^ift»»»ft;5;''' /Lie. -LI. IFram tl|p Ctbraro nf iBrqurattirb hg l|tm to tl|p ^library of JPrtnrrtan Slirnlngtral &rmtnarg sec 3:257 ox THE ORIGIN OF SPEGIES. " But with regard to tlic luatcrial world, vfc can at least go so far as tliia — wc can perceive that events are brought about not by insulated interpo- sitions of Divine power, exerted in each particular case, but by the estab- lishment of general laws." Whewkll: Bridjcwatcr Treatise. " The only distinct moaning of the word ' natural ' is slalnl, Jixcel, or tcllled ; since what is natural as much requires and presupposes an intelli- gent agent to render it so, i. c., to effect it continually or at stated times, as what is supernatural or miraculous does to effect it for once." Butler : An(do. A PPL ETON AND COMPANY, 5 4 9 & 5 5 1 BROADWAY. 1871. OOISTTEIS'TS nisTORiCAL Sketch ........ paoe 9 I.VTBOnUCTION ......... 17 CHAPTER I. VARIATION CNDE.I DOMESTICATION. Causes of Variability— EfTocts of Habit— Correlated Variation— Inheritance— Char- acter of Domestic Varieties — Diflftculty of distinguishing between Varieties and Species— Origin of Domestic Varieties from one or more Species — Domestic Pigeons, their Differences and Origin— Principles of Selection, anciently followed, their Effects — Methodical and Unconscious Selection— Unknown Origin of our Domestic Productions— Circumstances favorable to Man's Power of Selection 23 CHAPTER II. VARIATION UNDER NATCRE. Vailability- Individual Differences— Doubtful Species— Wide-ranging, much dif- fused, and Common Species, vary most— Species of the Larger Genera in each Country vary more frequently than the Species of the Smaller Genera— Many of the Species of the Larger Genera resemble Varieties in being very closely, but unequally, related to each other, and in having Restricted Ranges . 52 CHAPTER III. STRUGGLE FOR EXISTENCE. its bearing on Natural Selection— The Term used in a wide Sense— Geometrical rjutio of Increase — Rai)id Increase of Naturalized Animals and Plants — Nature of tlio Checks to Increase— Competition universal— Effects of Climate— Protection fri)ni the Number of Individuals— Complex Relations of all Animals and Plants through 6 CONTENTS. out Nature— Strugiclo for Life most severe hctwccn Individuals and Varieties of the same Species : often severe between Species of the same Genns— The Rela- tion of Organism to Organism the most important of all Relations faoe 69 CHAPTER IV. NATURAL PKLKCTION, OR THE SrUVIVAL OF TIIK FITTEST. Natural Selection— its Power compared with Man's Selection — its Power on Char- acters of trifling Importance— its Power at all Ages and on both Sexes — Sexual Selection— On the Generality of Intercrosses bi^tween Individuals of the same Species — Circumstances fiivorable and unfavorable to the Uesults of Natural Se- lection, namely, Intercrossing, Isolation, Number of Individuals — Slow Action — Extinction caused by Natural Selection— Divergence of Character related to the Diversity of Inhabitants of any Small Area, and to Naturalization— Action of Nat- ural Selection, through Divergence of Character and Extinction, on the Descend- ants from a Common Parent — Explains the Grouping of all Organic Beings— Ad- vance in Organization— Low Forms preserved — Objections considered— Uniform- ity of certain Characters due to their Unimportance and to their not having been acted on by Natural Selection— Indefinite Multiplication of Species— Summary. CHAPTER V. LAWS OF VARIATION. Effects of changed Conditions — Use and Disuse, combined with Natural Selection ; Organs of Flight and of Vision — Acclimatization — Correlated Variation — Com- pensation and Economy of Growtli — False Correlations — Multiple, Rudimentary, and Lowly-organized Structures variable — Parts developed in an Unusual Man- ner are highly variable: Specific Characters more variable than Generic: Second- ary Sexual Characters variable — Species of the same Genus vary in an analogous Manner- Reversions to long-lost Characters — Summary . . . 137 CHAPTER VI. DIFFICULTIES OF THE TIIEORV. Difficulties of the Theory of Descent with Modification— Transitions— Absence or Rarity of Transitional Varieties — Transitions in Habits of Life — Diversified Elal)- its in the same Species— Species with Habits widely diO'erent from those of their Allies— Organs of Extreme Perfection — Modes of Traifsition — Cases of Difficulty — Natura non facit saltum— Organs of small Importance— Organs not in all Cases absolutely perfect — The Law of Unity of Type and of the Conditions of Existence embraced by the Theory of Natural Selection .... 167 CHAPTER VII. INSTINCT. Instincts comjiarable with Ilabils, but different in tlu-ir Origin— Instincts graduated — Aphides and Ants— Instincts variable— Domestic Instincts, their Origin— Nat- CONTENTS. Y nral lOBtincls of the Cuckoo, Ostrich, and Parasitic Bees— Slave-making Ants — Hive-bee, its cell-making instinct— ChanKCs of In! lishing in detail all the facts, Avith references, on wliich my conclusions have been grounded ; and I hope in a future work to do this. For I am well aware that scarcely a single point is discussed in this volume on Avjiich facts cannot be adduced, often apparently leading to conclusions directly opposite to those at Avhich I have arrived. A fair result can be obtained only by fully stating and balancing tlie facts and arguments on both sides of each question ; and this is here imj^ossible. I much regret that Avant of space prevents my having the satisfaction of acknowledging the generous assistance Avhich I have received from A^ery many naturalists, some of them per- sonally unknoAvn to me. I cannot, hoAvevcr, let this 02')por- tunity pass Avithout expressing my deep obligations to Dr. Hooker, Avho for tlie last fifteen years has aided me in CA'ery [wssible Avay by his large stores of knoAAdedge and liis excellent judgment. In considering the Origin of Species, it is quite conceivable that a naturalist, reflecting on the mutual affinities of organic beings, on their embryological relations, their geographical distribution, geological succession, and other such facts, might come to the conclusion that species had not been independently created, l)ut had descended, like varieties, from other species. Nevertlielcss, such a conclusion, even if Avell founded, Avould be unsatisfactory, imtil it could be shoAvn how the innumerable species inhabiting this Avorld haA'e been modified, so as to ac- quin? that perfection of structure and coadaptation Avhich justly excites our admiration. Naturalists continually refer to exter- nal conditions, such as climate, food, etc., as the only possible cause of variation. In one limited sense, as Ave shall hereafter INTRODUCTION. 19 see, this may bo true ; but it is preposterous to attribute to mere exteriuil conditions, the structure, for instance, of the woodpecker, with its feet, tail, beak, and tonjTue, so ad- mirably adapted to catch insects under the bark of trees. In the case of the mistletoe, which draws its nourishment from certain trees, M'hich has seeds that must be transported by cer- tain birds, and which has flowers with separate sexes absolutely requiring the ag^ency of certain insects to brin<^ pollen from one flower to the other, it is equally preposterous to account for the structure of this parasite, with its relations to several dis- tinct organic beings, by the efl'ects of external conditions, or of habit, or of the volition of the plant itself. It is, therefore, of the highest importance to gain a clear insight into the means of modification and coadaptation. At the coramencem(mt of my observations it seemed to me prob- able that a carcfyl study of domesticated animals and of culti- vated plants would oiler the best chance of making out this obscure problem. Nor have I been disappointed; in this and in all other perplexing cases I have invariably found that our knowledge, imperfect though it be, of variation under domesti- cation, aflbrded the best and safest clew. I may venture to express my conviction of the high value of such studies, al- though they have been very commonly neglected by natural- ists. From these considerations, I shall devote the first chapter of this Aljstract to Variation under Domestication. We shall thus see that a large amount of hereditary modification is at least possible ; and, what is equally or more important, we shall see how great is the jiower of man in accumulating by his Selection successive slight variations. I will then pass on to the variability of species in a state of nature ; but I shall, unfortunately, be compelled to treat this subject far too briefly, as it can be treated properly only by giving long catalogues of facts. We shall, however, be enabled to discuss what cir- cumstances are most favorable to variation. In the next chap- ter the Struggle for Existence among all organic beings througliout the world, which inevitably follows from the high geometrical ratio of their increase, Avill be treated of. This is the doctrine of Malthus, applied to the whole animal and vege- table kingdoms. As many more individuals of each species are born than can possil)ly survive ; and as, consequently, there is a frequently-recurring struggle for existence, it follows that any being, if it vary however slightly in any manner profitable 20 INTRODUCTION. to itself, under tlie complex and sometimes varying conditions of life, uill have a better chance of survivinq-, and thus be nat- la'cdhj sdectcd. From the strong principle of inheritance, any selected variety will tend to propagate its new and modified form. This fundamental subject of Natural Selection will be treated at some length in tlie fourth chapter ; and we shall then see how Natural Selection almost inevitably causes much Extinction of the less improved forms of life, and leads to what I have called Divergence of Character. In the next chapter I shall discuss the com2:)lcx and little-known laws of variation. In the four succeeding chajitcrs, the most apparent and gravest difficulties in accepting the theory will be given : namely, first, the difficulties of transitions, or how a simple being or a simple organ can be changed and perfected into a highly-developed being or into an elaborately-constructed organ ; secondly, the subject of Instinct, or the mental powers of animals ; thirdly, Hybridism, or the infertility of species, and the fertility of va- rieties when intercrossed ; and fourthly, the imperfection of the Geological Record. In the next chapter I shall consider the geological succession of organic beings throughout time ; in the eleventh and twelfth, their geographical distribution throughout space ; in the thirteenth, their classification or mu- tual atlinities, both when mature and in an embrj-onic condi- tion. In the last chapter I shall give a brief recapitulation of the whole work, and a few concluding remarks. No one ought to feel surprise at much remaining as yet unexplained in regard to the origin of species and varieties, if he make due allowance for our profound ignorance in regard to the mutual relations of the many beings which live around us. AVho can explain why one species ranges Avidely and is very numerous, and why another allied species has a narrow range and is rare ? Yet these relations are of the highest im- ]')ortance, for they determine the present welfare, and, as I be- lieve, the future success and modification of every inhabitant of tins world. Still less do we know of tlic mutual relations of the innumerable inhabitants of the world during tlie man}"- past geological epochs in its history. Although much remains obscure, and Avill long remain obscure, I can entertain no doubt, after the most deliberate study and dispassionate judg- ment of which I am cajiable, that the view which most natu- ralists entertain, and which I fonnerly entertained — namely, that each species has been independently created — is erio- INTRODUCTION. 21 neons. I am fully convinced that species arc not immutable , but that those belonging to what are called the same genera are hncal descendants of some other and generally extinct spe- cies, in the same manner as the acknowledged varieties of any one sjiecies arc the descendants of that species. Further- more, I am convinced that Natural Selection has been the most important but not the exclusive means of modification. 22 VARIATION Chap. I. GIIAPTER I. VAEIATION UNDER DOMESTICATION. C!aii8PS of Variability— Effects of Habit— Correlated Variation— Inheritance— Char- acter of Domestic Varieties — Ditliculty of (liHtintruiehinj,' between Varieties and Species — Orii^in of Dome.stic Varieties from oiie or inorc (species — Domestic Pigeons, their Differences and Origin— Principles of Selection, anciently followed, their Effects — Methodical and Uuconscious Selection— Unknown Orisin of our Domestic Productions— Circumstances favorable to Man's Power of Selection. Causes of Yariahility. WnEN we compare the individuals of the same variety or sub-variety of our older cultivated plants and animals, one of the first points which strikes us is, that they generallj' differ from each other more than do the indi^aduals of any one spe- cies or variety in a state of nature. And if we reflect on the vast diversity of the plants and animals which have been cul- tivated, and which have varied during all ages under the most different climates and treatment, Ave are driven to conclude that this great variability is due to our domestic productions having been raised under conditions of life not so uniform as, and somewhat different from, those to which the parent-species had been exposed luider nature. There is also, I think, some probability in the view propounded by Andrew Knight, that this variability may be partly connected with excess of food. It seems clear that organic beings must be exposed during several generations to new conditions to cause any appreciable amount of variation ; and that, when the organization has once l)cgun to vary, it generally continues varying for many gencr- atitms. No case is on record of a variable organism ceasing to vary imder cultivation. Our oldest cultivated plants, siicli as wheat, still yield new varieties : our oldest domesticated animals arc still capable of rapid improvement or modifica- tion. As far as I am able to judge, after long attending to the subject, the conditions of life ajipcar to act in two ways — di- CuAP. I. U^'DER DOMESTICATION. 