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View of the Relations Which Exist amongst the Variations of the Several Organs
The preceding Article has pointed out the principal differences of which the organs, belonging to each animal function, are susceptible in their structure and operations. The number of these differences would have been much greater had we entered into details, and descended to the less important circumstances.
It is obvious, however, from the manner in which we have described them, that by supposing each of the differences of one organ united successively with those of every other, there would be produced a very considerable number of combinations, which would correspond with as many classes of animals. But these combinations which appear possible, when we consider them abstractedly, do not all exist in nature; because, in a state of life, the organs do not simply join their effects but act on each other, and concur altogether to one common object. Hence the modifications of any one of them exercise an influence on those of every other. Such of these modifications as cannot exist together, reciprocally exclude one another, while others are, as it were, called into the system; and this takes place, not only in the organs which have an immediate connexion, but in those which at the first view appear the most separate and independent.
In fact there is not one function which does not stand in need of the concurrence of almost all the others, and which is not more or less affected by their degree of energy.
Respiration, for example, cannot take place without the aid of the motion of the blood, since it consists in bringing that fluid in contact with the surrounding element; but as it is circulation that gives motion to the blood, it therefore is a necessary mean in producing respiration.
Circulation itself has its cause in the muscular action of the heart and arteries: it is produced, therefore, by the aid of irritability. That faculty, in its turn, derives its origin from the nervous fluid, and, consequently, from the function of sensibility which returns, by a kind of circle to the circulation of the blood, which is the cause of all the secretions, and of that of the nervous fluid as well as others.
Of what value would sensibility be, were it not aided by the muscular force, even in the most trifling circumstances? What would be the utility of the sense of feeling, were we not able to turn our hands towards palpable objects? And what would be the advantage of seeing, if we could not turn the head or eyes in every direction?
It is on this mutual dependence of the functions, and the aid they reciprocally yield to one another, that the laws which determine the relations of their organs are founded laws which have their origin in a necessity equal to that of metaphysical or mathematical laws: for it is evident that a suitable harmony between organs which act on one another, is a necessary condition of the existence of the being to which they belong; and that if any one of the functions were modified in a manner incompatible with the regulations of the others, that being could not exist.
Thus, among the animals that have blood-vessels, and enjoy a double circulation, those which respire the air by receiving it immediately into the cellular lungs, have always the two trunks of their arteries approximated, and furnished with muscular ventricles, but joined together in one mass; while those which respire only through the medium of water that passes between the folds of their branchiae, have always two separate trunks, whether they be both provided with ventricles as the Sepia [Squid] or have a ventricle for one only, like fishes and mollusca. ...
The nervous system has likewise its relations to respiration with respect to the varieties observed in both those functions. The external senses have much less energy, and the brain is considerably smaller in the animals that have cold blood, in which that organ occupies only a small part of the cranium, than in those of warm blood, in which the brain fills the whole cavity. Doubtless, the little irritability of the fibre in those animals requires but a small degree of activity in the organs that put it in motion: lively sensations and strong passions would have too much exhausted their muscular force. In this manner the organs of sensation are immediately connected with those of respiration.
But to what secret cause is it owing, that in all the animals which respire by distinct organs, the medullary masses form a small number, and are collected in the cranium, or, at least detached from the spinal marrow, while in those that respire by trachiae, nearly equal ganglions are distributed throughout the whole extent of their nervous cord? How does it happen, too, that there is no nervous system apparent in animals which have no organs particularly designed for respiration? These two relations must be included amongst those whose causes are unknown to us.
Digestion, also, has its connection with respiration: the latter being one of the functions which consume and expel, with the greatest rapidity, the substances of which the body is composed, the digestive power is generally the greater in proportion as respiration is more complete, in order that the quantity which is acquired may be equal to that which escapes.
In consequence of the connection, that subsists between the organs of respiration and the modifications of several other functions, some of the latter have relations to one another which at first sight did not appear necessary. This is the reason why birds have in general an exceedingly strong stomach, and a very quick digestion. This also is the reason why their repasts are so frequently repeated; while reptiles, which among the red-blooded animals seem to be contrasted to them in every respect, astonish us by the little aliment they take, and the length of time they abstain from food. These differences in the digestive powers do not depend upon the nature of the organs of motion which characterize these two classes, but upon that of the organs of respiration, the modifications of which have an immediate relation with those of motion.
It is easy to perceive that these two very different degrees of digestive powers depend on two dispositions equally different in the alimentary organs, and that each of these dispositions must be co-existent with a corresponding one in the respiratory organs. The latter also being always connected with a disposition equally determined in the organs of motion, in those of sensation, and in those of circulation, each of those five systems of organs may be said to regulate and govern the others.
The system of digestive organs has also immediate relations with those of motion and sensation. The disposition of the alimentary canal determines, in a manner perfectly absolute, the kind of food by which the animal is nourished; but if the animal did not possess, in its senses and organs of motion, the means of distinguishing the kinds of aliment suited to its nature, it is obvious it could not exist.
