THE PAST CONDITION OF ORGANIC NATURE

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

by Thomas H. Huxley

IN the lecture which I delivered last Monday evening, I endeavoured tosketch in a very brief manner, but as well as the time at my disposalwould permit, the present condition of organic nature, meaning bythat large title simply an indication of the great, broad, and generalprinciples which are to be discovered by those who look attentively atthe phenomena of organic nature as at present displayed. The generalresult of our investigations might be summed up thus: we found that themultiplicity of the forms of animal life, great as that may be, may bereduced to a comparatively few primitive plans or types of construction;that a further study of the development of those different formsrevealed to us that they were again reducible, until we at last broughtthe infinite diversity of animal, and even vegetable life, down to theprimordial form of a single cell. 

We found that our analysis of the organic world, whether animals orplants, showed, in the long run, that they might both be reduced into, and were, in fact, composed of, the same constituents. And we sawthat the plant obtained the materials constituting its substance bya peculiar combination of matters belonging entirely to the inorganicworld; that, then, the animal was constantly appropriating thenitrogenous matters of the plant to its own nourishment, and returningthem back to the inorganic world, in what we spoke of as its waste; andthat finally, when the animal ceased to exist, the constituents of itsbody were dissolved and transmitted to that inorganic world whence theyhad been at first abstracted. Thus we saw in both the blade of grass andthe horse but the same elements differently combined and arranged. Wediscovered a continual circulation going on, --the plant drawing in theelements of inorganic nature and combining them into food for the animalcreation; the animal borrowing from the plant the matter for its ownsupport, giving off during its life products which returned immediatelyto the inorganic world; and that, eventually, the constituent materialsof the whole structure of both animals and plants were thus returned totheir original source: there was a constant passage from one state ofexistence to another, and a returning back again. 

Lastly, when we endeavoured to form some notion of the nature of theforces exercised by living beings, we discovered that they--ifnot capable of being subjected to the same minute analysis as theconstituents of those beings themselves--that they were correlativewith--that they were the equivalents of the forces of inorganicnature--that they were, in the sense in which the term is now used, convertible with them. That was our general result. 

And now, leaving the Present, I must endeavour in the same manner to putbefore you the facts that are to be discovered in the Past history ofthe living world, in the past conditions of organic nature. We have, to-night, to deal with the facts of that history--a history involvingperiods of time before which our mere human records sink into utterinsignificance--a history the variety and physical magnitude of whoseevents cannot even be foreshadowed by the history of human life andhuman phenomena--a history of the most varied and complex character. 

We must deal with the history, then, in the first place, as we shoulddeal with all other histories. The historical student knows that hisfirst business should be to inquire into the validity of his evidence, and the nature of the record in which the evidence is contained, thathe may be able to form a proper estimate of the correctness of theconclusions which have been drawn from that evidence. So, here, we mustpass, in the first place, to the consideration of a matter which mayseem foreign to the question under discussion. We must dwell upon thenature of the records, and the credibility of the evidence they contain;we must look to the completeness or incompleteness of those recordsthemselves, before we turn to that which they contain and reveal. Thequestion of the credibility of the history, happily for us, will notrequire much consideration, for, in this history, unlike those of humanorigin, there can be no cavilling, no differences as to the reality andtruth of the facts of which it is made up; the facts state themselves, and are laid out clearly before us. 

But, although one of the greatest difficulties of the historical studentis cleared out of our path, there are other difficulties--difficultiesin rightly interpreting the facts as they are presented to us--whichmay be compared with the greatest difficulties of any other kinds ofhistorical study. 

What is this record of the past history of the globe, and what are thequestions which are involved in an inquiry into its completeness orincompleteness? That record is composed of mud; and the question whichwe have to investigate this evening resolves itself into a questionof the formation of mud. You may think, perhaps, that this is avast step--of almost from the sublime to the ridiculous--from thecontemplation of the history of the past ages of the world's existenceto the consideration of the history of the formation of mud! But, in nature, there is nothing mean and unworthy of attention; thereis nothing ridiculous or contemptible in any of her works; and thisinquiry, you will soon see, I hope, takes us to the very root andfoundations of our subject. 