03 rcctly ou the -wliole orn-anizalion or on certain parts alone, and indirectly by affecting the reproductive system. With respect to the direct action, Ave must bear in mind that in every case, as Prof. Weismann has lately insisted, and as I have incident- ally shown in my -work on " Variation under Domestication," there are two factors : namely, the nature of the organism, and the nature of the conditions. The former seems to be much the more important; for nearly similar variations sometimes arise under, as far as we can judge, dissimilar conditions ; and, on the other hand, dissimilar variations under conditions which appear to be nearly uniform. The effects on the offspring are cither defmite or indefmitc. They may be considered as defi- nite when all or nearly all the offspring of individuals exposed to certain conditions during several generations are modified in the same manner. It is extremely difficult to come to any conclusion in rcgjird to the extent of the changes which have been thus definitely induced. There can, however, be little doubt about many slight changes — such as size from the amoimt of food, color from the nature of the food, thickness of the skin and hair from climate, etc. Each of the endless vari- ations which we sec in the plumage of our fowls must hare had some efficient cause ; and if the same cause were to act uniformly during a long scries of generations on many indi- viduals, all probably would be modified in the same manner. Such facts as the complex and extraordinary outgrowths which invariably follow from the insertion of a minute drop of ])oison by a gall-producing insect, show us what singular mod- ifications might result in the case of plants from a chemical change in the nature of the sap. Indefinite variability is a much more common result of changed conditions than definite variability, and has probably played a more important part in the formation of our domestic races. We see indefinite Aariability in the endless slight pe- culiarities which distinguish the individuals of the same spe- cies, and which cannot be accounted for by inheritance from cither parent or from some more remote ancestor. Even strongly-marked differences occasionally appear in the young of the same litter, and in seedlings from the same seed-capsules. At long intervals of time, out of millions of individuals rc^ared in the same country and fed on nearly the same fooJ, dona- tions of structure so strongly pronounced as to deserve to be called monstrosities arise; but monstrosities cannot be sepa- rated by any distinct Une from slighter variations. All such 24 VARIATION Chap. I. changes of structure, ■whether extremely slight or strongly marked, which appear among many individuals living together, may be considered as the indefinite elTects of the conditions of life on each individual organism, in nearly the' same manner as a chill allxicts diflerent men in an indefinite manner, according to their state of body or constitution, causing coughs or colds, rheumatism, or inflammations of various organs. With respect to what I have called the indirect action of changed conditions, namely, through the reproductive system being afTected, Ave may infer that variability is thus induced, partly from the fact of this system being extremely sensitive to any change in the conditions, and partly from the similarity, as Kolreutcr and others have remarked, between the variabil- ity which follows from the crossing of distinct species, and that which may be observed with all plants and animals Avhen reared under new or unnatural conclitions. Many facts clearly show how eminently susceptible the reproductive sys- tem is to very slight changes in the surrounding conditions. Nothing is more easy than to tame an animal, and few things more difficult than to get it to breed freely under confinement, oven Avhen the male and female unite. How many animals there are Avhich Avill not breed, though kept in an almost free state in their native country ! This is generally, but errone- ously, attributed to vitiated instincts. Many cultivated plants display the utmost vigor, and yet rarely or never seed ! In some few cases it has been discovered that a very trifling change, such as a little more or less water at some particular period of growth, will determine whether or not a plant will produce seeds. I cannot here give the details which I liave collected and elscAvhere published on this curious subject ; but, to show how singular the laws are Avhich determine the repro- duction of animals under confinement, I may mention that car- nivorous animals, even from the tropics, breed in this country pretty freely under confinement, with the exception of the jilantigrades or bear family, Avhich seldom produce young; whereas carnivorous birds, with the rarest exceptions, hardly ever lay fertile eggs. Many exotic plants have jwllen utterly worthless, in the same condition as in the most sterile hybrids. When, on the one hand, we sec domesticated animals and jilants, though often weak and sickly, yet breechng freely un- der confinement ; and when, on the other hand, we see indi- viduals, though taken young from a state of nature, ]>erfectly tamed, long-lived, and healthy (of which I could give numerous Chap. I. UNDEIl DOMESTICATIOin. 25 instances), yet having their reproductive system so seriously anccted by uiiperceived causes as to fail to act, Ave need not he surprised at this system, when it does act under confine- UKMit, acting irregularly, and jiroducing offspring' somewhat imlike their parents. I may adil, that as some organisms breed fn^ely under the most unnatural conditions (for instance, rali- bits and ferrets kept in hutches), showing that their reproduc- tive organs arc not affected ; so will some animals and plants withstand domestication or cultivation, and vary very slightly — perhaps hardly more than in a state of nature. Some naturalists have maintained that all variations are connected with the act of sexual reproduction ; but this is cer- tainly an eiTor; for I have given in another woi'k a long list of " sporting plants," as they are called by gardeners — that is, of plants which have suddenly produced a single bud with a new and sometimes widely-diilerent character from that of the other buds on the same plant. These bud-variations, as they may be named, can be propagated by grafts, offsets, etc., and sometimes by seed. They occur rarely under nature, but far from rarely under culture. As a single bud out of the manj^ thousands produced year after year under uniform conditions on the same tree, has been known suddenly to assmne a new character ; and as buds on distinct trees growing und(^r differ- ent conditions, have sometimes yielded nearly the same variety — for instance, buds on peach-trees producing nectarines, and buds on common roses producing moss-roses — we clearly see tliat the nature of the conditions is of quite subordinate impor- tance in comparison with the nature of the organism in deter- mining each particular form of variation — of not more impor- tance than the nature of the spark by wliich a mass of combus- t \h\o matter is ignited, has in determining the nature of the flames. Effects of Habit ; Correlated Variation / Inheritance. Habits are inherited and have a decided influence ; as in the period of the flowering of plants when transported from one climate to another. In animals they have a more marktnl effect ; for instance, I llnd in the domestic duck that the bones of the wing weigh less and the bones of the leg more, in pro- portion to the whole skeleton, than do the same bones in the wild-duck ; and this change may be safely attributed to the do- mestic duck flying mucli less, and walking more, than its Mild ]):irents. The great and inheriteeen supposed. The lake-inhabitants of Switzerland cul- tivated several kinds of wheat and barley, the pea, the poppy t\)v oil, and llax ; and they possessed several domesticatcnl ani- mals ; they had also commerce with other nations. All this clearly shows, as Heer has remarked, that they had at this early ag-c progressed considerably in civilization ; and this again implies a long-continued previous period of less advanced civilization, during which the domesticated animals, kept by the different tribes in different districts, might have varied and given rise to distinct races. Since the discovery of flint tools or celts in the superficial formations in many parts of the world, all geologists believe that barbax-ian man existed at an enor- mously remote period ; and we know that at the present day there is hardly a tribe so barbarous as not to have domesti- cated at least the dog. The origin of most of our domestic animals will probably forever remain vague. But I may here state that, looking to the domestic dogs of the Avholc world, I have, after a la- borious collection of all known facts, come to the conclusion that several wild species of Canidre have been tamed, and that their blood, in some cases mingled together, llows in the A-eins of our domestic breeds. In regard to sheep and goats, I can form no decided opinion. From facts communicated to me by Mr. Blyth, on the habits, voice, constitution, and structure of the humped Indian cattle, it is almost certain that they are de- scended from a different aboriginal stock from our European cattle ; and some competent judges believe that these latter have had two or three wild progenitors — whether or not those deserve to be called species or races. This conclusion, as Avell as the specific distinction between the humped and common cattle, may, indeed, be looked at as established by the recent admirable researches of Prof. Kutimeyer. With respect to horses, from reasons which I cannot here give, I am doubtfully inclined to believe, in opposition to several authors, that all the races belong to the saine species. Having kept nearly all the English breeds of the fowl alive, having bred and crossed them, and examined their skeletons, it appears to me almost certain 32 CU^AKACTER OF Chap.*. that all are the descendants of the wild Indian fowl, Gallus bankiva; and Ihis is tlie conclusion of Mr. Bljth, and of others who have studied this bird in India. In reg'ard to ducks and rabbits, some breeds of which difler much from each otiier, the evidence is clear that they arc all descended from the common wild duck and rabbit. The doctrine of the origin of our several domestic races from several aborig'inal stocks, has been carried to an absurd ex- treme by some authors. They believe that every race which breeds true, let the distinctive characters be ever so slight, has had its wild protot^qDe. At this rate, there must have ex- isted at least a score of species of wild cattle, as many sheep, and several goats, in Europe alone, and several even within Great Britain. One author believes that there formerly existed in Great Britain eleven wild species of sheej? peculiar to it ! When v,e bear in mind that Britain has now hardly one pecu- liar mammal, and France but few distinct from those of Ger- many, and conversely, and so with Hungary, Spain, etc., but that each of these kingdoms possesses several peculiar breeds of cattle, sheep, etc., we must admit that many domestic breeds have originated in Europe ; for whence have they been derived, as these several countries could not possess so large a number of peculiar species for parent-stocks ? So it is in India. Even in the case of the domestic dogs of the whole world, which I admit to have descended from several wild species, it cannot be doubted that there has been an immense amount of inherited variation ; for who will believe tliat animals closely resembling the Italian greyhound, the bloodhound, the bull-dog, pug-dog, or Blenheim spaniel, etc. — so xmlike all Avild Canida? — ever ex- isted freely in a state of nature ? It has often been loosely said that all our races of dogs have been produced by the crossing of a few aboriginal species ; but by crossing Ave can only get forms in some degree intermediate between their par- ents ; and if we account for our several domestic races by this process, we must admit the former existence of the most ex- treme forms, as the Italian greyhound, bloodhound, bull-dog, etc., in the wild state. Moreover, the possibility of making distinct races by crossing has been greatly exaggerated. Many cases are on record, showing that a race may be modified by occasional crosses, if aided by the careful selection of the in- dividuals which present the desired character; but to obtaui a race nearly intermediate between two extronely dilTerent races or species, would be very dillicult. Sir J. Sebright expressly CnAP. I. DOMESTIC VARIETIES. 