THE PRESENT CONDITION OF ORGANIC NATURE

Lecture I. (of VI. ), "Lectures To Working Men", at the Museum of Practical Geology, 1863, On Darwin's work: "Origin of Species". 

By Thomas H. Huxley

EDITOR'S NOTE

Of the great thinkers of the nineteenth century, Thomas Henry Huxley, son of an Ealing schoolmaster, was undoubtedly the most noteworthy. Hisresearches in biology, his contributions to scientific controversy, hispungent criticisms of conventional beliefs and thoughts have probablyhad greater influence than the work of any other English scientist. Andyet he was a "self-made" intellectualist. In spite of the fact thathis father was a schoolmaster he passed through no regular course ofeducation. "I had, " he said, "two years of a pandemonium of a school(between eight and ten) and after that neither help nor sympathy in anyintellectual direction till I reached manhood. " When he was twelve acraving for reading found satisfaction in Hutton's "Geology, " and whenfifteen in Hamilton's "Logic. "

At seventeen Huxley entered as a student at Charing Cross Hospital, andthree years later he was M. B. And the possessor of the gold medal foranatomy and physiology. An appointment as surgeon in the navy proved tobe the entry to Huxley's great scientific career, for he was gazetted tothe "Rattlesnake", commissioned for surveying work in Torres Straits. Hewas attracted by the teeming surface life of tropical seas and his studyof it was the commencement of that revolution in scientific knowledgeultimately brought about by his researches. 

Thomas Henry Huxley was born at Ealing on May 4, 1825, and died atEastbourne June 29, 1895. 

LECTURES AND ESSAYS BY T. H. HUXLEY

ON OUR KNOWLEDGE OF THE CAUSES OF THE PHENOMENA OF ORGANIC NATURE

NOTICE TO THE FIRST EDITION. 

The Publisher of these interesting Lectures, having made an arrangementfor their publication with Mr. J. A. Mays, the Reporter, begs to appendthe following note from Professor Huxley:--

"Mr. J. Aldous Mays, who is taking shorthand notes of my 'Lectures toWorking Men, ' has asked me to allow him, on his own account, to printthose Notes for the use of my audience. I willingly accede to thisrequest, on the understanding that a notice is prefixed to the effectthat I have no leisure to revise the Lectures, or to make alterations inthem, beyond the correction of any important error in a matter of fact. "

THE PRESENT CONDITION OF ORGANIC NATURE. 

When it was my duty to consider what subject I would select for the sixlectures [*To Working Men, at the Museum of Practical Geology, 1863. ]which I shall now have the pleasure of delivering to you, it occurred tome that I could not do better than endeavour to put before you in a truelight, or in what I might perhaps with more modesty call, that which Iconceive myself to be the true light, the position of a book which hasbeen more praised and more abused, perhaps, than any book which hasappeared for some years;--I mean Mr. Darwin's work on the "Origin ofSpecies". That work, I doubt not, many of you have read; for I know theinquiring spirit which is rife among you. At any rate, all of you willhave heard of it, --some by one kind of report and some by another kindof report; the attention of all and the curiosity of all have beenprobably more or less excited on the subject of that work. All I cando, and all I shall attempt to do, is to put before you that kind ofjudgment which has been formed by a man, who, of course, is liableto judge erroneously; but, at any rate, of one whose business andprofession it is to form judgments upon questions of this nature. 

And here, as it will always happen when dealing with an extensivesubject, the greater part of my course--if, indeed, so small a number oflectures can be properly called a course--must be devoted to preliminarymatters, or rather to a statement of those facts and of those principleswhich the work itself dwells upon, and brings more or less directlybefore us. I have no right to suppose that all or any of youare naturalists; and even if you were, the misconceptions andmisunderstandings prevalent even among naturalists on these matterswould make it desirable that I should take the course I now propose totake, --that I should start from the beginning, --that I should endeavourto point out what is the existing state of the organic world, --that Ishould point out its past condition, --that I should state what is theprecise nature of the undertaking which Mr. Darwin has taken in hand;that I should endeavour to show you what are the only methods by whichthat undertaking can be brought to an issue, and to point out to you howfar 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. 

