On our knowledge of the causes of the phenomena of organic nature
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Nl a Oy sn gy, naa ty CE, 4M, in ain ai i el ee ee - 4 © it" Ss ae -e. —% \7 1% \ Bye? 35 “OE — ZOO PROFESSOR HUXLEY’S LECTURES TO WORKING MEN. ON OUR KNOWLEDGE OF THE CAUSES OF THE PHENOMENA OF ORGANIC NATURE. BY PROFESSOR HUXLEY, F.R.S. LONDON: ROBERT HARDWICKE, 192, PICCADILLY. 1862. ae NOTICE. — Tue Publisher of these interesting Lectures, having made an arrangement for their publication with Mr. J. A. Mays, the Reporter, begs to append the fol- lowing note from Professor Huxley :— “Mr. J. Aldous Mays, who is taking shorthand notes of my ‘ Lectures to Working Men,’ has asked ine to allow him, on his own account, to print those Notes for the use of my audience. I willingly accede to this request, on the understanding that a notice is prefixed to the effect that I have no leisure to revise the Lectures, or to make alterations in them, beyond the correction of any important error in a matter of fact.” Lied, hin e y ml eery tee: oho CONTENTS. eh atlas NO. PAGE I. The Present Condition of Organic Nature oe ine et II. The Past Condition of Organic Nature ... a oe eo II. The Method by which the Causes of the Present and Past Conditions of Organic Nature are to be Discovered — The Origination of Living Beings .., Ae dea 1 IV. The Perpetuation of Living Beings, Hereditary Trans- mission aud Variation .., ss ae Pe See V. The Conditions of Existence as affecting the Perpetua- tion of Living Beings ... oe io Be a se LOR VI. A Critical Examination of the Position of Mr. Darwin’s Work, “On the Origin of Species,” in relation to the complete Theory of the Causes of the Phenomena of Organic Nature... ad 55g rr dig ... 133 vi part i AEN a "ies * AY ain' ese. feens ieee pene a Heth vhs LECTURE IT. THE PRESENT CONDITION OF ORGANIC NATURE. WueEn it was my duty to consider what subject I would select for the six lectures which I shall now have the pleasure of delivering to you, it occurred to me that I could not do better than endeavour to put before you in a true light, or in what I might perhaps with more modesty call, that which I conceive myself to be the true light, the position of a book which has been more praised and more abused, perhaps, than any book which has appeared for some years;—I mean Mr. Darwin’s work on the “ Origin of Species.” That work, I doubt not, many of you have read; for I know the inquiring spirit which is rife among you. At any ‘rate, all of you will have heard of it,—some by one kind of report and some by another kind of report; the attention of all and the curiosity of all have been pro- bably more or less excited on the subject of that work. All I can do, and all I shall attempt to do, is to put before you that kind of judgment which has been formed by a man, who, of course, is liable to judge erroneously ; but at any rate, of one whose business and ~ profession it is to form judgments upon questions of this nature. 6 THE PRESENT CONDITION And here, as it will always happen when dealing with an extensive subject, the greater part of my course—if, indeed, so small a number of lectures can be properly called a course—must be devoted to preliminary matters, or rather to a statement of those facts and of those principles which the work itself dwells upon, and brings more or less directly before us. I have no right to suppose that all or any of you are naturalists; and even if you were, the misconceptions and misunder- standings prevalent even among naturalists on these matters would make it desirable that I should take the course I now propose to take,—that I should start from the beginning,—that I should endeavour to point out what is the existing state of the organic world—that I should point out its past condition,—that I should state what is the precise nature of the undertaking which Mr. Darwin has taken in hand; that I should endeavour to show you what are the only methods by which that undertaking can be brought to an issue, and to point out to you how far the author of the work in question has satisfied those conditions, how far he has not satisfied them, how far they are satisfiable by man, and how far they are not satisfiable by man. And for to-night, in taking up the first part of this question, I shall endeavour to put before you a sort of broad notion of our knowledge of the condition of the living world. There are many ways of doing this. I might deal with it pictorially and graphically. Following the, example of Humboldt in his “ Aspects of Nature,” I might endeavour to point out the infinite variety of organic life in every mode of its existence, with refer- ence to the variations of climate and the like ; and such OF ORGANIC NATURE. 