33 experimented for this object, and failed. The oflspriii^v from the first cross between two pure breeds is tolerably and some- times (as I have found with pip^eons) extremely uniform, and every thing seems simple enouji-h ; but when these mongrels are crossed one "with another for several generations, hardly two of them are alike ; and then the extreme dilEculty of the task becomes apparent. Certainly, a breed intermediate be- tween tico very distinct breeds could not be got without ex- treme care and long-continued selection ; nor can I find a case on record of a permanent race having been thus formed. Breeds of the Domestic Pigeon, their Differences and Origin. Belie\ing that it is alwa^'S best to study some special group, I have, after deliberation, taken up domestic pigeons. I have kept every breed whieli I could purchase or obtain, and have lieen most kindly fav(^red with skins from several quarters of the world, more especially by the Hon. W. Elliot from India, and by the Hon. C. Murray from Persia. Many treatises in different languages have been published on pigeons, and some of them are very important, as being of considerable antiquity. I have associated witli several eminent fanciers, and have been permitted to join two of the London Pigeon Clubs. The diver- sity of the breeds is something astonishing. Compare the Eng- lish carrier and the short-faced tumbler, and see the Avonderful difference in tlieir beaks, entailing corresponding differences in tlieir skulls. The carrier, more especially the male bird, is also remarlvable from the wonderful development of the caruncu- lated skin about the head; and this is accompanied by greatly- elongated eyelids, very large external orifices to the nostrils, and a wide gape of mouth. The short-faced tumbler has a b(>ak in outline almost like that of a finch; and the common tuin])ler has the singular inherited habit of flying at a great height in a compact flock, and tumbling in the air head over heels. The runt is a bird of great size, with long, massive beak and large feet ; some of tlie sub-ljreeds of runts have very long necks, others very long Avings and tails, others singularly short tails. The barb is allied to the carrier, but, instead of a long beak, has a very short and broad one. The ])outer has a much- elongated body, wings, and legs ; and its enormously devel- oped crop, which it glories in inllating, may well excite aston- ishment and even laughter. The turbit has a very short and conical beak, with a line of reversed feathers down the breast; 34 DOMESTIC PIGEONS. Chap. I. and it has the hal)it of continually expanding slightly the up- per part of tlie a?so2:)hagus. The Jacobin has the feathers so much reversed along the back of the neck that the}' form a hood ; and it has, proportionally to its size, much-elongated wing and tail feathers. The trumpeter and laugher, as their names express, utter a very different coo from the otlier breeds. The fantail has thirty or even forty tail-feathers, instead of twelve or fourteen — the normal number in all members of the great pigeon family ; and these feathers are kept expanded, and are carried so erect, that in good birds the head and tail touch : the oil-gland is quite aborted. Several other less dis- tinct breeds might be specified. In the skeletons of the several breeds, the development of the bones of the face in length and breadth and curvature dif- fers enormously. The shape, as Avell as the breadth and length of the ramus of the lower jaw, varies in a highly-remarkable manner. The caudal and sacral vertebra? vary in number ; as does the number of the ribs, together with their relative breadth and the presence of processes. The size and shape of the apertures in the sternum are highly variable ; so is the degree of divergence and relative size of the two arms of the furcula. The proportional width of the gape of mouth, the proportional length of the eyelids, of the orifice of the nostrils, of the tongue (not always in strict correlation with the length of beak), the size of the crop and of the upper part of the ccso})liagus ; the development and abortion of the oil-gland ; the number of the primary wing and caudal feathers ; the relative length of wing and tail to each other and to the body ; the relative length of leg and of the feet ; the number of scutcHa^ on the toes, the develop- ment of skin between the toes, arc all points of structure -which are variable. The period at which the perfect plumage is ac- quired, varies, as does the state of the do^^^l with which the nest- ling birds are clothed when hatched. The shape and size of the eggs vary. The manner of flight, and in some breeds the voice and disposition, differ remarkably. Lastly, in certain breeds, the males and females have come to dilVer in a slight degree from each other. Altogether, at least a score of pigeons might be chosen, which, if shown to an ornithologist, and he were told that they were wild birds, would certainly be ranked by him as well-de- fined species. Moreover, I do not believe that any ornitliolo- gist would place the English canicr, the short-faced tumbler, the runt, tlic barb, pouter and fantail in the same genus; Chat. I. DOMESTIC TIGEONS. ,35 more especially as in each of these breeds several truly-inher- ited sub-breeds, or s2Dccies as he would have called them, could be shown him. Great as the differences are between the breeds of pigeons, I am fully convinced that the common opinion of naturalists is correct, namely, that all are descended from the rock-pigeon (Columba livia), including under this term several geograplii- cal races or sul>specics, Avhich differ from each other in the most trifling respects. As several of the reasons which have led me to this belief are in some degree applicable in other cases, I will here Ijricfly give them. If the several breeds are not varieties, and have not proceeded from the rock-pigeon, they must have descended from at least seven or eight abori- ginal stocks ; for it is impossible to make the present domestic breeds by the crossing of any lesser number : how, for instance, could a pouter be produced by crossing two breeds unless one of the parent-stocks possessed the characteristic enonnous crop? The supposed aboriginal stocks must all have been rock-pigeons, that is, not breeding or willingly perching on trees. But besides Columba livia, Avith its geographical sub- species, only two or three other species of rock-pigeons are known ; and these have not any of the characters of the domestic breeds. Hence the supposed aboriginal stocks must either still exist in the countries where they were originally domesticated, and yet be unknown to ornithologists ; and this, considering their size, habits, and remarkable characters, seems improbable ; or they must have become extinct in the wild state. But birds breeding on precijiices, and good fliers, are unlikely to be exterminated ; and the common rock-pigeon, which has the same habits with the domestic breeds, has not been exterminated even on several of the smaller British isiecs, or on the shores of the Mediterranean. Hence the supposed extermination of so many species having similar habits with the rock-pigeon seems a very rash assumption. Moreover, the several above-named domesticated breeds have been trans- ported to all parts of tlie world, and, therefore, some of them must have been carried back again into their native country; but not one has ever become wild or feral, though the dovecot- pigeon, which is the rock-pigeon in a very slightly altered state, has become feral in several places. Again, all recent experience shows that it is difficult to get any wild animal to breed freely imder domestication : yet, on the hypothesis of tlie multiple oritrin of our pigeons, it must be assumed that at least 36 ^ DOMESTIC PIGEONS. Chap. I. seven or eight species were so thoroughly domesticated in ancient times by half-civilized man, as to be quite prolific un- der confinement. An argument of grcat_ weight, and applicable in several other cases, is, that the above-specified breeds, though agree- ing generally with the wild rock-pigeon in constitution, habits, voice, coloring, and in most parts of their structure, yet are certainly highly abnormal in other parts ; we may look in vain throughout the whole great family of Columbida? for a beak liUe that of the English carrier, or that of the short-faced tum- bler, or barb; for reversed feathers like those of the Jacobin; for a crop like that of the pouter; for tail-feathers like those of the fantail. Hence it must be assumed, not only that half- civilized man succeeded in thoroughly domesticating several species, but that he intentionally or by chance picked out ex- traordinarily abnormal species ; and further, that these very species have since all become extinct or unknown. So many strange contingencies are improbable in the highest degree. Some facts in regard to the coloring of pigeons Avell de- serve consideration. The rock-pigeon is of a slaty-blue, with white loins ; the Indian sub-species, C intermedia of Strick- land, having this part bluish ; the tail has a terminal dark bar, with the outer feathers externally edged at the base with white ; the wings have two black bars. Some semi-domestic breeds, and some truly wild breeds, have," besides the two black bars, the wings checkered with black. These several marks do not occur together in any other species of the Avhole family. Now, in every one of the domestic breeds, taking thoroughly well-bred birds, all the above marks, even to the white edging of the outer tail-feathers, sometimes concur per- fectly developed. Moreover, when birds belonging to two or more distinct breeds are crossed, none of Avhich are blue or have any of the above-specified marks, the mongrel offspring are very apt suddenly to acquire these characters. To give one instance out of several which I have observed : I crossed some white fantails, which breed very true, with some black barbs — and it so happens that blue varieties of barbs are so rare that I never heard of an instance in England; and the mongrels were black, brown, and mottled. I also crossed a barb with a spot, which is a white bird with a red tail and red spot on the forehead, and Avhich notoriously breeds very true ; the mongrels were dusky and mottled. I then crossed one of the mongrel barb-fanlails with a mongrel barb-spot, and Chap. I. DOMESTIC PIGEONS. 37 tlicy produced a bird of as beautiful a bhie color, with the white loius, double black Aviiif^-bar, and barred and Avliite- ed^cd tail-feathers, as any wild rock-pi2;'eon ! We can under- stand these facts, on the well-known principle of reversion to ancestral characters, if all the domestic breeds are descended from the rock-])igeoii. ]}ut if we deny this, we must make one of the two following; highly improbable suppositions : cither, first, that all the several imagined aboriginal stocks were colored and marked like the rock-pigeon, although no other existing species is thus colored and marked, so that in each separate breed there might be a tendency to revert to the very same colors and markings ; or, secondly, that each breed, even the purest, has within a dozen, or at most with- in a score, of generations, been crossed by the rock-pigeon : I say within a dozen or twenty generations, for no instance is known of crossed descendants reverting to an ancestor of foreign' blood, removed by a greater nmnbcr of genera- tions. In a breed which has been crossed only once, the tendency to revert to any character derived from such a cross will naturall}'^ become less and less, as in each succeeding generation there will be less of the foreign blood ; but when there has been no cross, and there is a tendency in the breed to revert to a character which was lost during some former generation, this tendency, for all that we can see to the contrary, may be transmitted imdiminishcd for an indefinite nunil)er of generations. These two distinct cases of reversion are often confounded together by those who have Avritten on inheritance. Lastly, the hybrids or mongrels from between all the domestic breeds of pigeons are perfectly fertile. I can state this from my own oljservationR, purposely made, on the most distinct breeds. Now, it is diflicult, perhaps impossible, to bring forward one case of the hybrid offspring of two animals clenrbj distbict being themselves perfectly fertile. Some authors believe that long-continu(?d domestication eliminates this strong tendency to sterilily: from the histor}' of tlie dog, and of some other domestic animals, there is great probability in this hypothesis, if applied to species closely related to each other, thougli it is imsupported by a single experiment. But A.O extend the h^11otIlesis so far as to suppose that sjx'cies, aboriginally as distinct as carriers, tumblers, pouters, and fan- tails now are, should yield offspring perfectly fertile inter se^ seems to me rash in the extreme. From these several reasons, namely, the improbability of 38 DOMESTIC PIGEONS. Chap. I. man having formerly got seven or eight supposed species of pigeons to breed freely under domestication ; these supposed species being quite unknown in a ■wild state, and their be- coming nowhere feral ; these species having very al^normal characters in certain respects, as conijoared with all other Co- lumbida?, thougli so like in most other respects to the rock- pigeon ; the bkic color and various black marks occasionally apjiearing in all the breeds, both when kept pure and when crossed ; the mongrel offspring being perfectly fertile — from these several reasons, taken together, we may safely conclude that all our domestic breeds have descended from the Columba livia Avith its geographical sub-species. In favor of this view, I may add, first, that C. livia, or the rock-pigeon, has been found capable of domestication in Europe and in India ; and that it agrees in habits and in a great num- ber of points of structure with all the domestic breeds. Second- ly, although an English carrier or a short-faced tumbler differs immensely in certain characters from the rock-pigeon, yet by comparing the several sub-breeds of these varieties, more especially those brought from distant countries, we can make in these two cases, and in some but not in all other cases, an almost perfect series between the extremes of stlncture. Thirdly, those characters which are mainly distinctive of each breed, for instance the wattle and length of beak of the car- rier, the shortness of that of the tumbler, and the number of tail-feathers in the fantail, ai'C in each breed eminently variable; and the explanation of this fact Avill be obvious Avhcn we come to treat of Selection. Fourthly, pigeons have been Avatched and tended Avith the utmost care, and loved by many people. They liaA'C been domesticated for thousands of years in several quarters of the Avorld ; the earliest knoAvn record of pigeons is in the fifth Eg^-ptian dynasty, about 3000 B. c, as Avas pointed out to me by Prol; Lcpsius ; but Mr. Birch informs me that pigeons are given in a bill of fare in the previous dynasty. In the time of the Romans, as AA'e hear from Pliny, immense prices Averc giAxn for pigeons ; " nay, they are come to this pass, that they can reckon up their pedigree and race." Pigeons Avere much A'alued by Akbar Ivlian in India, about the j-ear IGOO; never less than 20,000 pigeons Avere taken Avith the tourt. " The monarchs of Iran and Turan sent him some very rare birds;" and, continues the courtly historian, " his Majesty by crossing the breeds, Avhich method Avas ncA'cr practised before, has improved them astonishingly." About Chap. I. DOMESTIC PIGEONS. 