephalopods, which were all carnivorous. And when they nearly disappeared with the cretaceous period, there was created another race with carnivorous propensities and organs, called trachelipods; and those continue still to swarm in the ocean. Had they not appeared when the cephalopods pa.s.sed away, the herbivorous tribes would have multiplied to such an extent as ultimately to destroy marine vegetation, and bring on famine among themselves.
These examples are sufficient to prove the existence of the carnivorous and herbivorous races in all ages and in about the same relative numbers.
And it certainly furnishes most decisive evidence of the oneness of all these systems of organic life on the globe.
_In the fifth place, the laws of anatomy have always been the same since organic structures began to exist._
It had long been known that the organs of animals were beautifully adapted to perform the functions for which they were intended. But it was not till the investigations of Baron Cuvier, within the last half century, that it was known how mathematically exact is the relation between the different parts of the animal frame, nor how precise are the laws of variation in the different species, by which they are fitted to different elements, climates, and food. It is now well known, that each animal structure contains a perfect system of correlation, and yet the whole forms a harmonious part of the entire animal system on the globe. But the language of Cuvier himself will best elucidate this subject, so far as it is capable of popular explanation.
”Every organized individual,” says he, ”forms an entire system of its own; all the parts of which mutually correspond, and concur to produce a certain definite purpose, by reciprocal reaction, or by combining towards the same end. Hence none of these separate parts can change their forms without a corresponding change in the other parts of the same animal, and consequently each of these parts, taken separately, indicates all the other parts to which it has belonged. Thus, if the viscera of any animal are so organized as only to be fitted for the digestion of recent flesh, it is also requisite that the jaws should be so constructed as to fit them for devouring prey; the claws must be constructed for seizing and tearing it to pieces; the teeth for cutting and dividing its flesh; the entire system of the limbs, or organs of motion, for pursuing and overtaking it; and the organs of sense, for discovering it at a distance. Nature, also, must have endowed the brain of the animal with instinct sufficient for concealing itself, and for laying plans to catch its necessary victims.
”In order that the jaw may be well adapted for laying hold of objects, it is necessary that its condyle should have a certain form; that the resistance, the moving power, and the fulcrum, should have a certain relative position with respect to each other, and that the temporal muscles should be of a certain size; the hollow, or depression, too, in which these muscles are lodged, must have a certain depth; and the zygomatic arch, under which they pa.s.s, must not only have a certain degree of convexity, but it must be sufficiently strong to support the action of the ma.s.seter.
”To enable the animal to carry of its prey when seized, a corresponding force is requisite in the muscles which elevate the head; and this necessarily gives rise to a determinate form of the vertebr, to which these muscles are attached, and of the occiput into which they are inserted.
”In order that the teeth of a carnivorous animal may be able to cut the flesh, they require to be sharp, more or less so in proportion to the greater or less quant.i.ty of flesh which they have to cut. It is requisite that their roots should be solid and strong, in proportion to the greater quant.i.ty and size of the bones which they have to break to pieces. The whole of these circ.u.mstances must necessarily influence the development and form of all the parts which contribute to move the jaws.
”To enable the claws of a carnivorous animal to seize its prey, a considerable degree of mobility is necessary in their paws and toes, and a considerable strength in the claws themselves. From these circ.u.mstances, there necessarily result certain determinate forms in all the bones of their paws, and in the distribution of the muscles and tendons by which they are moved. The fore arm must possess a certain facility of moving in various directions, and consequently requires certain determinate forms in the bones of which it is composed. As the bones of the fore arm are articulated with the arm bone, or humerus, no change can take place in the form or structure of the former, without occasioning correspondent changes in the form of the latter. The shoulder-blade, also, or scapula, requires a correspondent degree of strength in all animals destined for catching prey, by which it likewise must necessarily have an appropriate form. The play and action of all these parts require certain proportions in the muscles which set them in motion, and the impressions formed by these muscles must still farther determine the form of all these bones.
”After these observations it will easily be seen that similar conclusions may be drawn with respect to the hinder limbs of carnivorous animals, which require particular conformations to fit them for rapidity of motion in general; and that similar considerations must influence the forms and connections of the vertebr and other bones const.i.tuting the trunk of the body, and to fit them for flexibility and readiness of motion in all directions. The bones, also, of the nose, of the orbit, and of the ears, require certain forms and structures to fit them for giving perfection to the senses of smell, sight, and hearing, so necessary to animals of prey.
In short, the shape and structure of the teeth regulate the forms of the condyle, of the shoulder-blade, and the claws, in the same manner as the equation of a curve regulates all its other properties; and as, in regard to a particular curve, all its properties may be ascertained by a.s.suming each separate property as the foundation of a particular equation, in the same manner a claw, a shoulder-blade, a condyle, a leg, an arm bone, or any other bone, separately considered, enables us to discover the description of teeth to which they have belonged; and so, also, reciprocally, we may determine the form of the other bones from the teeth.