THE CONDITIONS OF EXISTENCE AS AFFECTING THE PERPETUATION OF LIVINGBEINGS

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

By Thomas H. Huxley

IN the last Lecture I endeavoured to prove to you that, while, as ageneral rule, organic beings tend to reproduce their kind, there isin them, also, a constantly recurring tendency to vary--to vary to agreater or to a less extent. Such a variety, I pointed out to you, mightarise from causes which we do not understand; we therefore called itspontaneous; and it might come into existence as a definite and markedthing, without any gradations between itself and the form which precededit. I further pointed out, that such a variety having once arisen, might be perpetuated to some extent, and indeed to a very marked extent, without any direct interference, or without any exercise of that processwhich we called selection. And then I stated further, that by suchselection, when exercised artificially--if you took care to breed onlyfrom those forms which presented the same peculiarities of any varietywhich had arisen in this manner--the variation might be perpetuated, asfar as we can see, indefinitely. 

The next question, and it is an important one for us, is this: Is thereany limit to the amount of variation from the primitive stock which canbe produced by this process of selective breeding? In considering thisquestion, it will be useful to class the characteristics, in respect ofwhich organic beings vary, under two heads: we may consider structuralcharacteristics, and we may consider physiological characteristics. 

In the first place, as regards structural characteristics, I endeavouredto show you, by the skeletons which I had upon the table, and byreference to a great many well-ascertained facts, that the differentbreeds of Pigeons, the Carriers, Pouters, and Tumblers, might vary inany of their internal and important structural characters to a verygreat degree; not only might there be changes in the proportions of theskull, and the characters of the feet and beaks, and so on; but thatthere might be an absolute difference in the number of the vertebrae ofthe back, as in the sacral vertebrae of the Pouter; and so great is theextent of the variation in these and similar characters that I pointedout to you, by reference to the skeletons and the diagrams, that theseextreme varieties may absolutely differ more from one another in theirstructural characters than do what naturalists call distinct SPECIESof pigeons; that is to say, that they differ so much in structure thatthere is a greater difference between the Pouter and the Tumbler thanthere is between such wild and distinct forms as the Rock Pigeon orthe Ring Pigeon, or the Ring Pigeon and the Stock Dove; and indeedthe differences are of greater value than this, for the structuraldifferences between these domesticated pigeons are such as would beadmitted by a naturalist, supposing he knew nothing at all about theirorigin, to entitle them to constitute even distinct genera. 

As I have used this term SPECIES, and shall probably use it a good deal, I had better perhaps devote a word or two to explaining what I mean byit. 

Animals and plants are divided into groups, which become graduallysmaller, beginning with a KINGDOM, which is divided into SUB-KINGDOMS;then come the smaller divisions called PROVINCES; and so on from aPROVINCE to a CLASS from a CLASS to an ORDER, from ORDERS to FAMILIES, and from these to GENERA, until we come at length to the smallestgroups of animals which can be defined one from the other by constantcharacters, which are not sexual; and these are what naturalists callSPECIES in practice, whatever they may do in theory. 

If, in a state of nature, you find any two groups of living beings, which are separated one from the other by some constantly-recurringcharacteristic, I don't care how slight and trivial, so long as it isdefined and constant, and does not depend on sexual peculiarities, thenall naturalists agree in calling them two species; that is what is meantby the use of the word species--that is to say, it is, for the practicalnaturalist, a mere question of structural differences. [1]

We have seen now--to repeat this point once more, and it is veryessential that we should rightly understand it--we have seen thatbreeds, known to have been derived from a common stock by selection, maybe as different in their structure from the original stock as speciesmay be distinct from each other. 

But is the like true of the physiological characteristics of animals?Do the physiological differences of varieties amount in degree to thoseobserved between forms which naturalists call distinct species? This isa most important point for us to consider. 

As regards the great majority of physiological characteristics, thereis no doubt that they are capable of being developed, increased, andmodified by selection. 