How, then, is mud formed? Always, with some trifling exception, whichI need not consider now--always, as the result of the action of water, wearing down and disintegrating the surface of the earth and rocks withwhich it comes in contact--pounding and grinding it down, and carryingthe particles away to places where they cease to be disturbed by thismechanical action, and where they can subside and rest. For the ocean, urged by winds, washes, as we know, a long extent of coast, and everywave, loaded as it is with particles of sand and gravel as it breaksupon the shore, does something towards the disintegrating process. Andthus, slowly but surely, the hardest rocks are gradually ground down toa powdery substance; and the mud thus formed, coarser or finer, as thecase may be, is carried by the rush of the tides, or currents, till itreaches the comparatively deeper parts of the ocean, in which it cansink to the bottom, that is, to parts where there is a depth of aboutfourteen or fifteen fathoms, a depth at which the water is, usually, nearly motionless, and in which, of course, the finer particles of thisdetritus, or mud as we call it, sinks to the bottom. 

Or, again, if you take a river, rushing down from its mountain sources, brawling over the stones and rocks that intersect its path, loosening, removing, and carrying with it in its downward course the pebbles andlighter matters from its banks, it crushes and pounds down the rocks andearths in precisely the same way as the wearing action of the sea waves. The matters forming the deposit are torn from the mountain-side andwhirled impetuously into the valley, more slowly over the plain, thenceinto the estuary, and from the estuary they are swept into the sea. Thecoarser and heavier fragments are obviously deposited first, that is, as soon as the current begins to lose its force by becoming amalgamatedwith the stiller depths of the ocean, but the finer and lighterparticles are carried further on, and eventually deposited in a deeperand stiller portion of the ocean. 

It clearly follows from this that mud gives us a chronology; for it isevident that supposing this, which I now sketch, to be the sea bottom, and supposing this to be a coast-line; from the washing action of thesea upon the rock, wearing and grinding it down into a sediment of mud, the mud will be carried down, and at length, deposited in the deeperparts of this sea bottom, where it will form a layer; and then, whilethat first layer is hardening, other mud which is coming from the samesource will, of course, be carried to the same place; and, as it isquite impossible for it to get beneath the layer already there, itdeposits itself above it, and forms another layer, and in that way yougradually have layers of mud constantly forming and hardening one abovethe other, and conveying a record of time. 

It is a necessary result of the operation of the law of gravitation thatthe uppermost layer shall be the youngest and the lowest the oldest, andthat the different beds shall be older at any particular point or spotin exactly the ratio of their depth from the surface. So that if theywere upheaved afterwards, and you had a series of these different layersof mud, converted into sandstone, or limestone, as the case might be, you might be sure that the bottom layer was deposited first, and thatthe upper layers were formed afterwards. Here, you see, is the firststep in the history--these layers of mud give us an idea of time. 

The whole surface of the earth, --I speak broadly, and leave out minorqualifications, --is made up of such layers of mud, so hard, the majorityof them, that we call them rock whether limestone or sandstone, or othervarieties of rock. And, seeing that every part of the crust of the earthis made up in this way, you might think that the determination of thechronology, the fixing of the time which it has taken to form this crustis a comparatively simple matter. Take a broad average, ascertain howfast the mud is deposited upon the bottom of the sea, or in the estuaryof rivers; take it to be an inch, or two, or three inches a year, orwhatever you may roughly estimate it at; then take the total thicknessof the whole series of stratified rocks, which geologists estimate attwelve or thirteen miles, or about seventy thousand feet, make a sumin short division, divide the total thickness by that of the quantitydeposited in one year, and the result will, of course, give you thenumber of years which the crust has taken to form. 

Truly, that looks a very simple process! It would be so except forcertain difficulties, the very first of which is that of finding howrapidly sediments are deposited; but the main difficulty--a difficultywhich renders any certain calculations of such a matter out of thequestion--is this, the sea-bottom on which the deposit takes place iscontinually shifting. 

Instead of the surface of the earth being that stable, fixed thing thatit is popularly believed to be, being, in common parlance, the veryemblem of fixity itself, it is incessantly moving, and is, in fact, as unstable as the surface of the sea, except that its undulations areinfinitely slower and enormously higher and deeper. 

Now, what is the effect of this oscillation? Take the case to whichI have previously referred. The finer or coarser sediments that arecarried down by the current of the river, will only be carried out acertain distance, and eventually, as we have already seen, on reachingthe stiller part of the ocean, will be deposited at the bottom. 