7 an attempt would be fraught with interest to us all; but considering the subject before us, such a course would not be that best calculated to assist us. In an argument of this kind we must go further and dig deeper into the matter; we must endeavour to look into the foundations of living Nature, if I may so say, and discover the principles involved in some of her most secret operations. Few animals can be more familiar to you than that whose skeleton is shown on this diagram. You need not bother yourselves with this “ Hquus caballus”’ written under it; that is only the Latin name of it, and does not make it any better. It simply means the 8 THE PRESENT CONDITION common Horse. Suppose we wish to understand all about the Horse. Our first object must be to study the structure of the animal. The whole of his body is inclosed within a hide, a skin covered with hair; and if that hide or skin be taken off, we find a great mass of flesh, or what is technically called muscle, being the substance which by its power of contraction enables the animal to move. These muscles move the hard parts one upon the other, and so give that strength and power of motion which renders the Horse so useful to us in the performance of those services in which we employ him. And then, on separating and removing the whole of this skin and flesh, you have a great series of bones, hard structures, bound together with ligaments, and forming the skeleton which is represented here. In that skeleton there are a number of parts to be recognized. This long series of bones, beginning from the skull and ending in the tail, is called the spine, and these in front are the ribs ; and then there are two pairs limbs, one before and one behind; and these are what we all know as the fore-legs and the hind-legs. If we pursue our researches into the interior of this animal, we find within the framework of the skeleton a great cavity, or rather, I should say, two great cavities,—one cavity beginning in the skull and running through the neck-bones, along the spine, and ending in the tail, containing the brain and the spinal marrow, which are extremely important organs. The second great cavity, commencing with the mouth, contains the gullet, the stomach, the long intestine, and all the rest of those internal apparatus which are essential for digestion ; OF ORGANIC NATURE. 9 and then in the same great cavity, there are lodged the heart and all the great vessels going from it; and, be- sides that, the organs of respiration—the lungs; and then the kidneys, and the organs of reproduction, and so on. Let us now endeavour to reduce this notion of a horse that we now have, to some such kind of simple ex- pression as can be at once, and without difficulty, retained in the mind, apart from all minor details. If I make a transverse section, that is, if 1 were to saw a dead horse across, [ should find that, if I left out the details, and supposing I took my section through the anterior region, and through the fore-limbs, I should have here this kind of section of the body (Fig. 1). Here would be the upper part of the animal—that great mass of bones that we spoke of as the spine fe Bie: 1.) Here I should have the ali- mentary canal (0, Fig. 1). Here I should have the heart (c, Fig. 1); and then yousee, there »( would be a kind of Sg double tube, the whole being inclosed within the hide; the spinal marrow would be placed in the upper tube (a, Fig. 1), and in the lower tube (0, Fig. 1) there would be the alimentary canal and the heart; and here I shall have the legs proceeding from each side. For simplicity’s sake, I represent them merely as stumps (e e, Fig. 1). Now that is a horse— as mathematicians would say—reduced to its most sim- i0 THE PRESENT CONDITION ple expression. Carry that in your minds, if you please, as a simplified idea of the structure of the Horse. The considerations which I have now put before you belong to what we technically call the ‘ Anatomy’ of the Horse. Now, suppose we go to work upon these several parts,— flesh and hair, and skin and bone, and lay open these various organs with our scalpels, and examine them by means of our magnifying-glasses, and see what we can make of them. We shall find that the flesh is made up of bundles of strong fibres. The brain and nerves, too, we shall find, are made up of fibres, and these queer-looking things that are called ganglionic cor- puscles. If we take a slice of the bone and examine it, we shall find that it is very like this diagram of a section of the bone of an ostrich, though differing, of course, in some details; and if we take any part what- soever of the tissue, and examine it, we shall find it all has a minute structure, visible only under the microscope. All these parts constitute microscopic ana- tomy or ‘ Histology.’ These parts are constantly being changed; every part is constantly growing, decaying, and being replaced during the life of the animal. The ‘tissue is constantly replaced by new material; and if you go back to the young state of the tissue in the case of muscle, or in the case of skin, or any of the organs I have mentioned, you will find that they all come under the same condition. Every one of these microscopic filaments and fibres (I now speak merely of the general character of the whole process)—every one of these parts—could be traced down to some modification of a tissue which can be readily divided into little particles of fleshy matter, of that substance OF ORGANIC NATURE. ll which is composed of the chemical elements, carbon, hydrogen, oxygen, and nitrogen, having such a shape as this (Fig. 2). These particles, into which all primitive tissues break up, are called cells. If I were to make a section of apiece of the skin of my hand, I should find that it was made up of these cells. If I examine the fibres which form the Fie. 2. various organs of all living animals, I should find that all of them, at one time or other, had been formed out of a substance consisting of similar elements; so that you see, just as we reduced the whole body in the gross to that sort of simple expression given in Fig. 1, so we may reduce the whole of the microscopic structural elements to a form of even greater simplicity ; just as the plan of the whole body may be so represented in a sense (Fig. 1), so the primary structure of every tissue may be represented by a mass of cells (Fig. 2). Having thus, in this sort of general way, sketched to you what I may call, perhaps, the architecture of the body of the Horse (what we term technically its Morphology), I must now turn to another aspect. A horse is not a mere dead structure: it is an active, living, working machine. Hitherto we have, as it were, been looking at a steam-engine with the fires out, and nothing in the boiler; but the body of the living animal is a beautifully-formed active machine, and every part has its different work to do in the working of that machine, which is what we call its life. The Horse, if you see him after his day’s work is done, is cropping the grass in the fields, as it may be, or munch- ing the oats in his stable. What is he doing? His 12 THE PRESENT CONDITION jaws are working as a mill—and a very complex mill too—grinding the corn, or crushing the grass to a pulp. As soon as that operation has taken place, the food is passed down to the stomach, and there it is mixed with the chemical fluid called the gastric juice, a substance which has the peculiar property of making soluble and dissolving out the nutritious matter in the grass, and leaving behind those parts which are not nutritious ; so that you have, first, the mill, then a sort of chemical digester; and then the food, thus par- tially dissolved, is carried back by the muscular con- tractions of the intestines into the hinder parts of the body, while the soluble portions are taken up into the blood. The blood is contained in a vast system of pipes, spreading through the whole body, connected with a force-pump,—the heart,—which, by its position and by the contractions of its valves, keeps the blood constantly circulating in one direction, never allowing it to rest; and then, by means of this circulation of the blood, laden as it is with the products of digestion, the skin, the flesh, the hair, and every other part of the body, draws from it that which it wants, and every one of these organs derives those materials which are necessary to enable it to do its work. The action of each of these organs, the performance of each of these various duties, involve in their operation a continual absorption of the matters necessary for their support, from the blood, and a constant formation of waste products, which are returned to the blood, and conveyed by it to the lungs and the kidneys, which are organs that have allotted to them the office of extract- ing, separating, and getting rid of these waste pro- OF ORGANIC NATURE. 13 ducts; and thus the general nourishment, labour, and repair of the whole machine is kept up with order and regularity. But not only is it a’machine which feeds and appropriates to its own support the nourishment necessary to its existence—it is an engine for loco- motive purposes. The Horse desires to go from one place to another; and to enable it to do this, it has those strong contractile bundles of muscles attached to the bones of its limbs, which are put in motion by means of a sort of telegraphic apparatus formed by the brain and the great spinal cord running through the spine or backbone; and to this spinal cord are attached a number of fibres termed nerves, which proceed to all parts of the structure. By means of these the eyes, nose, tongue, and skin—all the organs of perception—transmit impressions or sensations to the brain, which acts as a sort of great central tele- graph-office, receiving impressions and sending mes sages to all parts of the body, and putting in motion the muscles necessary to accomplish any movement that may be desired. So that here you have an ex- tremely complex and beautifully-proportioned machine with all its parts working harmoniously together towards one common object—the preservation of the life of the animal. Now, note this: the Horse makes up its waste by feeding, and its food is grass or oats, or perhaps