There is no doubt that breeds may be made as different as species inmany physiological characters. I have already pointed out to you verybriefly the different habits of the breeds of Pigeons, all of whichdepend upon their physiological peculiarities, --as the peculiar habit oftumbling, in the Tumbler--the peculiarities of flight, in the "homing"birds, --the strange habit of spreading out the tail, and walking in apeculiar fashion, in the Fantail, --and, lastly, the habit of blowingout the gullet, so characteristic of the Pouter. These are all dueto physiological modifications, and in all these respects these birdsdiffer as much from each other as any two ordinary species do. 

So with Dogs in their habits and instincts. It is a physiologicalpeculiarity which leads the Greyhound to chase its prey by sight, --thatenables the Beagle to track it by the scent, --that impels the Terrier toits rat-hunting propensity, --and that leads the Retriever to itshabit of retrieving. These habits and instincts are all the results ofphysiological differences and peculiarities, which have been developedfrom a common stock, at least there is every reason to believe so. But it is a most singular circumstance, that while you may run throughalmost the whole series of physiological processes, without finding acheck to your argument, you come at last to a point where you do finda check, and that is in the reproductive processes. For there is a mostsingular circumstance in respect to natural species--at least about someof them--and it would be sufficient for the purposes of this argument ifit were true of only one of them, but there is, in fact, a great numberof such cases--and that is, that, similar as they may appear to beto mere races or breeds, they present a marked peculiarity in thereproductive process. If you breed from the male and female of the samerace, you of course have offspring of the like kind, and if you make theoffspring breed together, you obtain the same result, and if you breedfrom these again, you will still have the same kind of offspring; thereis no check. But if you take members of two distinct species, howeversimilar they may be to each other and make them breed together, you willfind a check, with some modifications and exceptions, however, which Ishall speak of presently. If you cross two such species with each other, then, --although you may get offspring in the case of the first cross, yet, if you attempt to breed from the products of that crossing, whichare what are called HYBRIDS--that is, if you couple a male and a femalehybrid--then the result is that in ninety-nine cases out of a hundredyou will get no offspring at all; there will be no result whatsoever. 

The reason of this is quite obvious in some cases; the male hybrids, although possessing all the external appearances and characteristicsof perfect animals, are physiologically imperfect and deficient in thestructural parts of the reproductive elements necessary to generation. It is said to be invariably the case with the male mule, the crossbetween the Ass and the Mare; and hence it is, that, although crossingthe Horse with the Ass is easy enough, and is constantly done, as far asI am aware, if you take two mules, a male and a female, and endeavour tobreed from them, you get no offspring whatever; no generation will takeplace. This is what is called the sterility of the hybrids between twodistinct species. 

You see that this is a very extraordinary circumstance; one does not seewhy it should be. The common teleological explanation is, that it isto prevent the impurity of the blood resulting from the crossing of onespecies with another, but you see it does not in reality do anything ofthe kind. There is nothing in this fact that hybrids cannot breed witheach other, to establish such a theory; there is nothing to prevent theHorse breeding with the Ass, or the Ass with the Horse. So that thisexplanation breaks down, as a great many explanations of this kind do, that are only founded on mere assumptions. 

Thus you see that there is a great difference between "mongrels, " whichare crosses between distinct races, and "hybrids, " which are crossesbetween distinct species. The mongrels are, so far as we know, fertilewith one another. But between species, in many cases, you cannot succeedin obtaining even the first cross: at any rate it is quite certain thatthe hybrids are often absolutely infertile one with another. 

Here is a feature, then, great or small as it may be, whichdistinguishes natural species of animals. Can we find any approximationto this in the different races known to be produced by selectivebreeding from a common stock? Up to the present time the answer to thatquestion is absolutely a negative one. As far as we know at present, there is nothing approximating to this check. In crossing the breedsbetween the Fantail and the Pouter, the Carrier and the Tumbler, or anyother variety or race you may name--so far as we know at present--thereis no difficulty in breeding together the mongrels. Take the Carrier andthe Fantail, for instance, and let them represent the Horse and the Assin the case of distinct species; then you have, as the result of theirbreeding, the Carrier-Fantail mongrel, --we will say the male and femalemongrel, --and, as far as we know, these two when crossed would not beless fertile than the original cross, or than Carrier with Carrier. Here, you see, is a physiological contrast between the races producedby selective modification and natural species. I shall inquire into thevalue of this fact, and of some modifying circumstances by and by; forthe present I merely put it broadly before you. 