Let C y (Fig. 4) be the sea-bottom, y D the shore, x y the sea-level, then the coarser deposit will subside over the region B, the finer overA, while beyond A there will be no deposit at all; and, consequently, norecord will be kept, simply because no deposit is going on. Now, supposethat the whole land, C, D, which we have regarded as stationary, goesdown, as it does so, both A and B go further out from the shore, whichwill be at y1; x1, y1, being the new sea-level. The consequence will bethat the layer of mud (A), being now, for the most part, further thanthe force of the current is strong enough to convey even the finest'debris', will, of course, receive no more deposits, and having attaineda certain thickness will now grow no thicker. 

We should be misled in taking the thickness of that layer, whenever itmay be exposed to our view, as a record of time in the manner in whichwe are now regarding this subject, as it would give us only an imperfectand partial record: it would seem to represent too short a period oftime. 

[Illustration: Fig. 4. ]

Suppose, on the other hand, that the land (C D) had gone on risingslowly and gradually--say an inch or two inches in the course of acentury, --what would be the practical effect of that movement? Why, thatthe sediment A and B which has been already deposited, would eventuallybe brought nearer to the shore-level, and again subjected to the wearand tear of the sea; and directly the sea begins to act upon it, itwould of course soon cut up and carry it away, to a greater or lessextent, to be re-deposited further out. 

Well, as there is, in all probability, not one single spot on the wholesurface of the earth, which has not been up and down in this way a greatmany times, it follows that the thickness of the deposits formed at anyparticular spot cannot be taken (even supposing we had at first obtainedcorrect data as to the rate at which they took place) as affordingreliable information as to the period of time occupied in its deposit. So that you see it is absolutely necessary from these facts, seeing thatour record entirely consists of accumulations of mud, superimposed oneon the other; seeing in the next place that any particular spots onwhich accumulations have occurred, have been constantly moving up anddown, and sometimes out of the reach of a deposit, and at other timesits own deposit broken up and carried away, it follows that our recordmust be in the highest degree imperfect, and we have hardly a traceleft of thick deposits, or any definite knowledge of the area that theyoccupied, in a great many cases. And mark this! That supposing eventhat the whole surface of the earth had been accessible to thegeologist, --that man had had access to every part of the earth, and hadmade sections of the whole, and put them all together, --even then hisrecord must of necessity be imperfect. 

But to how much has man really access? If you will look at this Map youwill see that it represents the proportion of the sea to the earth: thiscoloured part indicates all the dry land, and this other portion is thewater. You will notice at once that the water covers three-fifths of thewhole surface of the globe, and has covered it in the same manner eversince man has kept any record of his own observations, to say nothing ofthe minute period during which he has cultivated geological inquiry. So that three-fifths of the surface of the earth is shut out from usbecause it is under the sea. Let us look at the other two-fifths, and see what are the countries in which anything that may be termedsearching geological inquiry has been carried out: a good deal ofFrance, Germany, and Great Britain and Ireland, bits of Spain, ofItaly, and of Russia, have been examined, but of the whole great mass ofAfrica, except parts of the southern extremity, we know next to nothing;little bits of India, but of the greater part of the Asiatic continentnothing; bits of the Northern American States and of Canada, but ofthe greater part of the continent of North America, and in still largerproportion, of South America, nothing!

Under these circumstances, it follows that even with reference to thatkind of imperfect information which we can possess, it is only of aboutthe ten-thousandth part of the accessible parts of the earth that hasbeen examined properly. Therefore, it is with justice that the mostthoughtful of those who are concerned in these inquiries insistcontinually upon the imperfection of the geological record; for, Irepeat, it is absolutely necessary, from the nature of things, thatthat record should be of the most fragmentary and imperfect character. Unfortunately this circumstance has been constantly forgotten. Men ofscience, like young colts in a fresh pasture, are apt to be exhilaratedon being turned into a new field of inquiry, to go off at a hand-gallop, in total disregard of hedges and ditches, losing sight of the reallimitation of their inquiries, and to forget the extreme imperfection ofwhat is really known. Geologists have imagined that they could tell uswhat was going on at all parts of the earth's surface during a givenepoch; they have talked of this deposit being contemporaneous withthat deposit, until, from our little local histories of the changes atlimited spots of the earth's surface, they have constructed a universalhistory of the globe as full of wonders and portents as any other storyof antiquity. 