39 this saino period the Dutch were as easier about pigeons as were the old Romans. The paramount importance of these considerations in explainini^ the immense amount of variation Avhich pigeons have undcr2;one, will be obvious when Ave treat of Selection. We shall then, also, see how it is that the several breeds so often have a somewhat monstrous character. It is also a most favorable circumstance for the production of distinct breeds, that male and female pigeons can be easily mated for life ; and thus different breeds can be kept together in the same aviary. I have discussed tlie probable origin of domestic pigeons at some, yet quite insufiieient, length ; because when I first kept pigeons and watched the several kinds, avcU knowing how truly they breed, I felt fully as mucli dilliculty in believing that since they had been domesticated they had all proceeded from a common parent, as any naturalist could in coming to a similar conclusion in regard to the many species of finches, or other groups of birds, in nature. One circumstance has struck me much ; namely, that nearly all the breeders of the various domestic animals and the cultivators of plants, v/ith whom I have conversed, or Avhose treatises I have read, are firmly convinced that the several breeds to Avhich each has attended, are descended from so many aboriginally distinct species. Ask, as I have asked, a celebrated raiser of Here- ford cattle, whether his cattle might not have descended from Long-horns, or both from a common parent-stock, and he Avill laugh you to scorn, I have never met a pigeon, or })oultry, or duck, or rabljit fancier, Avho was not fully convinced that each main breed was descended from a distinct species. Van Mons, in his treatise on pears and apples, shows how utterly he disbelieves that tlie several sorts, for instance a Ribston- pippin or Codlin-apple, could ever have proceeded from the seeds of the same tree. Innumerable other examples could be given. The explanation, I think, is simple : from long- continued study they are strongly impressed with the differ- ences between the several races ; and though they well know that each race varies slightly, for they win their prizes by selecting such slight differences, yet they ignore all general arguments, and refuse to sum u[) in their minds slight differ- ences accumulated during many successive generations. May not those naturalists who, knowing far less of the laws of inheritance than does the breeder, and knowing no more than be docs of the intermediate links in the long lines of descent, 40 SELECTION BY MAN. Cn.sr. I. yet admit that many of our domestic races are descended from the same parents — may they not learn a lesson of caution, when tlicy deride the idea of species in a state of nature being lineal descendants of other species ? Princi2>les of Selection anciently foUoiccd, and their Effects. r^et us now briefly consider the steps by which domestic races have been produced, either from one or from several allied species. Some elTect may be attributed to the direct and definite action of the external conditions of life, and some little to habit ; but he would be a bold man who would ac- count by such agencies for the differences between a dray and race horse, a greyhound and bloodhound, a carrier and tumbler jiigeon. One of the most remarkable features in our domesti- cated races is that we see in them adaptation, not indeed to the animal's or plant's own good, but to man's use or fancy. Some variations useful to him have probably arisen suddenl}-, or by one step ; many botanists, for instance, ])elieve that the fuller's teasel, with its hooks, which cannot be rivalled by any mechan- ical contrivance, is only a variety of the wild Dipsacus ; and this amount of change may have suddenly arisen in a seedling. So it has probably been with tlie turnspit-tlog ; and tliis is known to have been the case with the ancon sheep. But when we compare the dray-horse and race-horse, the dromedary and camel, the various breeds of sheep fitted either for cultivated land or mountain-pasture, with the wool of one breed good for one purpose, and that of another breed for another purpose ; when we compare the many breeds of dogs, each good for man in very different ways ; when we compare the game-cock, so pertinacious in battle, with other breeds so little quarrelsome, with "everlasting layers" which never desire to sit, and Avith the bantam so small and elegant ; w^hen we compare the host of agricultural, culinary, orchard, and flower-garden races of plants, most useful to man at dillerent seasons and for differ- ent purposes, or so beautiful in his eyes, we must, I think, look fi'rther than to mere variability. "We cannot sup]")ose that all the breeds were suddenly produced as perfect and as useful as we now see them ; indeed, in many cases, we know that this has not been their history. The key is man's ])ower of accumulative selection: Nature gives successive variations; man abreed, superior to any thing existing in the country. But, for our purpose, a kind of Selection, which may be called Unconscious, and which results frojn every one trying to pos- sess and breed from the best individual animals, is more im- portant. Thus, a man who intends keeping pointere naturally tries to get as good dogs as he can, and afterward breeds from his own best dogs, but he has no wish or exjiectation of per- manently altering the breed. Nevertheless, we may infer that (4 DNCONSCIOUS SELECTION. Ciiai-. I. Iliis process, continued during centuries, Avould improve and modify any I)recd, in the same "way as Bakeivell, Collins, etc., by this very same process, only carried on more metliodically, did greatly modify, even during their own lifetimes, the forms and qualities of their cattle. Slow and insensible changes of this kind could never be recognized unless actual measure- ments or careful drawings of the breeds in cjuestion had been made long ago, which might serve for comparison. In some cases, however, imchanged or but little changed individuals of the same breed may be found in less civilized districts, Avhere the breed has been less improved. There is reason to believe that King Charles's spaniel has been unconsciously modified to a large extent since the time of that monarch. Some highly-competent authorities are convinced that the setter is directly derived from the spaniel, and has probably been slowly altered from it. It is known that the English pointer has been greatly changed within the last century, and in this case the change has, it is believed, been chiefly elfected by crosses Avith the fox-hound ; but what concerns us is, that the change has ]:)ccn elTccted unconsciously and gradually, and yet so eifectually, that, thougli the old Spanish pointer cer- tainly came from Spain, Mr. Boitow has not seen, as I am informed by him, any native dog in Spain like our pointer. By a similar process of selection, and by careful training, the whole body of English race-horses have come to surpass in fleetness and size the parent Arab stock, so that the latter, 1)}' the regulations for the Goodwood Races, are favored in the Aveights tlicy carry. Lord Sjicncer and others have shown how the cattle of England have increased in weight and in early maturity, compared with the stock formerly kept in this country. By comparing the accounts given in old pigeon treatises of carriers ami tumblers with these breeds as now existing in Britain, India, and Persia, Ave can, I think, clearly trace the stages through Avhich they have insensibly passed, and come to dilfer so greatly from the rock-pigeon. Youatt gives an excellent illustration of the effects of a course of selection, AA-hich may be considered as unconsciously followed, in so far that the breeders could never have expect- ed, or even have A\ished, to produce the result Avliich ensueil — namely, the production of tAvo distinct strains. The two Hocks of Leicester sheep ke])t by Mr. Buckley and INIr. Burgess, as Mr. Youatt remarks, " have been purely bred from the original stock of Mr. Bakewell for upAvard of lifty years. There is not Ci.vr. I. UNCONSCIOUS SELECTION. 45 a suspicion existing in the mind of any one at all acquainted with the subject, that the owner of cither of them has deviated in any one instance from tlie pure blood of Mr. BakewcU's flock, and yet the difference between tlie sheep possessed by these two f^entlemen is so f^reat that they have the appear- ance of being quite difl'erent varieties." If there exist savages so barbarous as never to think of the inherited character of the offspring of their domestic ani- mals, yet any one animal particularly useful to them, for any special purpose, would be carefully preserved during famines and other accidents, to Avhich savages are so liable, and such choice animals would thus generally leave more offspring than the inferior ones ; so that in this case there would be a kind * of unconscious selection going on. We see the value set on animals even by the barbarians of Tierra del Fuego, by their killing and devouring their old women, in times of dearth, as of less value than their dogs. In plants the same gradual process of improvement, through the occasional preservation of the best indi\4duals, whether or not sufBciently distinct to be ranked at their first appearance as distinct varieties, and whether or not two or more species or races have become blended together by crossing, may plainly be recognized in the increased size and beauty which we now see in the varieties of the heart's-easo, rose, pelargonium, dahlia, and other plants, when compared \\-ith the older varie- ties or with their parent-stocks. No one would ever expect to get a first-rate heart's-ease or dahlia from the seed of a wild plant. No one would expect to raise a first-rate melting ]jear from the seed of the wild pear, though he might succeed from a poor seedling growing wild, if it had come from a garden- stock. The pear, though cultivated in classical times, appears, from Pliny's description, to have been a fruit of very inferior quality. I have seen great surprise expressed in horticultm-al works at the wonderful skill of gardeners, in having produced such splendid results from such poor materials ; but the art has been simple, and, as far as tlie final result is concerned, lias been followed almost unconsciously. It has consisted in always cultivating the best-known variety, sowing its seeds, antl, when a sliglitly-better variety has chanced to appear, selecting it, and so onward. But the gardeners of the classi- cal period, who cultivated the best pear they could procure, never thought Avhat splendid fruit we should eat; though wo. owe our excellent fruit, in some small degree, to their having to UNCONSCIOUS SELECTION. Chap. I. naturally chosen and preserved the best varieties they could anywhere inid. A laro-e amount of chang'c in our cultivated plants, thus slowly and unconsciously accumulated, explains, as I believe, the well-known fact that, in a number of cases, we cannot recognize, and therefore do not know, the wild parent-stocks of the plants which have been longest cultivated in our flower and kitchen gardens. If it has taken centuries or thousands of 3'^ears to improve or modify most of our plants up to their present standard of usefulness to man, we can understand how it is that neither Australia, the Cape of Good Hope, nor any other region inhabited by quite uncivilized man, has aiforded us a single plant worth culture. It is not that these countries, so rich in species, do not by a strange chance possess the abori- ginal stocks of any useful plants, but that the native plants have not been improved by continued selection up to a stand- ard of perfection comparable with that given to tiie plants in countries anciently civilized. In regard to the domestic animals kept by uncivilized man, it should not be overlooked that they almost always have to struggle for their own food, at least during certain seasons. And, in two countries very diifcrently circumstanced, individ- \ials of the same species, having slightly-different constitutions or structure, would often succeed better in the one country than in the other; and thus by a process of "natural selec- tion," as will hereafter be more fully explained, two sub-breeds might be formed. This, perhaps, partly explains Avhat has been remarked by soiiic authors, namely, that the varieties kept by savages have more of the character of species than the varieties kept in civilized countries. On the view here given of the all-important part which selection by man has played, it becomes at once ob\-ious how it is that our domestic races show adaptation in their stnicture or in their habits to man's wants or fancies. We can, I think, further understand the frequently-abnormal character of our domestic races, and likewise their differences being so great in external characters, and relatively so slight in internal parts or organs. Man can hardly select, or only with much dilhculty, any deviation of structure, excepting such as is externally vis- ible ; and indeed he rarely cares for what is internal. He can never act l)y selection, excepting on variations which are first given to him in some slight degree by Nature. No man would ever try to make a fantail till he saw a pigeon with a tail de- CiiAr. I. UNCONSCIOUS SELECTION. 