other vegetable products; therefore, in the long run, the source of all this complex machinery lies in the vege- table kingdom. But where does the grass, or the oat, or any other plant, obtain this nourishing food- producing material? At first it is a little seed, which 14 THE PRESENT CONDITION soon begins to draw into itself from the earth and the surrounding air matters which in themselves contain no vital properties whatever; it absorbs into its own sub- stance water, an inorganic body; it draws into its sub- stance carbonicacid, an inorganic matter; and ammonia, another inorganic matter, found in the air; and then, by some wonderful chemical process, the details of which chemists do not yet understand, though they are near foreshadowing them, it combines them into one substance, which is known to us as ‘Protein, a complex compound of carbon, hydrogen, oxygen, and nitrogen, which alone possesses the property of mani- festing vitality and of permanently supporting animal life. So that, you see, the waste products of the animal economy, the effete materials which are con- tinually being thrown off by all living beings, in the form of organic matters, are constantly replaced by supplies of the necessary repairing and rebuilding materials drawn from the plants, which in their turn manufacture them, so to speak, by a mysterious com- bination of those same inorganic materials. Let us trace out the history of the Horse in another direction. After a certain time, as the result of sick- ness or disease, the effect of accident, or the consequence of old age, sooner or later, the animal dies. The mul- titudinous operations of this beautiful mechanism flag in their performance, the Horse loses its vigour, and after passing through the curious series of changes comprised in its formation and preservation, it finally decays, and ends its life by going back into that inor- ganic world from which all but an inappreciable frac- tion of its substance was derived. Its bones become OF ORGANIC NATURE. 15 mere carbonate and phosphate of lime; the matter of its flesh, and of its other parts, becomes, in the long run, converted into carbonic acid, into water, and into am- monia. You will now, perhaps, understand the curious relation of the animal with the plant, of the organic with the inorganic world, which is shown in this diagram. INORGANIC WORLD. Carbonic Acid. Water. Ammonia. Salines. ies - \ Cael Ps \ ae = : IC \ 74 — i WATER | SALINES VEGETABLE WORLD. (Fig. 3.) ANIMAL WORLD. The plant gathers these inorganic materials together and makes them up into its own substance. The animal eats the plant and appropriates the nutritious portions to its own sustenance, rejects and gets rid of the useless matters; and, finally, the animal itself dies, and its whole body is decomposed and veturned into the inorganic world. There is thus a constant circulation from one to the other, a continual formation of organic life from inorganic matters, and as constant a return of living bodies to the inorganic world; so that the materials of which our bodies are composed are largely, in all probability, the substances which con- stituted the matter of long extinct creations, but which have in the interval constituted a part of the imorganic world. 16 THE PRESENT CONDITION Thus we come to the conclusion, strange at first sight, that the Marrer constituting the living world is identical with that which forms the inorganic world. And not less true is it that, remarkable as are the powers or, in other words, as are the Forczs which are exerted by living beings, yet all these forces are either identical with those which exist in the inorganic world, or they are convertible into them; I mean in just the same sense as the researches of physical philosophers have shown that heat is convertible into electricity, that electricity is convertible into magnetism, magne- tism into mechanical force or chemical force, and any one of them with the other, each being measurable in terms of the other,—even so, I say, that great law is applicable to the living world. Consider why is the skeleton of this horse capable of supporting the masses of flesh and the various organs forming the living body, unless it is because of the action of the same forces of cohesion which combines together the particles of matter composing this piece of chalk? What is there in the muscular contractile power of the animal but the force which is expressible, and which is in a certain sense convertible, into the force of gravity which it overcomes? Or, if you go to more hidden processes, in what does the process of digestion differ from those processes which are carried on in the laboratory of the chemist? Even if we take the most recondite and most complex operations of animal life—those of the nervous system, these of late years have been shown to be—I do not say identical in any sense with the elec- trical processes—but this has been shown, that they are in some way or other associated with them; that is to OF ORGANIC NATURE. 