But while considering this question of the limitations of species, aword must be said about what is called RECURRENCE--the tendency of raceswhich have been developed by selective breeding from varieties to returnto their primitive type. This is supposed by many to put an absolutelimit to the extent of selective and all other variations. People say, "It is all very well to talk about producing these different races, but you know very well that if you turned all these birds wild, thesePouters, and Carriers, and so on, they would all return to theirprimitive stock. " This is very commonly assumed to be a fact, and it isan argument that is commonly brought forward as conclusive; but if youwill take the trouble to inquire into it rather closely, I think youwill find that it is not worth very much. The first question of courseis, Do they thus return to the primitive stock? And commonly as thething is assumed and accepted, it is extremely difficult to get anythinglike good evidence of it. It is constantly said, for example, that ifdomesticated Horses are turned wild, as they have been in some parts ofAsia Minor and South America, that they return at once to the primitivestock from which they were bred. But the first answer that you make tothis assumption is, to ask who knows what the primitive stock was; andthe second answer is, that in that case the wild Horses of Asia Minorought to be exactly like the wild Horses of South America. If they areboth like the same thing, they ought manifestly to be like each other!The best authorities, however, tell you that it is quite different. Thewild Horse of Asia is said to be of a dun colour, with a largish head, and a great many other peculiarities; while the best authorities onthe wild Horses of South America tell you that there is no similaritybetween their wild Horses and those of Asia Minor; the cut of theirheads is very different, and they are commonly chestnut or bay-coloured. It is quite clear, therefore, that as by these facts there ought tohave been two primitive stocks, they go for nothing in support of theassumption that races recur to one primitive stock, and so far as thisevidence is concerned, it falls to the ground. 

Suppose for a moment that it were so, and that domesticated races, whenturned wild, did return to some common condition, I cannot see that thiswould prove much more than that similar conditions are likely to producesimilar results; and that when you take back domesticated animals intowhat we call natural conditions, you do exactly the same thing as if youcarefully undid all the work you had gone through, for the purpose ofbringing the animal from its wild to its domesticated state. I do notsee anything very wonderful in the fact, if it took all that trouble toget it from a wild state, that it should go back into its original stateas soon as you removed the conditions which produced the variation tothe domesticated form. There is an important fact, however, forciblybrought forward by Mr. Darwin, which has been noticed in connection withthe breeding of domesticated pigeons; and it is, that however differentthese breeds of pigeons may be from each other, and we have alreadynoticed the great differences in these breeds, that if, among any ofthose variations, you chance to have a blue pigeon turn up, it will besure to have the black bars across the wings, which are characteristicof the original wild stock, the Rock Pigeon. 

Now, this is certainly a very remarkable circumstance; but I do not seemyself how it tells very strongly either one way or the other. I think, in fact, that this argument in favour of recurrence to the primitivetype might prove a great deal too much for those who so constantlybring it forward. For example, Mr. Darwin has very forcibly urged, that nothing is commoner than if you examine a dun horse--and I had anopportunity of verifying this illustration lately, while in the islandsof the West Highlands, where there are a great many dun horses--to findthat horse exhibit a long black stripe down his back, very often stripeson his shoulder, and very often stripes on his legs. I, myself, sawa pony of this description a short time ago, in a baker's cart, nearRothesay, in Bute: it had the long stripe down the back, and stripes onthe shoulders and legs, just like those of the Ass, the Quagga, and theZebra. Now, if we interpret the theory of recurrence as applied tothis case, might it not be said that here was a case of a variationexhibiting the characters and conditions of an animal occupyingsomething like an intermediate position between the Horse, the Ass, theQuagga, and the Zebra, and from which these had been developed? In thesame way with regard even to Man. Every anatomist will tell you thatthere is nothing commoner, in dissecting the human body, than to meetwith what are called muscular variations--that is, if you dissecttwo bodies very carefully, you will probably find that the modes ofattachment and insertion of the muscles are not exactly the same inboth, there being great peculiarities in the mode in which the musclesare arranged; and it is very singular, that in some dissections of thehuman body you will come upon arrangements of the muscles very similarindeed to the same parts in the Apes. Is the conclusion in that case tobe, that this is like the black bars in the case of the Pigeon, and thatit indicates a recurrence to the primitive type from which the animalshave been probably developed? Truly, I think that the opponents ofmodification and variation had better leave the argument of recurrencealone, or it may prove altogether too strong for them. 