47 veloped in some slis^lit degree in an unusual manner, or a pouter till lie saw a pid, ^^''hen further improved by the same slow 48 CIRCUMSTANCES FAVORABLE TO SELECTION. Chap. I. and gradual process, tliey will spread more widely, and will get recognized as sometliing distinct and valuable, and will then probably first receive a provincial name. In semi-civilized countries, with little free communication, the spreading of a new sub-breed would be a slow process. As soon as the points of value in a new strain are once acknowledged, the principle, as I have called it, of unconscious selection will always tend — per- haps more at one period than at another, as the breed rises or falls in fashion — perhaps more in one district than in another, according to the state of civilization of the inhabitants — slowly to add to the characteristic features of the breed, whatever they may be. But the chance may be infinitely small of any record having been jireserved of such slow, varying, and insen- sible changes. Circumstances favorable to Mail's Power of Selection. I must now say a few words on the circumstances, favor- able, or the reverse, to man's power of selection. A high de- gree of variability is obviously favorable, as freely giving the materials for selection to work on ; not that mere individual differences are not amply sufficient, with extreme care, to allow of the accumulation of a large amount of modification in almost any desired direction. But, as variations manifestly useful or pleasing to man appear only occasionally, the chance of their appearance will be much increased by a large number of indi- viduals being kept ; and hence tliis comes to be of the highest importance to success. On this principle Marshall formerly remarked, with respect to the sheep of parts of Yorkshu-e, that, " as they generally belong to poor people, and are mostly in small lots, they never can l)e improved." On the other hand, nurservmen, from raising large stocks of the same plants, arc generally far more successful than amateurs in getting new and valuable varieties. The keejiing of a large number of in- dividuals of a species in any country requires that the species should be placed under favorable conditions of life, so as to breed freely in that countr>', A\^hcn the indi\'iduals of any species are scanty, all the individuals, whatever their quality may be, will generally be allowed to l)reed, and this will effect- ually prevent selection. But probably the most important point of all is, that the animal or plant should be so highly useful to man, or so much valued hy him, that tlie closest at- Irntion is paid to even the slightest dmiation in the qualities Chap. I. CIRCUMSTANCES FAVORABLE TO SELECTION. 49 or structure of each individual. Unless such attention be paid nothinf^ can be effected. I have seen it gravely remarked, that it was most fortimate that tlic strawberry began to vary just when gardeners began to attend closely to this plant. No doubt the strawberry had always varied since it was cultivated, but the slight varieties had been neglected. As soon, however, as gardeners picked out individual plants with slightly larger, earlier, or better fruit, and raised seedlings from them, and again })ickcd out the best seedlings and bred from them, then, there appeared (aided by some crossing with distinct species) those many admirable varieties of the strawberry which have been raised during the last thirty or forty years. In the case of animals with separate sexes, facility in pre- venting crosses is an important element of success in the for- mation of new races — at least, in a country which is already stocked with -other races. In this respect, enclosure of the land plays a part. Wandering savages or the inhabitants of open plains rarely possess more than one breed of the same species. Pigeons can be mated for life, and this is a great convenience to the fancier, for thus many races may be im- proved and kept true, though mingled in the same aviary ; and this circumstance must have largely favored the formation of new breeds. Pigeons, I may add, can be propagated in great numbers and at a very quick rate, and inferior birds may be freely rejected, as when killed tliey serve for food. On the other hand, cats, from their nocturnal rambling habits, cannot be easily matched, and, although so much valued by women and children, we hardly ever see a distinct breed kept up ; such breeds as we do sometimes sec are iilmost always imported from some other country. Although I do not doubt that some domestic animals vary less than others, yet the rarity or ab- sence of distinct l)reeds of the cat, the donkey, peacock, goose, etc., may be attril)uted in main part to selection not having been l)rought into play : in cats, from tlie difhculty in pairing them : in donkeys, from onl}'^ a few ])oing kept by poor jieople, and little attention paid to their breeding; for recently, in cer- tain parts of Spain and of the United States, tliis animal has been surprisingly modified and improved by careful selection : in peacocks, from not being very easily reared and a large stock not kejit : in geese, from being valuable only for two puqioses, food and featliers, and more especially from no plccisure having lieen felt in the display of distinct breeds ; but the goose seems to have a singularly inflexible organization. 50 SUMMARY OF VAEIATION. Chap. I. Some authors have maintained that the amount of varia- tion in our domestic productions is soon reached, and can never afterward be exceeded. It would be somewhat rash to assert that the limit has been attained in any one case ; for almost all our animals and plants have been g'reatly improved in many ways within a recent period ; and this imjilies varia- tion. It would be equally rash to assert that characters now increased to their vitmost limit, could not, after remaining fixed for many centuries, again vary under new conditions of hfe. No doul)t, as Mr. Wallace has remarked with much truth, a limit will be at last reached. For instance, there must be a limit to the fleetness of any terrestrial animal, as this will be determined by the friction to be overcome, the weight of body to be carried, and the power of contraction in the muscular fibres. But what concerns us is that the domestic varieties of the same species differ from each other in almost every character, Mhich man has attended to and selected, more than do the distinct species of the same genera. Isidore Geoffroy St.-Hilaire has proved this in regard to size, and so it is with color, and probably Muth the length of hair. With respect to fleetness, which depends on many bodily characteristics. Eclipse was far fleeter, and a drajMiorse is incomparably stronger, than any two equine species. So with plants, the seeds of the dif- ferent varieties of the bean or maize difl'cr more in size than do the seeds of the distinct species in any one genus of the same two families. The same remark holds good in regard to the fruit of the several varieties c»f the plum, and still more so with the melon, as well as in endless other analogous cases. To sum up on the origin of our domestic races of animals and plants. Changed conditions of life are of the highest im- portance in causing variability, both directly l)y acting on the organization, and indirectly by aft'ccting the reproductive sj's- tem. It is not probable that variability is an inherent and necessary contingent, under all circumstances. The greater or less force of inheritance and reversion determine Avhether variations shall endure. Yariability is governed by many un- known laws, more especially by that of correlation. Some- Ihing may be attributed to the definite action of the conditions of life, but how much, we do not know. Something must be :iltiil)uted to use and disuse. The final result is thus rendered inllnilely comjilex. In some cases the intercrossing of abori- ginally distinct species, has probably played an important part in the orijxin of oin* domestic breeds. When several breeds Chap. I. SUMMARY OF VAEIATION. 51 have once been formed in any country, their occasional inter- crossing-, -with the aid of selection, has, no doubt, largely aided in forming new sub-breeds ; but the importance of crossing has been much exaggerated, both in regard to animals and to those plants which are propagated by seed. With plants which are temporarily propagated by cuttings, buds, etc, the importance of crossing is immense ; for the cultivator may here disregard the extreme variability both of hybrids and of mongrels, and the frequent sterility of hybrids ; but plants not propagated by seed are of little importance to us, for their endurance is only temporary. Over all these causes of Change the accumulative action of Selection, whether applied method- ically and quickly, or unconsciously and slowly, but more efB- ciently, is by far the predominant Power. VAEIATION UNDER NATURE. Chap. II. CHAPTER II. VARIATION UXDER NATURE. Variability— Individual Differences— Doubtful Species— Wide-ranging, much dif- fused, and Common Species, vary meet- Species of the Larger Genera in each Country vai"y more frequently than the Species of the Smaller Genera— Many of the Species of the Larger Genera resemble Varieties in being very closely, bat unequally, related to each other, and in having Restricted Ranges. Before applying the principles arrived at in the last cha|> ter to organic beings in a state of nature, we must briefly dis- cuss whether these latter are subject to any variation. To treat this subject at all properlj^ a long catalogue of dry facts ought to be given; but these I shall reserve for my future work. Nor shall I here discuss the various definitions which have been given of the term species. No one definition has as yet satisfied all naturalists ; yet every naturalist knows vaguely what he means when he speaks of a species. Gen- erally the term includes the unknown element of a distinct act of creation. The term "variety" is almost equally difficult to define ; but here community of descent is almost universally implied, though it can rarely be proved. We have also what are called monstrosities ; but they graduate into varieties. By a monstrosity I presume is meant some considerable devia- tion of structure, generally injurious to or not useful to the species. Some authors use the term "variation" in a techni- cal sense, as implying a modification directly due to the physi- cal conditions of life ; and " variations " in this sense are sup- posed not to be inherited : but who can say that the dwarfed condition of shells in the brackish waters of the Baltic, or dwarfed plants on Alpine summits, or the thicker fur of an animal from far northward, would not in some cases be inher- ited for at least some few generations ? and in this c;ise I presume that the form would be called a variety. It may be doubted whether sudden and great deviations of structure such as we occasionally see in our domestic produc- Chap. II. INDIVIDUAL DIFFERENCES. 53 tions, more especially with plants, are ever permanently prop- agated in a state of nature. Almost every part of every organic being is so beautifully related in its complex conditions of life that it seems as improbable that any part should have been suddenly produced perfect, as that a complex machine should have been invente4 by man in a perfect state. Under domestication monstrosities often occur which resemble noi mal structures in widely-different animals. Thus pigs have often been born with a sort of proboscis like that of tlie tapir or elephant. Now, if any wild species of the pig-genus had naturally possessed a proboscis, it might have been argued that tliis in like manner had suddenly appeared as a monstrosity ; but I have as yet failed to find, after diligent search, cases of monstrosities resembling normal structures in nearly-allied forms, and these alone would bear on the question. If mon- strous forms of this kind ever do appear in a state of nature and are capable of propagation (which is not always the case), as they occur rarely and singly, their preservation would de- pend on unusually favorable circumstances. They would, also, during the first and succeeding generations cross with the or- dinary form, and thus they would almost inevitably lose their abnormal character. But I shall have to retvu^n in a future cliapter to the preservation and perpetuation of occasional va- riations. Individual Differences. Tlie many slight differences Avhich frequently appear in the offspring from the same parents, or which may be presumed to have thus arisen, from being frequently observed in the indi- viduals of the same species inhabiting the same confined local- ity, may be called indiA-idual differences. No one supposes that all the indi\'iduals of the same species are cast in the same actual mould. These individual differences are of the highest importance for us, for they are often inherited, as must be fa- miliar to every one ; and they thus afford materials for natural selection to act on and accumulate, in the same manner as man accumulates in any given direction individual differences in his domesticated production.s. These individual differences generally affect what naturalists consider unimportant parts ; but I could show, by a long catalogue of facts, that parts which must be called important, whether viewed under a physiolo- gical or classificatory point of view, sometimes vary in the indi- viduals of the same species. I am convinced that the most ex- 54 INDIVIDUAL DIFFERENCES. Chap. II. perienced naturalist would be surprised at the number of the oases of variability, even in important parts of structure, which lie could collect on ^ood authority, as I have collected, during a course of years. It should be remembered that systematists are far from bcing^ pleased at finding variability in important characters, and that there are not many men who wiU labori- ously examine internal and important ort^ans, and compare tliem in many specimens of the same species. It Avould never have been expected that the branching of the main nerves close to the great central ganglion of an insect would have been variable in the same species ; it might have been thought that changes of this nature could have been effected only by slow degrees ; yet, recently, Sir J. Lubbock has shown a degree of . variability in these main nerves in Coccus, which ma}' almost be compared to the irregular branching of the stem of a tree. This philosophical naturalist, I may add, has also recently sliowu that the muscles in the larva? of certain insects are far from luiiform. Authors sometimes argue in a circle when they state that important organs never vary ; for these same authors practically rank those parts as important (as some few natu- ralists have honestly confessed), which do not vary ; and, under this point of view, no instance will ever be found of an impor- tant part varying : but under any other point of view many instances assuredly can be given. There is one point connected with individual differences, which is extremely perplexing : I refer to those genera which have been called " protean " or " polymorphic," in which the species present an inordinate amount of variation ; and about which hardly two naturalists agree whether to rank them as species or as varieties. We niay instance Rubus, Rosa, and Hieracium among plants, several genera of insects, several gen- era of Brachiopod shells, and the Ruff (Machetes pugnax) among birds. In most polpnorphic genera some of the species have fixed and definite characters. Genera which are poly- morpliic in one country seem to be, with some few exceptions, ])olymorphic in other countries, and likewise, judging from Brachiopod shells, at former periods of time. These facts are very perplexing, for they seem to sliow that this kind of varia- bility is independent of the conditions of life. I am inclined to suspect that we have, at least in some of these polymorphic gen(»ra, variations which are of no service or disservice to the Hpccics, and which, consequently, have not been seized on and reiiden^d delinite by natural selection, as hereafter to be ex- j)hiined. CiiAP. II. INDIVIDUAL DIFFERENCES. 