But what does this attempt to construct a universal history of the globeimply? It implies that we shall not only have a precise knowledge of theevents which have occurred at any particular point, but that we shallbe able to say what events, at any one spot, took place at the same timewith those at other spots. 

Let us see how far that is in the nature of things practicable. Supposethat here I make a section of the Lake of Killarney, and here thesection of another lake--that of Loch Lomond in Scotland for instance. The rivers that flow into them are constantly carrying down deposits ofmud, and beds, or strata, are being as constantly formed, one above theother, at the bottom of those lakes. Now, there is not a shadow ofdoubt that in these two lakes the lower beds are all older than theupper--there is no doubt about that; but what does 'this' tell us aboutthe age of any given bed in Loch Lomond, as compared with that of anygiven bed in the Lake of Killarney? It is, indeed, obvious that ifany two sets of deposits are separated and discontinuous, there isabsolutely no means whatever given you by the nature of the deposit ofsaying whether one is much younger or older than the other; but you maysay, as many have said and think, that the case is very much altered ifthe beds which we are comparing are continuous. Suppose two beds of mudhardened into rock, --A and B-are seen in section. (Fig. 5. )

[Illustration: Fig. 5. ]

Well, you say, it is admitted that the lowermost bed is always theolder. Very well; B, therefore, is older than A. No doubt, 'as a whole', it is so; or if any parts of the two beds which are in the same verticalline are compared, it is so. But suppose you take what seems a verynatural step further, and say that the part 'a' of the bed A is youngerthan the part 'b' of the bed B. Is this sound reasoning? If you find anyrecord of changes taking place at 'b', did they occur before any eventswhich took place while 'a' was being deposited? It looks all very plainsailing, indeed, to say that they did; and yet there is no proof ofanything of the kind. As the former Director of this Institution, SirH. De la Beche, long ago showed, this reasoning may involve an entirefallacy. It is extremely possible that 'a' may have been deposited agesbefore 'b'. It is very easy to understand how that can be. To returnto Fig. 4; when A and B were deposited, they were 'substantially'contemporaneous; A being simply the finer deposit, and B the coarserof the same detritus or waste of land. Now suppose that that sea-bottomgoes down (as shown in Fig. 4), so that the first deposit is carriedno farther than 'a', forming the bed Al, and the coarse no fartherthan 'b', forming the bed B1, the result will be the formation of twocontinuous beds, one of fine sediment (A A1) over-lapping another ofcoarse sediment (B B1). Now suppose the whole sea-bottom is raised up, and a section exposed about the point Al; no doubt, 'at this spot', theupper bed is younger than the lower. But we should obviously greatly errif we concluded that the mass of the upper bed at A was younger thanthe lower bed at B; for we have just seen that they are contemporaneousdeposits. Still more should we be in error if we supposed the upper bedat A to be younger than the continuation of the lower bed at Bl; forA was deposited long before B1. In fine, if, instead of comparingimmediately adjacent parts of two beds, one of which lies upon another, we compare distant parts, it is quite possible that the upper may be anynumber of years older than the under, and the under any number of yearsyounger than the upper. 

Now you must not suppose that I put this before you for the purpose ofraising a paradoxical difficulty; the fact is, that the great mass ofdeposits have taken place in sea-bottoms which are gradually sinking, and have been formed under the very conditions I am here supposing. 

Do not run away with the notion that this subverts the principle Ilaid down at first. The error lies in extending a principle which isperfectly applicable to deposits in the same vertical line to depositswhich are not in that relation to one another. 

It is in consequence of circumstances of this kind, and of others that Imight mention to you, that our conclusions on and interpretations ofthe record are really and strictly only valid so long as we confineourselves to one vertical section. I do not mean to tell you that thereare no qualifying circumstances, so that, even in very considerableareas, we may safely speak of conformably superimposed beds being olderor younger than others at many different points. But we can never bequite sure in coming to that conclusion, and especially we cannotbe sure if there is any break in their continuity, or any very greatdistance between the points to be compared. 