To sum up, --the evidence as far as we have gone is against the argumentas to any limit to divergences, so far as structure is concerned; andin favour of a physiological limitation. By selective breeding we canproduce structural divergences as great as those of species, but wecannot produce equal physiological divergences. For the present I leavethe question there. 

Now, the next problem that lies before us--and it is an extremelyimportant one--is this: Does this selective breeding occur in nature?Because, if there is no proof of it, all that I have been telling yougoes for nothing in accounting for the origin of species. Are naturalcauses competent to play the part of selection in perpetuatingvarieties? Here we labour under very great difficulties. In the lastlecture I had occasion to point out to you the extreme difficulty ofobtaining evidence even of the first origin of those varieties which weknow to have occurred in domesticated animals. I told you, that almostalways the origin of these varieties is overlooked, so that I could onlyproduce two of three cases, as that of Gratio Kelleia and of the Anconsheep. People forget, or do not take notice of them until they come tohave a prominence; and if that is true of artificial cases, under ourown eyes, and in animals in our own care, how much more difficult itmust be to have at first hand good evidence of the origin of varietiesin nature! Indeed, I do not know that it is possible by direct evidenceto prove the origin of a variety in nature, or to prove selectivebreeding; but I will tell you what we can prove--and this comes to thesame thing--that varieties exist in nature within the limits of species, and, what is more, that when a variety has come into existence innature, there are natural causes and conditions, which are amplycompetent to play the part of a selective breeder; and although thatis not quite the evidence that one would like to have--though it isnot direct testimony--yet it is exceeding good and exceedingly powerfulevidence in its way. 

As to the first point, of varieties existing among natural species, Imight appeal to the universal experience of every naturalist, and of anyperson who has ever turned any attention at all to the characteristicsof plants and animals in a state of nature; but I may as well take a fewdefinite cases, and I will begin with Man himself. 

I am one of those who believe that, at present, there is no evidencewhatever for saying, that mankind sprang originally from any more than asingle pair; I must say, that I cannot see any good ground whatever, oreven any tenable sort of evidence, for believing that there is more thanone species of Man. Nevertheless, as you know, just as there are numbersof varieties in animals, so there are remarkable varieties of men. Ispeak not merely of those broad and distinct variations which you see ata glance. Everybody, of course, knows the difference between a Negro anda white man, and can tell a Chinaman from an Englishman. They eachhave peculiar characteristics of colour and physiognomy; but you mustrecollect that the characters of these races go very far deeper--theyextend to the bony structure, and to the characters of that mostimportant of all organs to us--the brain; so that, among men belongingto different races, or even within the same race, one man shall have abrain a third, or half, or even seventy per cent. Bigger than another;and if you take the whole range of human brains, you will find avariation in some cases of a hundred per cent. Apart from thesevariations in the size of the brain, the characters of the skull vary. Thus if I draw the figures of a Mongul and of a Negro head on theblackboard, in the case of the last the breadth would be aboutseven-tenths, and in the other it would be nine-tenths of the totallength. So that you see there is abundant evidence of variation amongmen in their natural condition. And if you turn to other animals thereis just the same thing. The fox, for example, which has a very largegeographical distribution all over Europe, and parts of Asia, and on theAmerican Continent, varies greatly. There are mostly large foxes in theNorth, and smaller ones in the South. In Germany alone, the forestersreckon some eight different sorts. 