An animal, therefore, which can only digest flesh, must, to preserve its species, have the power of discovering its prey, of pursuing it, of seizing it, of overcoming it, and tearing it in pieces. It is necessary, then, that this animal should have a penetrating eye, a quick small, a swift motion, address, and strength in the claws and in the jaws. Agreeably to this necessity, a sharp tooth, fitted for cutting flesh, is never coexistent in the same species, with a foot covered with horn, which can only support the animal, but with which it cannot grasp any thing; hence the law by which all hoofed animals are herbivorous; and also those still more detailed laws which are but corollaries of the first, that hoofs indicate molar teeth, with flat crowns, a long alimentary canal, a capacious or multiplied stomach, and several other relations of the same kind.
Those laws which determine the relations of the organs belonging to the different functions, likewise exercise their powers on the different parts of the same system, and connect its variations with equal force. The application of these laws is particularly evident in the alimentary system, the parts of which are more numerous and distinct. The form of the teeth, the length, the convolutions, and the dilatations of the alimentary canal, and the number and abundance of the dissolving liquors poured into it, have always an admirable relation to each other, and to the nature, the hardness, and the solubility of the substances the animal eats. This connection is so evident, that the skillful anatomist, upon knowing one of those parts, may easily conjecture most of the others, and may, agreeably to the preceding laws, even guess the extent of the other functions.
The same harmony exists between all parts of the system of the organs of motion; as each of those organs acts upon the rest, and experiences their action in its turn, particularly when the animal is completely in motion, all their forms have relation to one another. Not a bone is varied in itssurfaces, in its curvatures, or in its eminences, without subjecting the other bones to proportionate variations: we may, therefore, on the view of one of them, form, with a certain degree of accuracy, an idea of the whole skeleton.
These laws of co-existence, which we have thus far pointed out, may be said to be reduced by reasoning from the knowledge we have of the reciprocal uses and functions of each organ. Observation having confirmed these laws, we are authorized to follow an opposite course under other circumstances; when, therefore, we observe constant relations of form, between certain organs, we may conclude that they exercise some influence on one another, and we may even make pretty accurate conjectures as to the uses of both ... . It is only by a profound study of those relations,and by the discovery of those which have hitherto escaped our observation, that we can hope to extend physiology. Comparative anatomy may, therefore, be regarded as one of the richest sources of observation for perfecting that important branch of knowledge.
Nature never oversteps the bounds which the necessary conditions of existence prescribe to her; but whenever she is unconfined by these conditions, she displays all her fertility and variety. Never departing from the small number of combinations that are possible, between the essential modifications of important organs, she seems to sport with infinite caprice in all the accessory parts. In these there appears no necessity for a particular form or disposition. It even frequently happens, that particular forms and dispositions are created without any apparent view to utility. It seems sufficient that they should be possible, that is to say, that they do not destroy the harmony of the whole. In proportion, therefore, as we turn our attention from the principal organs to those which are less important, we discover increasing variations; and when we arrive at the surface of bodies where the nature of things requires that the parts least essential, and the injury of which is least dangerous, should be placed, the number of varieties becomes so considerable, that all the labours of naturalists have not yet been able to give us an account of them.
Among these numerous combinations there are necessarily many which have common parts, and there is always a certain number which exhibits very few differences; by the comparison therefore of those which resemble each other, we may establish a kind of series, which will appear to descend gradually from a primitive type. These considerations are the foundations of the ideas from which certain naturalists have formed A Scale of Being, the object of which is to exhibit the whole in one series, commencing with the most simple kind of organization—with that which possesses the least numerous and most common properties; so that the mind passes from one link of the chain to the other, almost without perceiving any interval, and, as it were, by insensible shades.
Indeed, when we confine ourselves within certain limits, and particularly when we consider each organ separately, and follow it through all the species of one class, we observe that its progression in the scale is preserved with a singular regularity; we even perceive the organ partially, or some vestige of it in species, in which it is not longer of any use; so that Nature seems to have left it there only to show how strictly she adheres to the law of doing nothing by sudden transitions: but, on the one hand, the organs do not all follow the same order of gradation; one is found in its highest degree of perfection in one species, while another is most perfect in a species altogether different. If, therefore, we were to class the different species according to each organ considered separately, it would be necessary to form as many series as we should adopt regulating organs; and to make a general scale of perfection, it would be necessary to calculate the effect resulting from each combination. This, however, is far from being practicable.
On the other hand, the gentle and insensible shades of gradation prevail so long as we confine ourselves to the same combination of the principal organs, and so long as the great central springs remain the same. All the animals in which this takes place, seem to be formed upon one common plan, which serves as the basis of all the little external modifications: but the moment we turn our attention to those animals in which other principal combinations take place, there is no longer any resemblance, and an interval or marked transition is obvious to every one.
Whatever arrangement may be given to vertebral animals, and those which have no vertebrae, we never shall succeed in placing at the bottom of one of those great classes, and at the head of the other, two animals which sufficiently resemble each other to serve as a link between them.
Translated by William Ross
Reading and Discussion Questions
1.What does Cuvier mean in stating that the parts of animals are “correlated”? What limitations does this place on the degree to which an animal can vary? What kinds of things can change, and what cannot?
2.How can we use comparison of parts between animals to assist in our efforts at classification? What does he mean by saying that most animals are constructed on a “common plan”?