55 Individuals of the same species often present ^cat difTcr- eiices of structure, as in the two sexes of various animals, iuthe two or three castes of sterile females or workers amon^^ insects, and in the immature and larval states of many of the lower ani- mals. Tlierc are, however, other cases, namely, of dimori)hism and trimorphism, which might easily be, and have frecjucntly l)een, confounded with varial^ility, but which are quite distinct. I refer to the two or three dillerent forms which certain animals of either sex, and certain hermaphrodite plants, habitually pre- sent. Thus, Mr. Wallace, who has lately called special atten- tion to the subject, has shown that the females of certain spe- cies of butterflies, in the Malayan archipelaj^o, regularly ap- pear under two or even three consjoicuously distinct forms, not connected together by intermediate varieties. The winged and frequently wingless states of so many Hemipterous insects may probably be included as a case of dimorphism, and not of mere variability. Fritz Muller, also, has recently described analo- gous but more extraordinary cases with the males of certain Brazilian Crustaceans : thus, the male of a Tanais regularly occurs under two widely-dillerent forms, not connected by any intermediate links ; one of these forms has much stronger and differently-shaped pincers for seizing the female, and the other, as if for compensation, has anlennaj much more abundantly furnished with smelhng-hairs, so as to have a better chance of finding the female. Again, the males of another Crustacean, an Orchestia, occur under two distinct forms, with pincers dif- fering much more from each other in structure, than do the pincers of most species of the same genus. With respect to plants, I have recently shown that in several -widely-distinct orders, the species present two or even three forms, which arc abruptly distinguished from each other in several important ]ioints, as in the size and color of the pollen-grains ; and these forms, though all hermaphrotlites, differ from each other in their reproductive power, so that for full fertility, or indeed in some cases for any fertility, they must reciprocally impregnate each other. Although the forms of the few dimorphic and tri- morphic animals and plants which have been studied, are not now connected together by intermi^diate links, it is ]irol)able that thi^ will be found to occur in other cases ; for Mr. \\''allacc observed a certain Ijutterlly which jiresentcd in the same island a great range of varieties connected by intermediate links, and the extreme links of the chain closely resembled the two forms of an allied dimorphic species inhabiting another part of the 5G DOUBTFUL SPECIES. CnAP. II, Malay archipelago. Tlius also with ants, the several worker- castes are generally cjuite distinct ; but in some cases, as we shall hereafter see, the castes are connected together by gradu- ated varieties. It certainly at first appears a highly-remark- able fact that the same female butterfly should have the power of producing at the same time three distinct female forms and a male ; that a male Crustacean should generate two male forms and a female form, all widely different from each other ; and that an hermaphrodite plant should produce from the same seed-capsule three distinct hermaplu-odite forms, bearing three different kinds of females and three or even six different kinds of males. Nevertheless these cases are only exaggerations of the universal fact that every female produces males and females, Avhich in some instances differ in a wonderful manner fj-omeach other. Doubtful jSpecies. The forms which possess in some considerable degree the character of species, but which are so closely similar to other forms, or are so closely linked to them by intermediate grada- tions, that naturalists do not like to rank them as distinct species, are in several respects the most important for us. We have every reason to believe that many of these doubtful and closely-allied forms have permanently retained their characters in their own country for a long time ; for as long, as far as we knoAV, as have good and true species. Practically, when a naturalist can unite two forms together by othei*s having inter- mediate characters, he treats the one as a variety of the other, ranking the most common, but sometimes the one first de- scribed, as the species, and the other as the variety. But cases of great difliculty, which I will not here enumerate, sometimes occur in deciding whether or not to rank one form as a variety of another, even when they are closely connected by intermediate links ; nor will the commonly-assumed hybrid nature of the intermediate links always remove the difficulty. In very many cases, however, one form is ranked as a variety of another, not because the intermediate links have actually been found, but because analogy leads the observer to suppose either that they do now somewhere exist, or may formerly have existed ; and here a wide door for tlie entry of doubt and conjecture is opened. Hence, in determining whether a form should be ranked as a species or a variety, the opinion of naturalists having sound Chap. II. DOUBTFUL SPECIES. 57 judgment and wide experience seems the only guide to follow. itVe must, however, in many cases, decide by a majority of naturalists, for few well-marked and well-known varieties can be named which have not been ranked as species by at least some competent judges. That varieties of this doubtful nature are far from uncom- mon cannot be disputed. Compare the several floras of Great Britain, of France, or of the United States, drawn up by dif- ferent botanists, and see Avhat a surprising number of forms have been ranked by one botanist as good species, and by an- other as mere varieties. ^Ir. H. C. Watson, to whom I lie inider deep obligation for assistance of all kinds, has marked fur me 18"v British plants, which are generally considered as varieties, but which have all been ranked by botanists as spe- cies ; and in making this list he has omitted many trifling varie- ties, but which nevertheless have been ranked by some bota- nists as specie's, and he has entirely omitted several bighly-poly- morj^hic genera. Under genera, including the most polymor- {)hic forms, Mr. Babington gives 251 species, whereas Mr. Benthara gives only 112 — a difference of 139 doubtful forms ! Among animals which unite for each birth, and which are highly locomotive, doubtful forms, ranked by one zoologist as a species and by another as a variety, can rarely be found within the same country, but are common in separated areas. How many of those birds and insects in North America and Europe, which differ very slightly from each other, have been ranked by one eminent naturalist as undoubted species, and by another as varieties, or, as they are often called, as geographical races ! Mr. Wallace, in several valuable papers on the various animals, especially on the Lepidoptera, inhabiting the islands of the great Malayan archipelago, shows that they may be classed under four heads, namely, as variable forms, as local forms, as geographical races or sul)-species, and as true representative species. The first or variable forms vary much within the limits of the same island. The local forms are moderately constant and distinct in eqch separate island; but when all the forms from the several islands arc compared, the differ- ences are seen to be so slight and graduated, that it is impos- sible to define or describe them, though at the same time the extreme forms are sufficiently distinct. The geographical races or sul>speoies are local forms completely fixed and iso- lated; but as they do not differ from each other by strongly- marked and important characters, " there is no possible test but 58 DOUBTFUL SPECIES. Cuap. II. individual opinion to determine which of them shall be consid- ered as species and -which as varieties." Lastly, representa- tive species fill the same place in the natural economy of each island as do the local forms and sub-species ; but, as they are distinp^uished from each other by a greater amount of differ- ence than that between the local forms and sub-species, they are almost imiversally ranked by naturalists as true species. Nevertheless, no certain criterion can possibly be given by which variable forms, local forms, sub-species, and representa- tive species can be recognized. Many years ago, when comparing, and seeing others com- pare, the birds from the closely-neighboring islands of the Galapagos archipelago, both one with another, and with those from the American main-land, I was much struck how entirely vague and arbitrary is the distinction between species and varieties. On the islets of the little Madeira group there are many insects Avhich are characterized as varieties in Mr. Wol- laston's admirable work, but which would certainly be ranked as distinct species by many entomologists. Even Ireland has a few animals, now generally regarded as varieties, but which have been ranked as species by some zoologists. Several experienced ornithologists consider our British red grouse as only a strongly-marked race of a Norwegian species, whereas the greater number rank it as an undoubted species peculiar to Great Britain. A wide distance between the homes of two doubtful forms leads many naturalists to rank them as distinct species ; but what distance, it has been Avell asked, Avill suf- fice ; if that between America and Europe is ample, will that lietween Europe and the Azores, or Madeira, or tlie Canaries, or between the several islets of these small archipelagos, be sufficient ? Mr. B. D. Walsh, a distinguished entomologist of the United States, has lately described what he calls Phytophagic varieties and Phytophagic species. Most vegetable-feeding insects live on one kind of plant or on one group of plants ; some feed in- discriminately on many kinds, but do not in consequence vary. In several cases, however, insects found living on different plants have been observed by Mr. Walsh to present, either ex- el usivel}' in their larval or mature state, or in both states, slight though constant differences in color, size, or in the nature of their secretions. In some instances the males alone, in other instances l)()th males and females, liave been olisen'ed to be thus alTected in a shght degree. When the diflerencos are Chap. II. DOUBTFUL SPECIES. 59 rather more strongly marked, and when both sexes and all aj^os are afTectcd, the forms -would be ranked by all entomolo<^ists as species. But no observer can determine for others, even if he can do so for himself, which of these Phytophagic forms ought to be called species and which varieties. Mr. Walsh ranks the forms which it may be supposed would freely intercross together, as varieties ; and those Avhich appear to have lost this jjower, as species. As the ditfcrences depend on the insects having long fed on distinct plants, it cannot be expected that inter- mediate links connecting tlie several forms should now be found. The naturalist thus loses his best guide in determining whether to rank such doubtful forms as varieties or sjiecies. This likewise necessarily occurs with closely-allied organisms, which inhabit distinct continents or islands. When, on the other hand, an animal or plant ranges over the same continent, or inhabits many islands in the same archipelago, and presents different forms in the different areas, there is always a good chance that intermediate forms may be discovered which shall link together the extreme states ; and these arc then degraded to the rank of varieties. Some few naturalists maintain that animals never present varieties ; but then these same; naturalists rank the slightest differences as of specific value; and when the same identical form is met with in two distant countries, or in tM'o geological formations, they believe that two distinct species are hidden under the same dress. Tlie term species thus comes to be a mere useless mental abstraction, implying and assuming a sepa rate act of creation. It is certain that many forms, considered by highly-competent judges as varieties, so completely resemble species in character, that they have been thus ranked by other highly-competent judges. But to discuss whether they ought to lie called species or varieties, before any definition of these terms has been generally accepted, is A'ainly to beat the air. Many of the cases of strongly-marked varieties or doul)tful species well deserve consideration ; for several interesting lines of argument, from geographical distribution, analogical varia- tion, hybridism, etc., have been brought to l)ear on tlie attempt to determine their rank ; but space does not here permit me to discuss them. Close investigation, in many cases, will bring naturalists to an agreement how to rank doubtfid forms. Yet it must be confessed that it is in the best-known countries that we find the greatest number of forms of doubtful value. I have been struck Avith the fact that, if any animal or plant in a CO DOUBTFUL SPECIES. Chap. II. state of nature be liigbly useful to man, or from any cause closely attract his attention, varieties of it will almost univer- sally be found recorded. These varieties, moreover, will often be ranked by some authors as species. Look at the common oak, how closely it has been studied; yet a German author makes more than a dozen species out of forms, which are almost imiversally considered as varieties ; and in this