Of the tiger, no one supposes that there is more than one species; theyextend from the hottest parts of Bengal, into the dry, cold, bittersteppes of Siberia, into a latitude of 50 degrees, --so that they mayeven prey upon the reindeer. These tigers have exceedingly differentcharacteristics, but still they all keep their general features, so thatthere is no doubt as to their being tigers. The Siberian tiger has athick fur, a small mane, and a longitudinal stripe down the back, whilethe tigers of Java and Sumatra differ in many important respects fromthe tigers of Northern Asia. So lions vary; so birds vary; and so, ifyou go further back and lower down in creation, you find that fishesvary. In different streams, in the same country even, you will find thetrout to be quite different to each other and easily recognisable bythose who fish in the particular streams. There is the same differencesin leeches; leech collectors can easily point out to you the differencesand the peculiarities which you yourself would probably pass by; so withfresh-water mussels; so, in fact, with every animal you can mention. 

In plants there is the same kind of variation. Take such a case even asthe common bramble. The botanists are all at war about it; some ofthem wanting to make out that there are many species of it, and othersmaintaining that they are but many varieties of one species; and theycannot settle to this day which is a species and which is a variety!

So that there can be no doubt whatsoever that any plant and anyanimal may vary in nature; that varieties may arise in the way I havedescribed, --as spontaneous varieties, --and that those varieties may beperpetuated in the same way that I have shown you spontaneous varietiesare perpetuated; I say, therefore, that there can be no doubt as to theorigin and perpetuation of varieties in nature. 

But the question now is:--Does selection take place in nature? is thereanything like the operation of man in exercising selective breeding, taking place in nature? You will observe that, at present, I say nothingabout species; I wish to confine myself to the consideration of theproduction of those natural races which everybody admits to exist. Thequestion is, whether in nature there are causes competent to produceraces, just in the same way as man is able to produce by selection, suchraces of animals as we have already noticed. 

When a variety has arisen, the CONDITIONS OF EXISTENCE are such as toexercise an influence which is exactly comparable to that of artificialselection. By Conditions of Existence I mean two things, --there areconditions which are furnished by the physical, the inorganic world, and there are conditions of existence which are furnished by the organicworld. There is, in the first place, CLIMATE; under that head I includeonly temperature and the varied amount of moisture of particular places. In the next place there is what is technically called STATION, whichmeans--given the climate, the particular kind of place in which ananimal or a plant lives or grows; for example, the station of a fishis in the water, of a fresh-water fish in fresh water; the station of amarine fish is in the sea, and a marine animal may have a station higheror deeper. So again with land animals: the differences in their stationsare those of different soils and neighbourhoods; some being best adaptedto a calcareous, and others to an arenaceous soil. The third conditionof existence is FOOD, by which I mean food in the broadest sense, thesupply of the materials necessary to the existence of an organic being;in the case of a plant the inorganic matters, such as carbonic acid, water, ammonia, and the earthy salts or salines; in the case of theanimal the inorganic and organic matters, which we have seen theyrequire; then these are all, at least the two first, what we maycall the inorganic or physical conditions of existence. Food takes amid-place, and then come the organic conditions; by which I mean theconditions which depend upon the state of the rest of the organiccreation, upon the number and kind of living beings, with which ananimal is surrounded. You may class these under two heads: there areorganic beings, which operate as 'opponents', and there are organicbeings which operate as 'helpers' to any given organic creature. Theopponents may be of two kinds: there are the 'indirect opponents', whichare what we may call 'rivals'; and there are the 'direct opponents', those which strive to destroy the creature; and these we call 'enemies'. By rivals I mean, of course, in the case of plants, those which requirefor their support the same kind of soil and station, and, among animals, those which require the same kind of station, or food, or climate; thoseare the indirect opponents; the direct opponents are, of course, thosewhich prey upon an animal or vegetable. The 'helpers' may also beregarded as direct and indirect: in the case of a carnivorous animal, for example, a particular herbaceous plant may in multiplying be anindirect helper, by enabling the herbivora on which the carnivore preysto get more food, and thus to nourish the carnivore more abundantly;the direct helper may be best illustrated by reference to some parasiticcreature, such as the tape-worm. The tape-worm exists in the humanintestines, so that the fewer there are of men the fewer there will beof tape-worms, other things being alike. It is a humiliating reflection, perhaps, that we may be classed as direct helpers to the tape-worm, butthe fact is so: we can all see that if there were no men there would beno tape-worms. 