Well now, so much for the record itself, --so much for itsimperfections, --so much for the conditions to be observed ininterpreting it, and its chronological indications, the moment we passbeyond the limits of a vertical linear section. 

Now let us pass from the record to that which it contains, --from thebook itself to the writing and the figures on its pages. This writingand these figures consist of remains of animals and plants which, in thegreat majority of cases, have lived and died in the very spot in whichwe now find them, or at least in the immediate vicinity. You must all ofyou be aware--and I referred to the fact in my last lecture--that thereare vast numbers of creatures living at the bottom of the sea. Thesecreatures, like all others, sooner or later die, and their shells andhard parts lie at the bottom; and then the fine mud which is beingconstantly brought down by rivers and the action of the wear and tear ofthe sea, covers them over and protects them from any further changeor alteration; and, of course, as in process of time the mud becomeshardened and solidified, the shells of these animals are preservedand firmly imbedded in the limestone or sandstone which is being thusformed. You may see in the galleries of the Museum up stairs specimensof limestones in which such fossil remains of existing animals areimbedded. There are some specimens in which turtles' eggs have beenimbedded in calcareous sand, and before the sun had hatched the youngturtles, they became covered over with calcareous mud, and thus havebeen preserved and fossilized. 

Not only does this process of imbedding and fossilization occur withmarine and other aquatic animals and plants, but it affects those landanimals and plants which are drifted away to sea, or become buried inbogs or morasses; and the animals which have been trodden down by theirfellows and crushed in the mud at the river's bank, as the herd havecome to drink. In any of these cases, the organisms may be crushed or bemutilated, before or after putrefaction, in such a manner that perhapsonly a part will be left in the form in which it reaches us. It is, indeed, a most remarkable fact, that it is quite an exceptional caseto find a skeleton of any one of all the thousands of wild land animalsthat we know are constantly being killed, or dying in the course ofnature: they are preyed on and devoured by other animals or die inplaces where their bodies are not afterwards protected by mud. There areother animals existing in the sea, the shells of which form exceedinglylarge deposits. You are probably aware that before the attempt was madeto lay the Atlantic telegraphic cable, the Government employed vesselsin making a series of very careful observations and soundings of thebottom of the Atlantic; and although, as we must all regret, up to thepresent time that project has not succeeded, we have the satisfactionof knowing that it yielded some most remarkable results to science. The Atlantic Ocean had to be sounded right across, to depths of severalmiles in some places, and the nature of its bottom was carefullyascertained. Well, now, a space of about 1, 000 miles wide from east towest, and I do not exactly know how many from north to south, but atany rate 600 or 700 miles, was carefully examined, and it was found thatover the whole of that immense area an excessively fine chalky mud isbeing deposited; and this deposit is entirely made up of animals whosehard parts are deposited in this part of the ocean, and are doubtlessgradually acquiring solidity and becoming metamorphosed into a chalkylimestone. Thus, you see, it is quite possible in this way to preserveunmistakable records of animal and vegetable life. Whenever thesea-bottom, by some of those undulations of the earth's crust that Ihave referred to, becomes upheaved, and sections or borings are made, or pits are dug, then we become able to examine the contents andconstituents of these ancient sea-bottoms, and find out what manner ofanimals lived at that period. 

Now it is a very important consideration in its bearing on thecompleteness of the record, to inquire how far the remains containedin these fossiliferous limestones are able to convey anything like anaccurate or complete account of the animals which were in existenceat the time of its formation. Upon that point we can form a very clearjudgment, and one in which there is no possible room for any mistake. There are of course a great number of animals--such as jelly-fishes, and other animals--without any hard parts, of which we cannot reasonablyexpect to find any traces whatever: there is nothing of them topreserve. Within a very short time, you will have noticed, after theyare removed from the water, they dry up to a mere nothing; certainlythey are not of a nature to leave any very visible traces of theirexistence on such bodies as chalk or mud. Then again, look at landanimals; it is, as I have said, a very uncommon thing to find a landanimal entire after death. Insects and other carnivorous animals veryspeedily pull them to pieces, putrefaction takes place, and so, out ofthe hundreds of thousands that are known to die every year, it is therarest thing in the world to see one imbedded in such a way that itsremains would be preserved for a lengthened period. Not only is this thecase, but even when animal remains have been safely imbedded, certainnatural agents may wholly destroy and remove them. 