To-night, in taking up the first part of this question, I shallendeavour to put before you a sort of broad notion of our knowledge ofthe condition of the living world. There are many ways of doing this. Imight deal with it pictorially and graphically. Following the example ofHumboldt in his "Aspects of Nature", I might endeavour to point out theinfinite variety of organic life in every mode of its existence, withreference to the variations of climate and the like; and such an attemptwould be fraught with interest to us all; but considering the subjectbefore 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 thematter; we must endeavour to look into the foundations of living Nature, if I may so say, and discover the principles involved in some of hermost secret operations. I propose, therefore, in the first place, totake some ordinary animal with which you are all familiar, and, byeasily comprehensible and obvious examples drawn from it, to show whatare the kind of problems which living beings in general lay before us;and I shall then show you that the same problems are laid open to us byall kinds of living beings. But first, let me say in what sense I haveused the words "organic nature. " In speaking of the causes which leadto our present knowledge of organic nature, I have used it almost as anequivalent of the word "living, " and for this reason, --that in almostall living beings you can distinguish several distinct portions setapart to do particular things and work in a particular way. These aretermed "organs, " and the whole together is called "organic. " And as itis universally characteristic of them, this term "organic" has been veryconveniently employed to denote the whole of living nature, --the wholeof the plant world, and the whole of the animal world. 

Few animals can be more familiar to you than that whose skeleton isshown on our diagram. You need not bother yourselves with this "Equuscaballus" written under it; that is only the Latin name of it, and doesnot make it any better. It simply means the common Horse. Suppose wewish to understand all about the Horse. Our first object must be tostudy the structure of the animal. The whole of his body is inclosedwithin a hide, a skin covered with hair; and if that hide or skin betaken off, we find a great mass of flesh, or what is technically calledmuscle, being the substance which by its power of contraction enablesthe animal to move. These muscles move the hard parts one upon theother, and so give that strength and power of motion which renders theHorse so useful to us in the performance of those services in which weemploy 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 withligaments, and forming the skeleton which is represented here. 

[Illustration: FIGURE 1. (Section through a horse. )]

[Illustration: FIGURE 2. (Section through a cell. )]

In that skeleton there are a number of parts to be recognized. The longseries of bones, beginning from the skull and ending in the tail, iscalled the spine, and those in front are the ribs; and then there aretwo pairs of limbs, one before and one behind; and there are what weall know as the fore-legs and the hind-legs. If we pursue our researchesinto the interior of this animal, we find within the framework ofthe skeleton a great cavity, or rather, I should say, two greatcavities, --one cavity beginning in the skull and running through theneck-bones, along the spine, and ending in the tail, containing thebrain and the spinal marrow, which are extremely important organs. Thesecond great cavity, commencing with the mouth, contains the gullet, the stomach, the long intestine, and all the rest of those internalapparatus which are essential for digestion; and then in the same greatcavity, there are lodged the heart and all the great vessels going fromit; and, besides that, the organs of respiration--the lungs: and thenthe kidneys, and the organs of reproduction, and so on. Let us nowendeavour to reduce this notion of a horse that we now have, tosome such kind of simple expression as can be at once, and withoutdifficulty, retained in the mind, apart from all minor details. IfI make a transverse section, that is, if I were to saw a dead horseacross, I should find that, if I left out the details, and supposing Itook 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 wouldbe the upper part of the animal--that great mass of bones that we spokeof as the spine (a, Fig. 1). Here I should have the alimentary canal(b, Fig. 1). Here I should have the heart (c, Fig. 1); and then you see, there would be a kind of double tube, the whole being inclosed withinthe hide; the spinal marrow would be placed in the upper tube (a, Fig. 1), and in the lower tube (d d, Fig. 1), there would be the alimentarycanal (b), and the heart (c); and here I shall have the legs proceedingfrom 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--reducedto its most simple expression. Carry that in your minds, if you please, as a simplified idea of the structure of the Horse. The considerationswhich 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 severalparts, --flesh and hair, and skin and bone, and lay open thesevarious organs with our scalpels, and examine them by means of ourmagnifying-glasses, and see what we can make of them. We shall find thatthe 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-lookingthings that are called ganglionic corpuscles. If we take a slice of thebone and examine it, we shall find that it is very like this diagramof a section of the bone of an ostrich, though differing, of course, in some details; and if we take any part whatsoever of the tissue, andexamine it, we shall find it all has a minute structure, visible onlyunder the microscope. All these parts constitute microscopic anatomyor 'Histology. ' These parts are constantly being changed; every part isconstantly growing, decaying, and being replaced during the life of theanimal. The tissue is constantly replaced by new material; and if you goback to the young state of the tissue in the case of muscle, or in thecase of skin, or any of the organs I have mentioned, you will find thatthey all come under the same condition. Every one of these microscopicfilaments and fibres (I now speak merely of the general character of thewhole process)--every one of these parts--could be traced down tosome modification of a tissue which can be readily divided into littleparticles of fleshy matter, of that substance which is composed of thechemical elements, carbon, hydrogen, oxygen, and nitrogen, having sucha shape as this (Fig. 2). These particles, into which all primitivetissues 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 thesecells. If I examine the fibres which form the various organs of allliving animals, I should find that all of them, at one time or other, had been formed out of a substance consisting of similar elements; sothat you see, just as we reduced the whole body in the gross to thatsort of simple expression given in Fig. 1, so we may reduce the wholeof the microscopic structural elements to a form of even greatersimplicity; just as the plan of the whole body may be so representedin a sense (Fig. 1), so the primary structure of every tissue may berepresented by a mass of cells (Fig. 2). 