17 say, that every amount of nervous action is accompanied by a certain amount of electrical disturbance in the particles of the nerves in which that nervous action is carried on. In this way the nervous action is related to electricity in the same way that heat is related to electricity ; and the same sort of argument which demonstrates the two latter to be related to one an- other shows that the nervous forces are correlated to electricity ; for the experiments of M. I said just now that the Horse eventually died and became converted into the same inorganic substances from whence all but an inappreciable fraction of its substance demonstrably originated, so that the actual wanderings of matter are as remarkable as the trans- migrations of the soul fabled by Indian tradition. But before death has occurred, in the one sex or the other, and in fact in both, certain products or parts of the organism have been set free, certain parts of the organisms of the two sexes have come into contact 18 THE PRESENT CONDITION with one another, and from that conjunction, from that union which then takes place, there results the forma- tion of a new being. At stated times the mare, from a particular part of the interior of her body, called the evary, gets rid of a minute particle of matter comparable in all essential respects with that which we called a cell a little while since, which cell contains a kind of nucleus in its centre, surrounded by a clear space and by a viscid mass of protein substance (Fig. 2); and though it is different in appearance from the eggs which we are mostly acquainted with, it is really an egg. After a time this minute particle of matter, which may only be a small fraction of a grain in weight, undergoes a series of changes,— wonderful, complex changes. Finally, upon its surface there is fashioned a little elevation, which afterwards becomes divided and marked by a groove. The lateral boundaries of the groove extend upwards and downwards, and at length give rise to a double tube. In the upper smaller tube the spinal marrow and brain are fashioned; in the lower, the alimentary canal and heart, and at length two pairs of buds shoot out at the sides of the body, which are the rudiments of the limbs. In fact a true drawing of a section of the embryo in this state would in all essential respects resemble that diagram of a horse reduced to its simplest expression, which I first placed before you (Fig. 1). Slowly and gradually these changes take place. The whole of the body, at first, can be broken up into “cells,” which become in one place metamorphosed into muscle, —in another place into gristle and bone,—in another place into fibrous tissue,—and in another into hair; every part becoming gradually and slowly fashioned, as OF ORGANIC NATURE. 19 if there were an artificer at work in each of these complex structures that we have mentioned. This embryo, as it is called, then passes into other conditions. This dia- gram represents the embryo of a dog; and I should tell you that there is a time when the embryos of neither dog, nor horse, nor porpoise, nor monkey, nor man, can be distinguished by any essential feature one from the other; there is a time when they each and all of them resemble this one of the Dog. But as development advances, all the parts acquire their speciality, till at length you have the embryo converted into the form of the parent from which it started. So that you see, this living animal, this horse, begins its existence as a minute particle of introgenous matter, which, being supplied with nutriment (derived, as I have shown, from the inorganic world), grows up according to the special type and construction of its parents, works and undergoes a constant waste, and that waste is made good by nutriment derived from the inorganic world ; the waste given off in this way being directly added to the inorganic world; and eventually the animal itself dies, and, by the process of decomposi- tion, its whole body is returned to those conditions of inorganic matter in which its substance origi- nated. This, then, is that which is true of every living form, from the lowest plant to the highest animal—to man himself. You might define the life of every one in exactly the same terms as those which I have now used; the difference between the highest and the lowest being simply in the complexity of the develop- mental changes, the variety of the structural forms, 20 THE PRESENT CONDITION the diversity of the physiological functions which are exerted by each. If I were to take an oak tree as a specimen of the plant world, I should find that it originated in an acorn, which, too, commenced in a cell; the acorn is placed in the ground, and it very speedily begins to absorb the inorganic matters I have named, adds enormously to its bulk, and we can see it, year after year, extending itself upward and downward, attract- ing and appropriating to itself inorganic materials, which it vivifies, and eventually, as