Almost all the hard parts of animals--the bones and so on--are composedchiefly of phosphate of lime and carbonate of lime. Some years ago, Ihad to make an inquiry into the nature of some very curious fossilssent to me from the North of Scotland. Fossils are usually hard bonystructures that have become imbedded in the way I have described, andhave gradually acquired the nature and solidity of the body with whichthey are associated; but in this case I had a series of 'holes' in somepieces of rock, and nothing else. Those holes, however, had a certaindefinite shape about them, and when I got a skilful workman to makecastings of the interior of these holes, I found that they were theimpressions of the joints of a backbone and of the armour of a greatreptile, twelve or more feet long. This great beast had died and gotburied in the sand; the sand had gradually hardened over the bones, butremained porous. Water had trickled through it, and that water beingprobably charged with a superfluity of carbonic acid, had dissolved allthe phosphate and carbonate of lime, and the bones themselves had thusdecayed and entirely disappeared; but as the sandstone happened to haveconsolidated by that time, the precise shape of the bones was retained. If that sandstone had remained soft a little longer, we should haveknown nothing whatsoever of the existence of the reptile whose bones ithad encased. 

How certain it is that a vast number of animals which have existedat one period on this earth have entirely perished, and left no tracewhatever of their forms, may be proved to you by other considerations. There are large tracts of sandstone in various parts of the world, inwhich nobody has yet found anything but footsteps. Not a bone of anydescription, but an enormous number of traces of footsteps. There is noquestion about them. There is a whole valley in Connecticut covered withthese footsteps, and not a single fragment of the animals which madethem has yet been found. Let me mention another case while upon thatmatter, which is even more surprising than those to which I have yetreferred. There is a limestone formation near Oxford, at a place calledStonesfield, which has yielded the remains of certain very interestingmammalian animals, and up to this time, if I recollect rightly, therehave been found seven specimens of its lower jaws, and not a bit ofanything else, neither limb-bones nor skull, or any part whatever; nota fragment of the whole system! Of course, it would be preposterousto imagine that the beasts had nothing else but a lower jaw!The probability is, as Dr. Buckland showed, as the result of hisobservations on dead dogs in the river Thames, that the lower jaw, notbeing secured by very firm ligaments to the bones of the head, and beinga weighty affair, would easily be knocked off, or might drop away fromthe body as it floated in water in a state of decomposition. The jawwould thus be deposited immediately, while the rest of the body wouldfloat and drift away altogether, ultimately reaching the sea, andperhaps becoming destroyed. The jaw becomes covered up and preservedin the river silt, and thus it comes that we have such a curiouscircumstance as that of the lower jaws in the Stonesfield slates. Sothat, you see, faulty as these layers of stone in the earth's crustare, defective as they necessarily are as a record, the account ofcontemporaneous vital phenomena presented by them is, by the necessityof the case, infinitely more defective and fragmentary. 

It was necessary that I should put all this very strongly before you, because, otherwise, you might have been led to think differently of thecompleteness of our knowledge by the next facts I shall state to you. 

The researches of the last three-quarters of a century have, in truth, revealed a wonderful richness of organic life in those rocks. Certainlynot fewer than thirty or forty thousand different species of fossilshave been discovered. You have no more ground for doubting that thesecreatures really lived and died at or near the places in which we findthem than you have for like scepticism about a shell on the sea-shore. The evidence is as good in the one case as in the other. 

Our next business is to look at the general character of these fossilremains, and it is a subject which it will be requisite to considercarefully; and the first point for us is to examine how much theextinct 'Flora' and 'Fauna' as a 'whole'--disregarding altogetherthe 'succession' of their constituents, of which I shall speakafterwards--differ from the 'Flora' and 'Fauna' of the present day;--howfar they differ in what we 'do' know about them, leaving altogether outof consideration speculations based upon what we 'do not' know. 

I strongly imagine that if it were not for the peculiar appearance thatfossilised animals have, any of you might readily walk through a museumwhich contains fossil remains mixed up with those of the present formsof life, and I doubt very much whether your uninstructed eyes wouldlead you to see any vast or wonderful difference between the two. Ifyou looked closely, you would notice, in the first place, a great manythings very like animals with which you are acquainted now: you wouldsee differences of shape and proportion, but on the whole a closesimilarity. 