Having thus, in this sort of general way, sketched to you what I maycall, perhaps, the architecture of the body of the Horse (what we termtechnically its Morphology), I must now turn to another aspect. A horseis 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 thefires out, and nothing in the boiler; but the body of the living animalis a beautifully-formed active machine, and every part has its differentwork to do in the working of that machine, which is what we callits life. The Horse, if you see him after his day's work is done, iscropping the grass in the fields, as it may be, or munching the oats inhis stable. What is he doing? His jaws are working as a mill--and a verycomplex mill too--grinding the corn, or crushing the grass to a pulp. Assoon as that operation has taken place, the food is passed down tothe stomach, and there it is mixed with the chemical fluid called thegastric juice, a substance which has the peculiar property of makingsoluble and dissolving out the nutritious matter in the grass, andleaving 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 partially dissolved, is carried back by the muscular contractionsof the intestines into the hinder parts of the body, while the solubleportions are taken up into the blood. The blood is contained in a vastsystem of pipes, spreading through the whole body, connected with aforce pump, --the heart, --which, by its position and by the contractionsof its valves, keeps the blood constantly circulating in one direction, never allowing it to rest; and then, by means of this circulation ofthe blood, laden as it is with the products of digestion, the skin, theflesh, the hair, and every other part of the body, draws from it thatwhich it wants, and every one of these organs derives those materialswhich are necessary to enable it to do its work. 

The action of each of these organs, the performance of each of thesevarious duties, involve in their operation a continual absorption ofthe matters necessary for their support, from the blood, and a constantformation of waste products, which are returned to the blood, andconveyed by it to the lungs and the kidneys, which are organs that haveallotted to them the office of extracting, separating, and getting ridof these waste products; and thus the general nourishment, labour, andrepair of the whole machine is kept up with order and regularity. Butnot only is it a machine which feeds and appropriates to its ownsupport the nourishment necessary to its existence--it is an engine forlocomotive purposes. The Horse desires to go from one place to another;and to enable it to do this, it has those strong contractile bundles ofmuscles attached to the bones of its limbs, which are put in motion bymeans of a sort of telegraphic apparatus formed by the brain and thegreat spinal cord running through the spine or backbone; and to thisspinal cord are attached a number of fibres termed nerves, which proceedto all parts of the structure. By means of these the eyes, nose, tongue, and skin--all the organs of perception--transmit impressionsor sensations to the brain, which acts as a sort of great centraltelegraph-office, receiving impressions and sending messages to allparts of the body, and putting in motion the muscles necessary toaccomplish any movement that may be desired. So that you have here anextremely complex and beautifully-proportioned machine, with all itsparts working harmoniously together towards one common object--thepreservation of the life of the animal. 