It is extremely difficult to estimate, in a proper way, the importanceand the working of the Conditions of Existence. I do not think therewere any of us who had the remotest notion of properly estimating themuntil the publication of Mr. Darwin's work, which has placed them beforeus with remarkable clearness; and I must endeavour, as far as I can inmy own fashion, to give you some notion of how they work. We shall findit easiest to take a simple case, and one as free as possible from everykind of complication. 

I will suppose, therefore, that all the habitable part of thisglobe--the dry land, amounting to about 51, 000, 000 square miles, --I willsuppose that the whole of that dry land has the same climate, and thatit is composed of the same kind of rock or soil, so that there will bethe same station everywhere; we thus get rid of the peculiar influenceof different climates and stations. I will then imagine that there shallbe but one organic being in the world, and that shall be a plant. Inthis we start fair. Its food is to be carbonic acid, water and ammonia, and the saline matters in the soil, which are, by the supposition, everywhere alike. We take one single plant, with no opponents, nohelpers, and no rivals; it is to be a "fair field, and no favour". Now, I will ask you to imagine further that it shall be a plant which shallproduce every year fifty seeds, which is a very moderate number for aplant to produce; and that, by the action of the winds and currents, these seeds shall be equally and gradually distributed over the wholesurface of the land. I want you now to trace out what will occur, andyou will observe that I am not talking fallaciously any more than amathematician does when he expounds his problem. If you show that theconditions of your problem are such as may actually occur in nature anddo not transgress any of the known laws of nature in working out yourproposition, then you are as safe in the conclusion you arrive at asis the mathematician in arriving at the solution of his problem. Inscience, the only way of getting rid of the complications with which asubject of this kind is environed, is to work in this deductive method. What will be the result, then? I will suppose that every plant requiresone square foot of ground to live upon; and the result will be that, in the course of nine years, the plant will have occupied every singleavailable spot in the whole globe! I have chalked upon the blackboardthe figures by which I arrive at the result:--

Plants. Plants 1 x 50 in 1st year = 50 50 x 50 " 2nd " = 2, 500 2, 500 x50 " 3rd " = 125, 000 125, 000 x 50 " 4th " = 6, 250, 000 6, 250, 000 x 50" 5th " = 312, 500, 000 312, 500, 000 x 50 " 6th " = 15, 625, 000, 00015, 625, 000, 000 x 50 " 7th " = 781, 250, 000, 000 781, 250, 000, 000 x 50 "8th " = 39, 062, 500, 000, 000 39, 062, 500, 000, 000 x 50& " 9th " =1, 953, 125, 000, 000, 000

51, 000, 000 sq. Miles--the dry surface of the earth x 27, 878, 400--thenumber of sq. Ft. In 1 sq. Mile = sq. Ft. 1, 421, 798, 400, 000, 000 being531, 326, 600, 000, 000 square feet less than would be required at the endof the ninth year. 

You will see from this that, at the end of the first year the singleplant will have produced fifty more of its kind; by the end of thesecond year these will have increased to 2, 500; and so on, in succeedingyears, you get beyond even trillions; and I am not at all sure that Icould tell you what the proper arithmetical denomination of the totalnumber really is; but, at any rate, you will understand the meaning ofall those noughts. Then you see that, at the bottom, I have taken the51, 000, 000 of square miles, constituting the surface of the dry land;and as the number of square feet are placed under and subtracted fromthe number of seeds that would be produced in the ninth year, you cansee at once that there would be an immense number more of plants thanthere would be square feet of ground for their accommodation. This iscertainly quite enough to prove my point; that between the eighth andninth year after being planted the single plant would have stocked thewhole available surface of the earth. 

This is a thing which is hardly conceivable--it seems hardlyimaginable--yet it is so. It is indeed simply the law of Malthusexemplified. Mr. Malthus was a clergyman, who worked out thissubject most minutely and truthfully some years ago; he showed quiteclearly, --and although he was much abused for his conclusions at thetime, they have never yet been disproved and never will be--he showedthat in consequence of the increase in the number of organic beings ina geometrical ratio, while the means of existence cannot be made toincrease in the same ratio, that there must come a time when the numberof organic beings will be in excess of the power of production ofnutriment, and that thus some check must arise to the further increaseof those organic beings. At the end of the ninth year we have seen thateach plant would not be able to get its full square foot of ground, andat the end of another year it would have to share that space with fiftyothers the produce of the seeds which it would give off. 