country the highest botanical authorities and practical men can Ix) quoted to show that the sessile and pedunculated oaks are either good and distinct species or mere varieties. I may here allude to a remarkable memoir lately published by A. de Candolle, on the oaks of the whole world. No one ever had more ample materials for the discrimination of the species, or could have worked on them with more zeal and sagacity. He first gives in detail all the many points of struc- ture which vary in the species, and estimates numerically the relative frequency of the variations. He specifics above a dozen characters Avliich niay be found varying even on the same branch, sometimes according to age or development, sometimes without any assignable reason. Such characters of course are not of specific value, but they are, as Asa Gray has remarked in commenting on this memoir, such as generally enter into specific definitions. De Candolle then goes on to saj^- that he gives the rank of species to the forms that differ by characters never varying on the same tree, and never found connected by intermediate states. After this discussion, the result of so much labor, he emphatically remarks : " They are mistaken, who repeat that the greater part of our species arc clearly limited, and that the doubtful species are in a feeble minority. This seemed to be true, so long as a genus was imperfectly known, and its species Avere fovuidcd upon a few specimens, that is to say, were provisional. Just as we come to know them better, intermediate forms flow in, and doubts as to spe- cific limits augment." He also adds that it is the best-known species which present the greatest number of spontaneous varieties and sub-varieties. Thus.Quercus robur has twenty- eight varieties, all of which, excepting six, are clustered round three sub-species, namely, Q. peduuculata, sessilifiora, and pubescens. The forms which connect these three sulvspecies are comparatively rare ; and, as Asa Gray remarks, if these connecting forms, whi(;h are now rare, were to become wholly extinct, the three sub-species Avould hold exactly the same relation to each other, as do the four or five provisionally- Cnxr. II. DOUBTFUL SPECIES. 61 admitted species which closely surround the typical Qucnuis rol)ur. Finally, De Candolle admits that, out of the three hun- dred species, Avhicli will l^e enumerated in his Prodromus as belonging to the oak family, at least two-thirds are provisional species, that is, are not known strictly to fulfil the definition above given of a true species. For it should be added that De Candolle no longer believes that species are immutable crea- tions, but concludes that the derivative theory of the succession of forms is the most natural one, " and the most accordant with tlie known facts in paleontology, geographical botany and zo- ology, of anatomical structure and classification ; " but, he adds, direct proof is still wanting. When a young naturalist commences the study of a group of organisms quite unknown to him, he is at first much per- plexed to determine what difiercnces to consider as specific, and what as varietal ; for he knows nothing of the amount and kind of variation to which the group is subject ; and this shows, at least, how very generally there is some variation. But if he confine his attention to one class within one country, he will soon make up his mind how to rank most of the doubtful forms. His general tendency will be to make many species, for he will become impressed, just like the pigeon or poultry fancier l)eforc alluded to, with the amount of difference in the forms which he is continually studying ; and he has little general knowledge of analogical variation in other groups and in other countries, by which to corrett his first impressions. As he extends the range of his oljservations, he will meet Avith more cases of diffi- culty ; for ho will encounter a greater number of closely-allied forms. But if his observations be widely extended, he will in the end generally l)e enaljled to make up his own mind which to call varieties and wliich species ; but he will succeed in this at the expense of admitting much variation — and the truth of this admission will often be disputed by other naturalists. AV'hen, moreover, he comes to study allied forms brought from countries not now continuous, in which case he cannot hope to find tlie intermediate links between his doubtful forms, he will have to trust almost entirely to analog}-, and his difliculties rise to a climax. Certainl}^ no clear line of demarcation has as yet been drawn between species and sub-sju^cies — that is, the forms which in the ojjinion of some naturalists come very near to, but do not quite arrive at, the rank of species: or, again, between sub- species and well-marked varieties, or between lesser varieties and G2 DOMINANT SPECIES VARY MOST. Chap. II. individual differences. These differences blend into each other in an insensible scries ; and a seiies impresses the mind with the idea of an actual passage. Hence, I look at individual differences, though of small interest to the systematist, as of the highest importance for us, as being the first steps toward such slight varieties as are barely thought worth recording in works on natural history. And I look at varieties which are in any degree more distinct and permanent, as steps toward more strongly-marked and permanent varieties ; and at the latter, as leading to sub- species or species. The passages from one stage of difference to another may, in some cases, be the simple result of the long-continued action of different physical conditions ; but in most cases they may be attributed to the gradual accumulative 'action of natural selection, as hereafter to be more fullj" ex- plained, on fluctuating variability. • Hence a well-marked variety may be called an incipient species ; but whether this belief is justifiable must be judged of by the general weight of the facts and considerations given throughout this Avork. It need not be supposed that all varieties or incipient spe- cies necessarily attain the rank of species. They may become extinct, or they may endure as varieties for very long periods, as has been shown to be the case by Mr. Wollaston with the varieties of certain fossil land-shells in Madeira, and with plants by Gaston de Saporta, If a variety were to flourish so as to exceed in immbers the parent-species, it would then rank as the species, and the species as the variety ; or it might come 1o supplant and exterminate the parent-species ; or both might coexist, and both rank as independent species. But we shall hereafter return to this subject. From these remarks it will be seen that I look at the term species as one arbitrarily given, for tlie sake of convenience, to a set of individuals closely resembling each other, and that it does not essentially differ from the term variety, which is given to less distinct and more fluctuating forms. The term variety, again, in comparison with mere individual differences, is also applied arbitrarily, and for mere convenience' sake. Wide-ranging^ much-d {fused, and Commo7i Siyecics, vary most. Guided by theoretical considerations, I thought that some interesting results might be obtained in regard to the nature Chat. II. DOMINANT SPECIES VARY MOST. 63 and relations of the species which vary most, by tabulatinj^ all the varieties in several Avell-workcd floras. At first this seemed a simple task ; but Mr. H. C. W^atson, to whom I am much indebted for valuable advice and assistance on this sub- ject, soon convinced me that there were many difficulties, as did subsequently Dr. Hooker, even in strono^er terms. I shall reserve • for my future work the discussion of these difficulties, and the tables themselves of the proportional numbers of the varying species. Dr. Hooker permits me to add that, after liaviufT' carefully read my manuscript, and examined the tables, he tliinks tliat the follo\\ing statements are fairly well estab- lished. The whole subject, however, treated as it necessarily here is with much brevity, is rather pcrplexin;T, and allusions cannot be avoided to the "struggle for existence," "divergence of character," and other cjuestions, hereafter to be discussed. Alphonse de Candollc and others have shown that plants which have very wide ranges generally present varieties ; and this might have been expected, as they become exposed to diverse physical conditions, and as they come into competition (which, as we shall hereafter see, is an equally or more im- portant circumstance, with different sets of organic beings. But my ta])les further show that, in any limited country, the species winch arc most common, that is, abound most in individuals, and the species Avhich are most widely diifused within their own country (and this is a different consideration from wide range, and to a certain extent from commonness) oftenest give rise to varieties sufficiently well marked to have been recorded in botanical works. Hence it is the most flourishing, or, as they may be called the dominant species — those which range widely, are the most diffused in their own country, and arc the most numerous in individuals — wln'ch oftenest produce well- marked varieties, or, as I consider them, incipient species. And this, perhaps, might have been anticipated; for, as varie- ties, in order to become in any degree permanent, necessarily have to struggle with the other inhabitants of the coimtry, the species which are already dominant will be the most likely to yield olTspring, which, though in some slight degree modi- fied, still inlu^rit those advantages tliat enabled their parents to become dominant over their compatriots. In these remai'ks on predominance, it should l)e understood that reference is marie only to those forms which come into competition Avith each other, and more especially to the members of the same genus or class having nearly similar habits of life. With re- 64 SPECIES OF LARGER GENERA VARIABLE. Chap. II. spect to cominoiinoss or the number of individuals of any spe- cies, the comparison, of course, relates only to the members of the same group. A plant may be said to be dominant if it be more numerous in individuals and more widely diffused than the other plants of the same country, not livini^^ under widely- different conditions of Hfe. Such a plant is not the less domi- nant in the sense here used, because some conferva inhabiting the water or some parasitic fungus is infinitely more numerous in individuals, and more widely diffused ; if one kind of conferva or parasitic fungus exceeded its allies in the above respects, it would be a dominant form within its own class. JSjjecies of the Larger Genera in each Country vary more frequently than the Species of the Smaller Genera. If the plants inhabiting a country and descrilicd in any Flora be divided into two ecjual masses, all those in the larger genera (i. e., those including many species) being placed on one side, and all those in the smaller genera on the other side, a somewhat larger number of the very common and much-dif- fused or dominant species will be found on the side of the larger genera. This, again, might have been anticipated ; for the mere fact of many species of the same genus inhabiting any country, shows that there is something in the organic or inor- ganic conditions of that country favorable to the genus ; and, consequently, we might have expected to have found in the larger genera, or those including many species, a large propor- tional number of dominant species. But so many causes tend to obscure this result, that I am surprised that my tables show even a small majority on the side of the larger genera. I will here allude to only two causes of obscurity. Fresh-water and salt-loving plants have generally very wide ranges and are much diffused, but this seems to be connected with the nature of the stations inhabited by them, and has little or no relation to the size of the genera to which the species belong. Again, plants low in the scale of organization are generally much more widely diffused than plants higher in the scale ; and here, again, there is no close relation to the size of the genera. Tlie cause of lowly-organized plants ranging widely will be discussed in our chapter on Geographical Distribution. From looking at species as only strongly-marked and well- defined varieties, I was led to anticipate tliat the species of the larger genera in each country would oftener present varieties, CnAP. 11. SPECIES OF LARGER GENERA VARIABLE. 