Now, note this: the Horse makes up its waste by feeding, and its foodis grass or oats, or perhaps other vegetable products; therefore, in thelong run, the source of all this complex machinery lies in the vegetablekingdom. But where does the grass, or the oat, or any other plant, obtain this nourishing food-producing material? At first it is a littleseed, which soon begins to draw into itself from the earth and thesurrounding air matters which in themselves contain no vital propertieswhatever; it absorbs into its own substance water, an inorganic body;it draws into its substance carbonic acid, an inorganic matter; andammonia, another inorganic matter, found in the air; and then, by somewonderful chemical process, the details of which chemists do not yetunderstand, though they are near foreshadowing them, it combinesthem into one substance, which is known to us as 'Protein, ' a complexcompound of carbon, hydrogen, oxygen, and nitrogen, which alonepossesses the property of manifesting vitality and of permanentlysupporting animal life. So that, you see, the waste products of theanimal economy, the effete materials which are continually being thrownoff by all living beings, in the form of organic matters, are constantlyreplaced by supplies of the necessary repairing and rebuilding materialsdrawn from the plants, which in their turn manufacture them, so tospeak, by a mysterious combination of those same inorganic materials. 

Let us trace out the history of the Horse in another direction. Aftera certain time, as the result of sickness or disease, the effect ofaccident, or the consequence of old age, sooner or later, the animaldies. The multitudinous operations of this beautiful mechanism flag intheir performance, the Horse loses its vigour, and after passingthrough the curious series of changes comprised in its formation andpreservation, it finally decays, and ends its life by going back intothat inorganic world from which all but an inappreciable fraction of itssubstance was derived. Its bones become mere carbonate and phosphate oflime; the matter of its flesh, and of its other parts, becomes, in thelong run, converted into carbonic acid, into water, and into ammonia. You will now, perhaps, understand the curious relation of the animalwith the plant, of the organic with the inorganic world, which is shownin this diagram (Fig. 3). 

[Illustration: FIGURE 3. (Diagram showing material relationship of theVegetable, Animal and Inorganic Worlds. )]

The plant gathers these inorganic materials together and makes them upinto its own substance. The animal eats the plant and appropriates thenutritious portions to its own sustenance, rejects and gets rid of theuseless matters; and, finally, the animal itself dies, and its wholebody is decomposed and returned into the inorganic world. There is thusa constant circulation from one to the other, a continual formation oforganic life from inorganic matters, and as constant a return of thematter of living bodies to the inorganic world; so that the materialsof which our bodies are composed are largely, in all probability, thesubstances which constituted the matter of long extinct creations, butwhich have in the interval constituted a part of the inorganic world. 

Thus we come to the conclusion, strange at first sight, that the MATTERconstituting the living world is identical with that which forms theinorganic world. And not less true is it that, remarkable as are thepowers or, in other words, as are the FORCES which are exerted by livingbeings, yet all these forces are either identical with those which existin the inorganic world, or they are convertible into them; I mean injust the same sense as the researches of physical philosophers haveshown that heat is convertible into electricity, that electricity isconvertible into magnetism, magnetism into mechanical force or chemicalforce, and any one of them with the other, each being measurable interms of the other, --even so, I say, that great law is applicable tothe living world. Consider why is the skeleton of this horse capable ofsupporting the masses of flesh and the various organs forming the livingbody, unless it is because of the action of the same forces of cohesionwhich combines together the particles of matter composing this piece ofchalk? What is there in the muscular contractile power of the animalbut the force which is expressible, and which is in a certain senseconvertible, into the force of gravity which it overcomes? Or, if you goto more hidden processes, in what does the process of digestion differfrom those processes which are carried on in the laboratory of thechemist? Even if we take the most recondite and most complex operationsof animal life--those of the nervous system, these of late yearshave been shown to be--I do not say identical in any sense with theelectrical processes--but this has been shown, that they are in someway or other associated with them; that is to say, that every amountof nervous action is accompanied by a certain amount of electricaldisturbance in the particles of the nerves in which that nervous actionis carried on. In this way the nervous action is related to electricityin the same way that heat is related to electricity; and the same sortof argument which demonstrates the two latter to be related to oneanother shows that the nervous forces are correlated to electricity; forthe experiments of M. Dubois Reymond and others have shown that whenevera nerve is in a state of excitement, sending a message to the musclesor conveying an impression to the brain, there is a disturbance of theelectrical condition of that nerve which does not exist at other times;and there are a number of other facts and phenomena of that sort; sothat we come to the broad conclusion that not only as to living matteritself, but as to the forces that matter exerts, there is a closerelationship between the organic and the inorganic world--the differencebetween them arising from the diverse combination and disposition ofidentical forces, and not from any primary diversity, so far as we cansee. 