What, then, takes place? Every plant grows up, flourishes, occupies itssquare foot of ground, and gives off its fifty seeds; but notice this, that out of this number only one can come to anything; there is thus, as it were, forty-nine chances to one against its growing up; it dependsupon the most fortuitous circumstances whether any one of these fiftyseeds shall grow up and flourish, or whether it shall die and perish. This is what Mr. Darwin has drawn attention to, and called the "STRUGGLEFOR EXISTENCE"; and I have taken this simple case of a plant becausesome people imagine that the phrase seems to imply a sort of fight. 

I have taken this plant and shown you that this is the result of theratio of the increase, the necessary result of the arrival of a timecoming for every species when exactly as many members must be destroyedas are born; that is the inevitable ultimate result of the rate ofproduction. Now, what is the result of all this? I have said that thereare forty-nine struggling against every one; and it amounts to this, that the smallest possible start given to any one seed may give it anadvantage which will enable it to get ahead of all the others; anythingthat will enable any one of these seeds to germinate six hours beforeany of the others will, other things being alike, enable it to chokethem out altogether. I have shown you that there is no particular inwhich plants will not vary from each other; it is quite possiblethat one of our imaginary plants may vary in such a character as thethickness of the integument of its seeds; it might happen that one ofthe plants might produce seeds having a thinner integument, and thatwould enable the seeds of that plant to germinate a little quickerthan those of any of the others, and those seeds would most inevitablyextinguish the forty-nine times as many that were struggling with them. 

I have put it in this way, but you see the practical result of theprocess is the same as if some person had nurtured the one and destroyedthe other seeds. It does not matter how the variation is produced, solong as it is once allowed to occur. The variation in the plant oncefairly started tends to become hereditary and reproduce itself; theseeds would spread themselves in the same way and take part in thestruggle with the forty-nine hundred, or forty-nine thousand, with whichthey might be exposed. Thus, by degrees, this variety, with some slightorganic change or modification, must spread itself over the wholesurface of the habitable globe, and extirpate or replace the otherkinds. That is what is meant by NATURAL SELECTION; that is the kind ofargument by which it is perfectly demonstrable that the conditions ofexistence may play exactly the same part for natural varieties as mandoes for domesticated varieties. No one doubts at all that particularcircumstances may be more favourable for one plant and less so foranother, and the moment you admit that, you admit the selective power ofnature. Now, although I have been putting a hypothetical case, you mustnot suppose that I have been reasoning hypothetically. There are plentyof direct experiments which bear out what we may call the theory ofnatural selection; there is extremely good authority for the statementthat if you take the seed of mixed varieties of wheat and sow it, collecting the seed next year and sowing it again, at length you willfind that out of all your varieties only two or three have lived, orperhaps even only one. There were one or two varieties which were bestfitted to get on, and they have killed out the other kinds in justthe same way and with just the same certainty as if you had taken thetrouble to remove them. As I have already said, the operation of natureis exactly the same as the artificial operation of man. 

But if this be true of that simple case, which I put before you, wherethere is nothing but the rivalry of one member of a species with others, what must be the operation of selective conditions, when you recollectas a matter of fact, that for every species of animal or plant thereare fifty or a hundred species which might all, more or less, becomprehended in the same climate, food, and station;--that every planthas multitudinous animals which prey upon it, and which are its directopponents; and that these have other animals preying upon them, --thatevery plant has its indirect helpers in the birds that scatter abroadits seed, and the animals that manure it with their dung;--I say, whenthese things are considered, it seems impossible that any variationwhich may arise in a species in nature should not tend in some way orother either to be a little better or worse than the previous stock;if it is a little better it will have an advantage over and tend toextirpate the latter in this crush and struggle; and if it is a littleworse it will itself be extirpated. 