I explained what I meant by ORDERS the other day, when I described theanimal kingdom as being divided in sub-kingdoms, classes and orders. Ifyou divide the animal kingdom into orders, you will find that there areabout one hundred and twenty. The number may vary on one side or theother, but this is a fair estimate. That is the sum total of the ordersof all the animals which we know now, and which have been known in pasttimes, and left remains behind. 

Now, how many of those are absolutely extinct? That is to say, how manyof these orders of animals have lived at a former period of the world'shistory, but have at present no representatives? That is the sense inwhich I meant to use the word "extinct. " I mean that those animals didlive on this earth at one time, but have left no one of their kindwith us at the present moment. So that estimating the number of extinctanimals is a sort of way of comparing the past creation as a whole withthe present as a whole. Among the mammalia and birds there are noneextinct; but when we come to the reptiles there is a most wonderfulthing: out of the eight orders, or thereabouts, which you can make amongreptiles, one-half are extinct. These diagrams of the plesiosaurus, the ichthyosaurus, the pterodactyle, give you a notion of some of theseextinct reptiles. And here is a cast of the pterodactyle and bones ofthe ichthyosaurus and the plesiosaurus, just as fresh as if it had beenrecently dug up in a churchyard. Thus, in the reptile class, there areno less than half of the orders which are absolutely extinct. If we turnto the 'Amphibia', there was one extinct order, the Labyrinthodonts, typified by the large salamander-like beast shown in this diagram. 

No order of fishes is known to be extinct. Every fish that we find inthe strata--to which I have been referring--can be identified and placedin one of the orders which exist at the present day. There is not knownto be a single ordinal form of insect extinct. There are only two ordersextinct among the 'Crustacea'. There is not known to be an extinct orderof these creatures, the parasitic and other worms; but there aretwo, not to say three, absolutely extinct orders of this class, the'Echinodermata'; out of all the orders of the 'Coelenterata' and'Protozoa' only one, the Rugose Corals. 

So that, you see, out of somewhere about 120 orders of animals, takingthem altogether, you will not, at the outside estimate, find above tenor a dozen extinct. Summing up all the orders of animals which have leftremains behind them, you will not find above ten or a dozen which cannotbe arranged with those of the present day; that is to say, that thedifference does not amount to much more than ten per cent. : and theproportion of extinct orders of plants is still smaller. I think thatthat is a very astounding, a most astonishing fact, seeing the enormousepochs of time which have elapsed during the constitution of the surfaceof the earth as it at present exists; it is, indeed, a most astoundingthing that the proportion of extinct ordinal types should be soexceedingly small. 

But now, there is another point of view in which we must look at thispast creation. Suppose that we were to sink a vertical pit through thefloor beneath us, and that I could succeed in making a section rightthrough in the direction of New Zealand, I should find in each of thedifferent beds through which I passed the remains of animals which Ishould find in that stratum and not in the others. First, I should comeupon beds of gravel or drift containing the bones of large animals, suchas the elephant, rhinoceros, and cave tiger. Rather curious things tofall across in Piccadilly! If I should dig lower still, I should comeupon a bed of what we call the London clay, and in this, as you will seein our galleries upstairs, are found remains of strange cattle, remainsof turtles, palms, and large tropical fruits; with shell-fish such asyou see the like of now only in tropical regions. If I went belowthat, I should come upon the chalk, and there I should find somethingaltogether different, the remains of ichthyosauri and pterodactyles, andammonites, and so forth. 

I do not know what Mr. Godwin Austin would say comes next, but probablyrocks containing more ammonites, and more ichthyosauri and plesiosauri, with a vast number of other things; and under that I should meet withyet older rocks, containing numbers of strange shells and fishes; and inthus passing from the surface to the lowest depths of the earth's crust, the forms of animal life and vegetable life which I should meet within the successive beds would, looking at them broadly, be the moredifferent the further that I went down. Or, in other words, inasmuchas we started with the clear principle, that in a series ofnaturally-disposed mud beds the lowest are the oldest, we should cometo this result, that the further we go back in time the more differenceexists between the animal and vegetable life of an epoch and that whichnow exists. That was the conclusion to which I wished to bring you atthe end of this Lecture.