I said just now that the Horse eventually died and became convertedinto the same inorganic substances from whence all but an inappreciablefraction of its substance demonstrably originated, so that the actualwanderings of matter are as remarkable as the transmigrations of thesoul fabled by Indian tradition. But before death has occurred, in theone sex or the other, and in fact in both, certain products or parts ofthe organism have been set free, certain parts of the organisms ofthe two sexes have come into contact with one another, and from thatconjunction, from that union which then takes place, there results theformation of a new being. At stated times the mare, from a particularpart of the interior of her body, called the ovary, gets rid of a minuteparticle of matter comparable in all essential respects with that whichwe called a cell a little while since, which cell contains a kind ofnucleus in its centre, surrounded by a clear space and by a viscid massof protein substance (Fig. 2); and though it is different in appearancefrom 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 asmall fraction of a grain in weight, undergoes a series ofchanges, --wonderful, complex changes. Finally, upon its surface thereis fashioned a little elevation, which afterwards becomes divided andmarked by a groove. The lateral boundaries of the groove extend upwardsand downwards, and at length give rise to a double tube. In the uppersmaller tube the spinal marrow and brain are fashioned; in the lower, the alimentary canal and heart; and at length two pairs of buds shootout at the sides of the body, which are the rudiments of the limbs. Infact a true drawing of a section of the embryo in this state would inall essential respects resemble that diagram of a horse reduced to itssimplest 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 placemetamorphosed into muscle, --in another place into gristle and bone, --inanother place into fibrous tissue, --and in another into hair; every partbecoming gradually and slowly fashioned, as if there were an artificerat work in each of these complex structures that we have mentioned. Thisembryo, as it is called, then passes into other conditions. I shouldtell you that there is a time when the embryos of neither dog, norhorse, nor porpoise, nor monkey, nor man, can be distinguished by anyessential feature one from the other; there is a time when they each andall of them resemble this one of the Dog. But as development advances, all the parts acquire their speciality, till at length you have theembryo converted into the form of the parent from which it started. Sothat you see, this living animal, this horse, begins its existence asa minute particle of nitrogenous matter, which, being supplied withnutriment (derived, as I have shown, from the inorganic world), grows upaccording to the special type and construction of its parents, worksand undergoes a constant waste, and that waste is made good by nutrimentderived from the inorganic world; the waste given off in this way beingdirectly added to the inorganic world; and eventually the animal itselfdies, and, by the process of decomposition, its whole body is returnedto those conditions of inorganic matter in which its substanceoriginated. 

This, then, is that which is true of every living form, from the lowestplant to the highest animal--to man himself. You might define the lifeof 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 thecomplexity of the developmental changes, the variety of the structuralforms, the diversity of the physiological functions which are exerted byeach. 

If I were to take an oak tree as a specimen of the plant world, I shouldfind 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 absorbthe inorganic matters I have named, adds enormously to its bulk, andwe can see it, year after year, extending itself upward and downward, attracting and appropriating to itself inorganic materials, which itvivifies, and eventually, as it ripens, gives off its own proper acorns, which again run the same course. But I need not multiply examples, --fromthe 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, whichyou can understand and comprehend, so long as you confine yourself toone 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--areconfined to a certain area on the surface of the earth on which welive, --and, as that is the simpler matter, I may take that first. In itswild state, and before the discovery of America, when the natural stateof things was interfered with by the Spaniards, the Horse was only tobe found in parts of the earth which are known to geographers as the OldWorld; that is to say, you might meet with horses in Europe, Asia, orAfrica; but there were none in Australia, and there were none whatsoeverin the whole continent of America, from Labrador down to Cape Horn. Thisis an empirical fact, and it is what is called, stated in the way I havegiven it you, the 'Geographical Distribution' of the Horse. 