I know nothing that more appropriately expresses this, than the phrase, "the struggle for existence"; because it brings before your minds, in avivid sort of way, some of the simplest possible circumstances connectedwith it. When a struggle is intense there must be some who are sure tobe trodden down, crushed, and overpowered by others; and there will besome who just manage to get through only by the help of the slightestaccident. I recollect reading an account of the famous retreat ofthe French troops, under Napoleon, from Moscow. Worn out, tired, anddejected, they at length came to a great river over which there wasbut one bridge for the passage of the vast army. Disorganised anddemoralised as that army was, the struggle must certainly have been aterrible one--every one heeding only himself, and crushing through theranks and treading down his fellows. The writer of the narrative, whowas himself one of those who were fortunate enough to succeed in gettingover, and not among the thousands who were left behind or forced intothe river, ascribed his escape to the fact that he saw striding onwardthrough the mass a great strong fellow, --one of the French Cuirassiers, who had on a large blue cloak--and he had enough presence of mind tocatch and retain a hold of this strong man's cloak. He says, "I caughthold of his cloak, and although he swore at me and cut at and struck meby turns, and at last, when he found he could not shake me off, fell toentreating me to leave go or I should prevent him from escaping, besidesnot assisting myself, I still kept tight hold of him, and would not quitmy grasp until he had at last dragged me through. " Here you see wasa case of selective saving--if we may so term it--depending for itssuccess on the strength of the cloth of the Cuirassier's cloak. It isthe same in nature; every species has its bridge of Beresina; it hasto fight its way through and struggle with other species; and when wellnigh overpowered, it may be that the smallest chance, something in itscolour, perhaps--the minutest circumstance--will turn the scale one wayor the other. 

Suppose that by a variation of the black race it had produced the whiteman at any time--you know that the Negroes are said to believe this tohave been the case, and to imagine that Cain was the first white man, and that we are his descendants--suppose that this had ever happened, and that the first residence of this human being was on the West Coastof Africa. There is no great structural difference between the white manand the Negro, and yet there is something so singularly different in theconstitution of the two, that the malarias of that country, which do nothurt the black at all, cut off and destroy the white. Then you see therewould have been a selective operation performed; if the white man hadrisen in that way, he would have been selected out and removed by meansof the malaria. Now there really is a very curious case of selection ofthis sort among pigs, and it is a case of selection of colour too. In the woods of Florida there are a great many pigs, and it is a verycurious thing that they are all black, every one of them. ProfessorWyman was there some years ago, and on noticing no pigs but these blackones, he asked some of the people how it was that they had no whitepigs, and the reply was that in the woods of Florida there was a rootwhich they called the Paint Root, and that if the white pigs were to eatany of it, it had the effect of making their hoofs crack, and they died, but if the black pigs eat any of it, it did not hurt them at all. Herewas a very simple case of natural selection. A skilful breeder could notmore carefully develope the black breed of pigs, and weed out all thewhite pigs, than the Paint Root does. 

To show you how remarkably indirect may be such natural selectiveagencies as I have referred to, I will conclude by noticing a casementioned by Mr. Darwin, and which is certainly one of the most curiousof its kind. It is that of the Humble Bee. It has been noticed thatthere are a great many more humble bees in the neighbourhood of towns, than out in the open country; and the explanation of the matter is this:the humble bees build nests, in which they store their honey and depositthe larvae and eggs. The field mice are amazingly fond of the honey andlarvae; therefore, wherever there are plenty of field mice, as in thecountry, the humble bees are kept down; but in the neighbourhood oftowns, the number of cats which prowl about the fields eat up the fieldmice, and of course the more mice they eat up the less there are to preyupon the larvae of the bees--the cats are therefore the INDIRECT HELPERSof the bees! [2] Coming back a step farther we may say that the oldmaids are also indirect friends of the humble bees, and indirect enemiesof the field mice, as they keep the cats which eat up the latter! Thisis an illustration somewhat beneath the dignity of the subject, perhaps, but it occurs to me in passing, and with it I will conclude thislecture. 

[Footnote 1: I lay stress here on the 'practical' signification of"Species. " Whether a physiological test between species exist or not, itis hardly ever applicable by the practical naturalist. ]

[Footnote 2: The humble bees, on the other hand, are direct helpers ofsome plants, such as the heartsease and red clover, which are fertilizedby the visits of the bees; and they are indirect helpers of the numerousinsects which are more or less completely supported by the heartseaseand red clover. ]