it ripens, gives off its own proper acorns, which again run the same course. But I need not multiply examples,—from the highest to the lowest the essential features of life are the same, as I have described in each of these cases. So much, then, for these particular features of the organic world, which you can understand and com- prehend, so long as you confine yourself to one sort of living being, and study that only. But, as you know, horses are not the only living creatures in the world; and again, horses, like all other animals, have certain limits—are confined to a certain area on the surface of the earth on which we live,—and, as that is the simpler matter, I may take that first. In its wild state, and before the discovery of America, when the natural state of things was interfered with by the Spaniards, the Horse was only to be found in parts of the earth which are known to geographers as the Old World; that is to say, you might meet with horses in Europe, Asia, or Africa ; but there were none in Australia, and there were none whatsoever in the whole continent of America, from OF ORGANIC NATURE. 21 Labrador down to Cape Horn. This is an empirical fact, and it is what is called, stated in the way I have given it you, the ‘Geographical Distribution’ of the Horse. Why horses should be found in Europe, Asia, and Africa, and not in America, is not obvious; the expla- nation that the conditions of life in America are un- favourable to their existence, and that, therefore, they had not been created there, evidently does not apply ; for when the invading Spaniards, or our own yeomen farmers, conveyed horses to these countries for their own use, they were found to thrive well and multiply very rapidly ; and many are even now running wild in those countries, and in a perfectly natural condition. Now, suppose we were to do for every animal what we have here done for the Horse,—that is, to mark off and distinguish the particular district or region to which each belonged ; and supposing we tabulated all these results, that would be called the Geographical Distri- bution of animals, while a corresponding study of plants would yield as a result the Geographical Distri- bution of plants. I pass on from that now, as I merely wished to explain to you what I meant by the use of the term ‘Geographical Distribution.’ As I said, there is an- other aspect, and a much more important one, and that is, the relations of the various animals to one another. The Horse is a very well-defined matter-of- fact sort of animal, and we are all pretty familiar with its structure. I dare say it may have struck you, that it resembles very much no other member of the animal kingdom, except perhaps the Zebra or the Ass. Oe THE PRESENT CONDITION But let me ask you to look along these diagrams. Here is the skeleton of the Horse; and here the skeleton of the Dog. You will notice that we have in the Horse a skull, a backbone and ribs, shoulder- blades and haunch-bones. In the fore-limb, one upper arm-bone, two fore arm-bones, wrist-bones (wrongly called knee), and middle hand-bones, ending in the three bones of a finger, the last of which is sheathed in the horny hoof of the fore-foot: in the hind-limb, one thigh-bone, two leg-bones, ankle-bones, and middle foot-bones, ending in the three bones of a toe, the last of which is encased in the hoof of the hind-foot. Now turn to the Dog’s skeleton. We find identically the same bones, but more of them, there being more toes in each foot, and hence more toe-bones. Well, that is a very curious thing! The fact is that the Dog and the Horse—when one gets a look at them without the outward impediments of the skin— are found to be made in very much the same sort of fashion. And if I were to make a transverse section of the Dog, I should find the same organs that I have already shown you as forming parts of the Horse. Well, here is another skeleton—that of a kind of Lemur —yousee he has just the same bones; and if I were to make a transverse section of it, it would be just the same again. In your mind’s eye turn him round, so as to put his backbone in a position inclined obliquely upwards and forwards, just as in the next three dia- grams, which represent the skeletons of an Orang, a Chimpanzee, a Gorilla, and you find you have no trouble in identifying the bones throughout ; and lastly turn to OF ORGANIC NATURE. 