Why horses should be found in Europe, Asia, and Africa, and not inAmerica, is not obvious; the explanation that the conditions of life inAmerica are unfavourable to their existence, and that, therefore, theyhad not been created there, evidently does not apply; for when theinvading Spaniards, or our own yeomen farmers, conveyed horses to thesecountries for their own use, they were found to thrive well and multiplyvery rapidly; and many are even now running wild in those countries, andin a perfectly natural condition. Now, suppose we were to do for everyanimal what we have here done for the Horse, --that is, to mark off anddistinguish the particular district or region to which each belonged;and supposing we tabulated all these results, that would be called theGeographical Distribution of animals, while a corresponding study ofplants would yield as a result the Geographical Distribution of plants. 

I pass on from that now, as I merely wished to explain to you what Imeant by the use of the term 'Geographical Distribution. ' As I said, there is another aspect, and a much more important one, and that is, the relations of the various animals to one another. The Horse is avery well-defined matter-of-fact sort of animal, and we are all prettyfamiliar with its structure. I dare say it may have struck you, thatit resembles very much no other member of the animal kingdom, exceptperhaps the Zebra or the Ass. But let me ask you to look along thesediagrams. Here is the skeleton of the Horse, and here the skeleton ofthe Dog. You will notice that we have in the Horse a skull, a backboneand ribs, shoulder-blades and haunch-bones. In the fore-limb, one upperarm-bone, two fore arm-bones, wrist-bones (wrongly called knee), andmiddle hand-bones, ending in the three bones of a finger, the last ofwhich is sheathed in the horny hoof of the fore-foot: in the hind-limb, one thigh-bone, two leg-bones, anklebones, and middle foot-bones, endingin the three bones of a toe, the last of which is encased in the hoof ofthe hind-foot. Now turn to the Dog's skeleton. We find identically thesame bones, but more of them, there being more toes in each foot, andhence more toe-bones. 

Well, that is a very curious thing! The fact is that the Dog and theHorse--when one gets a look at them without the outward impediments ofthe 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 thesame organs that I have already shown you as forming parts of the Horse. Well, here is another skeleton--that of a kind of Lemur--you see he hasjust 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 upwardsand forwards, just as in the next three diagrams, which represent theskeletons of an Orang, a Chimpanzee, a Gorilla, and you find you have notrouble in identifying the bones throughout; and lastly turn to the endof the series, the diagram representing a man's skeleton, and still youfind no great structural feature essentially altered. There are thesame bones in the same relations. From the Horse we pass on and on, withgradual steps, until we arrive at last at the highest known forms. Onthe other hand, take the other line of diagrams, and pass from the Horsedownwards in the scale to this fish; and still, though the modificationsare vastly greater, the essential framework of the organization remainsunchanged. Here, for instance, is a Porpoise: here is its strongbackbone, with the cavity running through it, which contains the spinalcord; here are the ribs, here the shoulder blade; here is the littleshort 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-lookingaffair--its flapper (as it is called), the same fundamental elements asthe fore-leg of the Horse or the Dog, or the Ape or Man; and here youwill 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 theforearm, in this large pectoral fin--carrying your mind's eye onwardfrom the flapper of the Porpoise. And here you have the hinder limbsrestored in the shape of these ventral fins. If I were to make atransverse section of this, I should find just the same organs thatwe have before noticed. So that, you see, there comes out this strangeconclusion as the result of our investigations, that the Horse, whenexamined and compared with other animals, is found by no means tostand alone in nature; but that there are an enormous number of othercreatures which have backbones, ribs, and legs, and other parts arrangedin the same general manner, and in all their formation exhibiting thesame broad peculiarities. 