23 the end of the series, the diagram representing a man’s skeleton, and still you find no great structural feature essentially altered. There are the same bones in the same relations. From the Horse we pass on and on, with gradual steps, until we arrive at last at the highest known forms. On the other hand, take the other line of dia- grams, and pass from the Horse downwards in the scale to this fish; and still, though the modifications are vastly greater, the essential framework of the organization remains unchanged. Here, for instance, is a Porpoise ; here is its strong backbone, with the cavity running through it, which contains the spinal cord; here are the ribs, here the shoulder-blade; here is the little short upper-arm bone, here are the two forearm bones. the wrist-bone, and the finger-bones. Strange, is it not, that the Porpoise should have in this queer-looking affair—its flapper (as it is called), the same fundamental elements as the fore-leg of the Horse or the Dog, or the Ape or Man; and here you will notice a very curious thing,—the hinder limbs are absent. Now, let us make another jump. Let us go to the Codfish: here you see is the forearm, in this large pectoral fin—carrying your mind’s eye onward from the flapper of the Porpoise. And here you have the hinder limbs restored in the shape of these ventral fins. If I were to make a transverse section of this, I should find just the same organs that we have before noticed. So that, you see, there comes out this strange conclusion as the result of our investigations, that the Horse, when examined and compared with other ani- mals, is found by no means to stand alone in nature ; but that there are an enormous number of other 24 THE PRESENT CONDITION creatures which have backbones, ribs, and legs, and other parts arranged inthe same general manner, and in all their formation exhibiting the same broad pecu- liarities. Iam sure that you cannot have followed me even in this extremely elementary exposition of the struc- tural relations of animals, without seeing what I have been driving at all through, which is, to show you that, step by step, naturalists have come to the idea of a unity of plan, or conformity of construction, among animals which appeared at first sight to be extremely dissimilar. And here you have evidence of such a unity of plan among all the animals which have backbones, and which we technically call Vertebrata. But there are multitudes of other animals, such as crabs, lobsters, spiders, and so on, which we term Annulosa. In these I could not point out to you the parts that correspond with those of the Horse,--the backbone, for instance,— as they are constructed upon a very different prin- ciple, which is also common to all of them; that is to say, the Lobster, the Spider, and the Centipede, have a common plan running through their whole arrange- ment, in just the same way that the Horse, the Dog, and the Porpoise assimilate to each other. Yet other creatures—whelks, cuttlefishes, oysters, snails, and all their tribe (Mollusca)—resemble one another in the same way, but differ from both Verte- brata and Annulosa ; and the like is true of the animals called Coelenterata (Polypes) and Protozoa (animalcules and sponges). Now, by pursuing this sort of comparison, naturalists OF ORGANIC NATURE. 25 have arrived at the conviction that there are,—some think five, and some seven,—but certainly not more than the latter number—and perhaps it is simpler to assume five —distinct plans or constructions in the whole of the animal world; and that the hundreds of thousands of species of creatures on the surface of the earth, are all reducible to those five, or, at most, seven, plans of organization. But can we go no further than that? When one has got so far, one is tempted to go on a step and in- quire whether we cannot go back yet further and bring down the whole to modifications of one primordial unit. The anatomist cannot do this; but if he call to his aid the study of development, he can do it. For we shall find that, distinct as those plans are, whether it bea porpoise or man, or lobster, or any of those other kinds I have mentioned, every one begins its existence with one and the same primitive form,—that of the egg, consisting, as we have seen, of a introgenous substance, having a small particle or nucleus in the centre of it. _ Furthermore, the earlier changes of each are sub- stantially the same. And it is in this that lies that true “unity of organization” of the animal kingdom which has been guessed at and fancied for many years ; but which it has been left to the present time to be demonstrated by the careful study of develop- ment. But is it possible to go another step further still, and to show that in the same way the whole of the organic world is reducible to one primitive con- dition of form? Is there among the plants the same primitive form of organization, and is that identical with that of the animal kingdom? The reply to that 26 THE PRESENT CONDITION question, too, is not uncertain or doubtful. It is now proved that every plant begins its existence under the same form; that is to say, in that of a cell—a particle of introgenous matter having substantially the same conditions. So that if you trace back the oak to its first germ, or a man, or a horse, or lobster, or oyster, or any other animal you choose to name, you shall find each and all of these commencing their existence in forms essentially similar to each other: and, furthermore, that the first processes of growth, and many of the subsequent modifications, are essentially the same in principle in almost all. 