I am sure that you cannot have followed me even in this extremelyelementary exposition of the structural relations of animals, withoutseeing what I have been driving at all through, which is, to show youthat, step by step, naturalists have come to the idea of a unity ofplan, or conformity of construction, among animals which appeared atfirst sight to be extremely dissimilar. 

And here you have evidence of such a unity of plan among all the animalswhich have backbones, and which we technically call "Vertebrata". Butthere are multitudes of other animals, such as crabs, lobsters, spiders, and so on, which we term "Annulosa". In these I could not point out toyou the parts that correspond with those of the Horse, --the backbone, for instance, --as they are constructed upon a very different principle, which is also common to all of them; that is to say, the Lobster, theSpider, and the Centipede, have a common plan running through theirwhole arrangement, in just the same way that the Horse, the Dog, and thePorpoise assimilate to each other. 

Yet other creatures--whelks, cuttlefishes, oysters, snails, and alltheir tribe ("Mollusca")--resemble one another in the same way, butdiffer from both "Vertebrata" and "Annulosa"; and the like is true ofthe animals called "Coelenterata" (Polypes) and "Protozoa" (animalculesand sponges). 

Now, by pursuing this sort of comparison, naturalists have arrived atthe conviction that there are, --some think five, and some seven, --butcertainly not more than the latter number--and perhaps it is simpler toassume five--distinct plans or constructions in the whole of the animalworld; and that the hundreds of thousands of species of creatures onthe 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 istempted to go on a step and inquire whether we cannot go back yetfurther and bring down the whole to modifications of one primordialunit. The anatomist cannot do this; but if he call to his aid the studyof development, he can do it. For we shall find that, distinct as thoseplans are, whether it be a porpoise or man, or lobster, or any of thoseother kinds I have mentioned, every one begins its existence with oneand the same primitive form, --that of the egg, consisting, as we haveseen, of a nitrogenous substance, having a small particle or nucleusin the centre of it. Furthermore, the earlier changes of each aresubstantially the same. And it is in this that lies that true "unityof organization" of the animal kingdom which has been guessed at andfancied for many years; but which it has been left to the presenttime to be demonstrated by the careful study of development. But is itpossible to go another step further still, and to show that in thesame way the whole of the organic world is reducible to one primitivecondition of form? Is there among the plants the same primitive form oforganization, and is that identical with that of the animal kingdom?The reply to that question, too, is not uncertain or doubtful. It is nowproved that every plant begins its existence under the same form; thatis to say, in that of a cell--a particle of nitrogenous matter havingsubstantially the same conditions. So that if you trace back the oakto its first germ, or a man, or a horse, or lobster, or oyster, or anyother animal you choose to name, you shall find each and all of thesecommencing their existence in forms essentially similar to each other:and, furthermore, that the first processes of growth, and many of thesubsequent modifications, are essentially the same in principle inalmost all. 

In conclusion, let me, in a few words, recapitulate the positions whichI have laid down. And you must understand that I have not been talkingmere theory; I have been speaking of matters which are as plainlydemonstrable as the commonest propositions of Euclid--of facts that mustform the basis of all speculations and beliefs in Biological science. We have gradually traced down all organic forms, or, in other words, wehave analyzed the present condition of animated nature, until we foundthat each species took its origin in a form similar to that under whichall the others commence their existence. We have found the whole of thevast array of living forms, with which we are surrounded, constantlygrowing, increasing, decaying and disappearing; the animal constantlyattracting, modifying, and applying to its sustenance the matter of thevegetable kingdom, which derived its support from the absorption andconversion of inorganic matter. And so constant and universal is thisabsorption, waste, and reproduction, that it may be said with perfectcertainty that there is left in no one of our bodies at the presentmoment a millionth part of the matter of which they were originallyformed! We have seen, again, that not only is the living matter derivedfrom the inorganic world, but that the forces of that matter are all ofthem correlative with and convertible into those of inorganic nature. 

This, for our present purposes, is the best view of the presentcondition of organic nature which I can lay before you: it gives youthe great outlines of a vast picture, which you must fill up by your ownstudy. 

In the next lecture I shall endeavour in the same way to go back intothe past, and to sketch in the same broad manner the history of life inepochs preceding our own.