65 than the species of the smaller g'enera ; for wherever many closely-related species (i. e., species of the same genus) have been fornietl, many varieties or incipient species ought, as a general rule, to be now forming. Where many large trees grow, we expect to find saplings. Where many species of a genus have been formed tkrough variation, circumstances have been favorable for variation ; and hence we might expect thai the circumstances would generally be still favorable to varia- tion. On the other hand, if we look at each species as a special act of creation, there is no apparent reason why more varieties should occur in a group having many species, than in one having few. To test the truth of this anticipation, I have arranged the j^lants of twelve countries, and the coleopterous insects of two districts, into two nearly equal masses, the species of the larger genera on one side, and those of the smaller gCnera on the other sid(?, and it has invariably proved to be the case that a larger proportion of ±he species on the side of the larger genera presented varieties, than on the side of the smaller genera. Moreover, the species of the large genera which pre- sent any varieties, invariably present a larger average number of varieties than do the species of the small genera. Both these results follow when another diWsion is made, and when all the least genera, with from only one to four species, are absolutely excluded from the tables. These facts are of plain significa- tion on the view that species are only strongly-marked and permanent varieties ; for wherever many species of the same genus have been formed, or where, if we may use the expres- sion, the manufactory of species has been active, we ought generally to find the manufactory still in action, more espe- cially as we have every reason to believe the process of manu- facluring new species to be a slow one. And this certainly is the case, if varieties be looked at as incipient species ; for my tables clearly show as a general rule that, wherever many speci(^s of a genus have been formed, the species of that genus present a nvunber of varieties, that is, of incipient species, be- yond the average. It is not that all large genera are now varying nuich, and are thus increasing in the lunnber of their species, or that no small genera are now varying and increas- ing; for if this had l)een so, it would have been fatal to my theory ; inasmuch as geology plainly tells us that small genera have in the lapse of time often increased greatly in size; and that large genera have often come to their maxima, declined, OG SPECIES OF LARGER GENERA Chap. II. aiul disappeared. All that we -want to show is, that, where many species of a genus have been formed, on an average many arc still forming ; and this certainly holds good. Many of tJic Sjiccies included iclthln the Larger Genera re- semble Varieties in being very closely, but unequally, related to each other, and in having liestricted Manges. There are other relations between the species of large gen- era and their recorded varieties which deserve notice. We have seen that there is no infallible criterion by which to dis- tinguish species and well-marked varieties ; and in those cases in which intermediate links have not been found between doubtful forms, naturalists are compelled to come to a deter- mination by the amount of difference between them, judging by analogy whether or not the amoimt suffices to raise one or both to the rank of species. Hence the amount of difference is one very important criterion in settling whether two forms should be ranked as species or varieties. Now Fries has remarked in regard to plants, and Westwood in regard to insects, that in large genera the amount of difference between the sjiecies is often exceedingly small. I have endeavored to test this nu- merically by averages, and, as far as my imperfect results go, they confirm the view. I have also consulted some sagacious and experienced observers, and, after deliberation, they concur in this view. In this respect, therefore, tlie species of the larger genera resemble varieties, more than do the species of the smaller genera. Or the case may be put in another way, and it may be said that in the larger genera, in which a num- ber of varieties or incipient species greater than the average are now manufacturing, many of the species already manufac- tured still to a certain extent resemble varieties, for they differ from each other by less than the usual amount of difference. Moreover, the species of the large genera are related to each other, in the same manner as the varieties of any one species are related to each other. No naturalist pretends that all the species of a genus arc equally distinct from each other ; they may generally be divided into sub-genera, or sections, or lesser groups. As Fries has well remarked, little groups of species are generally clustered like satellites around certain other species. And what are varieties but groups of forms, imequally related to each other, and clustered round certain forms — that is, round their parent-species ? Undoubtedly there Chap. II. RESEMBLE VARIETIES. 07 is one most important point of dilFerencc between varieties and species ; namely, that the amount of diflerencc between varieties, M'hen compared with each other or with their parent-species, is much less than that betw'een the species of the same genus. But ■when WG come to discuss the principle, as I call it, of Diver- gence of Character, we shall see how this may be explained, and how the lesser diiferences between varieties tend to in- crease into the greater differences between species. There is one other point which is worth notice. Varieties generally have much-restricted ranges : this statement is in- deed scarcely more than a truism, for, if a variety were found to have a wider range thaii that of its sujiposed parent-species, their denominations would be reversed. But there is also reason to believe that those species which are very closely allied to other species, and in so far resemble varieties, often have much-restricted ranges. For instance, Mr. II. C. Watson has marked for me, in the well-sifted London Catalogue of plants (fourth edition), G3 plants which are therein ranked as species, but which he considers as so closely allied to other species as to be of doubtful value : these 63 reputed species range on an average over G.9 of the provinces into which Mr. Watson has divided Great Britain. Now, in this same cata- logue, 53 acknowledged varieties are recorded, and these range over 7.7 jirovinces ; whereas, the species to Avhich these vari- eties belong range over 14.3 provinces. So that the acknowl- edged varieties have very nearly the same restricted average range as have those very closely allied forms, marked for me liy Mr. Watson as doubtful species, but which are almost uni- versally ranked by British botanists as good and true species. Summart/. Finally, then, varieties caimot be distinguished from species — except, first, by the discovery of intermediate linking forms; and, secondly, by a certain indefinite amount of difference be- tween them ; for two forms, if differing very little, are gener- ally ranked as varieties, notwithstanding that they cannot be closely connected ; but the amount of difference considered necessary to give to any two forms the rank of species cannot be defined. In genera having more than the average number of species in any country, the species of these genera have more than the average number of varieties. In large genera the species are apt to be closely, but uncfiually, allied together. 68 SUMMARY. Chap. II. forming little clusters round certain other species. Species very closoly allied to other species apparently have restricted rang'cs. In all these several respects the species of large genera present a strong analogy with varieties. And we can clearly understand these analogies, if species once existed as varieties, and thus originated ; whereas, these analogies are utterly in- explicable if species are independent creations. We have, also, seen that it is the most nourishing or dom- inant species of the larger genera within each class which on an average 3'ield the greatest number of varieties ; and varie- ties, as we shall hereafter see, tend to become converted into new and distinct species. Thus the larger genera tend to be- come larger ; and throughout Nature the forms of life which are now dominant tend to become still more dominant by leav- ing many modified and dominant descendants. But by steps hereafter to be explained, the larger genera also tend to break up into smaller genera. And thus, the forms of life through- out the universe become divided into groups subordhiate to groups. Chap. III. STRUGGLE FOB EXISTENCE. 69 CHAPTER III. STRUGGLE FOR EXISTENCE. Its bearing on Natural Selection— The Term used In a wide Sense— Geometrical Ratio oriucroase — Rapid Iiicreaao of Nuturalized Animals and Plants— Nature of tho Checks to Incroase— Competition universal— EftectR of Climate — Protection from the Number of Individuals — Complex Relations of all Animals and Plants throush- out Nature— St ru;;srlo for Life most severe between Individuals and Varieties of the same *Specics: often severe between Species of the same Genus — The Rela- tion of Organism to Or^^anism the most important of all Relations. Before entering on the subject of this chapter, I must make a few prehminary remarks, to show how the struggle for existence bears on Natural Selection. It has been seen in the last chapter that among organic beings in a state of nature there is some individual variability : indeed, I am not aware that this has ever been disputed. It is immaterial for us whether a multitude of doubtful forms be called species or sub-species or varieties ; what rank, for instance, the two or three hundred doubtful forms of British plants are entitled to hold, if the existence of any well-marked varieties be admitted. But the mere existence of individual variability and of some few well-marked varieties, though necessary as the foundation for the work, helps us but little in understanding how species arise in Nature. How have all those exquisite adaptations of one part of the organization to another part, and to the condi- tions of life, and of one organic being to another being, been perfected ? We see these beautiful coadaptations most plain- ly in the woodpecker and the mistletoe ; and only a little less ])lainly in the hinnblest parasite which clings to the hairs of a quadruped or feathers of a bird; in the structure of the beetle wliich dives through the water ; in the ])lumed seed which is wafted by the gentlest breeze ; in short, we see beautiful adaptations everywhere and in every part of the organic world. Again, it may be asked, how is it that varieties, which I have called incipient species, become ultimately converted into 70 STRUGGLE FOR EXISTENCE. Cuap. III. good and distinct species, which in most cases obviously differ from each oilier far more than do the varieties of the same species? How do those groups of species, wliich constitute what are called distinct genera, and which differ from each other more than do the species of the same genus, arise ? All these results, as we shall more fully see in the next chapter, follow from the struggle for life. 0\\4ng to this struggle, variations, however slight, and from whatever cause proceed- ing, if they be in any degree profitable to the individuals of a species, in their infinitely complex relations to other organic beings and to their physical conditions of life, will tend to the preservation of such individuals, and will generally be inherited by the offspring. The offspring, also, wUl thus have a better chance of surviving, for, of the many individuals of an}^ species Avhich are periodically born, but a small number can survive. I have called this principle, by which each slight variation, if useful, is preserved, by the term Natural Selection, in order to mark its relation to man's power of selection. But the expres- sion often used by Mr. Herbert Spencer of the Survival of the Fittest is more accurate, and is sometimes equally convenient. 'SVe have seen that man by selection can certainly produce great results, and can adapt organic beings to his own uses, through the accumulation of slight but useful variations, given to him by the hand of Nature. But Natural Selection, as we shall hereafter see, is a power incessantly ready for action, and is as immeasurably superior to man's feeble efforts as the works of Nature are to those of Art. We will now discuss in a little more detail the struggle for existence. In my future work this subject will be treated, as it well deserves, at greater length. The elder De Candolle and Ijyell have largely and philosophically shown that all organic beings are exposed to severe competition. In regard to plants, no one has treated this subject Avith more spirit and ability than AV. Herbert, Dean of Manchester, evidently the result of his great horticultural knowledge. Nothing is easier than to admit in words the truth of the universal struggle for life, or more difficult — at least I have found it so — than con- stantly to bear this conclusion in mind. Yet unless it be thoroughly engrained in the mind, the whole economy of Nature, with every fact on distriljution, rarity, abundance, ex- tinction, and variation, will be dimly seen or quite misunder- stood. We behold the face of Nature bright Avith gladness, Ave often sec superabundance of food ; Ave do not see, or Ave CuAP. HI. GEOMETRICAL KATIO OF INCREASE. 71 forget, tliat the birds which arc idly singing round us mostly live on insects or seeds, and are thus constantly destroying life ; or we forget how largely these songsters, or their eggs, or their nestlings, are destroyed by birds and beasts of prey ; we do not always bear in mind, that, though food may be now superabundant, it is not so at all seasons of each recurring year. Tlie 2h'm, Stu-uggle for Existence^ used in a Large Sense. I should premise that I use this term in a large and meta- phorical sense, including dependence of one being on another, and including (which is more important) not only the life of the indi\-idual, but success in lea\'ing progeny. Two canine animals, in a time of dearth, may be truly said to struggle with each -other which shall get food and live. But a plant on the edge of a desert is said to struggle for life against the drought, though more properly it should be said to be dependent on the moisture. A plant which annually produces a thousand seeds, of which on an average only one comes to maturity, may be more truly said to struggle with the plants of the same and other kinds which already clothe the ground. The mistletoe is dependent on the apple and a few other trees, but can only in a far-fetched sense be said to struggle with these trees, for, if too many of these parasites grow on the same tree, it will languish and die. But several seedling mistletoes, growing close together on the same branch, may more truly be said to struggle with each other. As the mistletoe is disseminated by birds, its existence depends on birds ; and it may metaphori- cally be said to struggle with other fruit-bearing plants, in order to tempt birds to devour and thus disseminate its seeds rather than those of other plants. In these several senses, which pass into each other, I use for convenience' sake the general term of struggle for existence. Geometrical Ratio of Increase. A struggle for existence inevitably follows from the high rate at which all organic beings tend to increase. Every being which during its natural lifetime produces several eggs or seeds, must suffer destruction during some period of its life, and during some season or occasional year, otherwise, on the principle of geometrical increase, its numbers would quickly 72 GEOMETRICAL RATIO OF INCREASE. Chap. III. become so inordinately great that no country could support the product. Hence, as more individuals are produced than can possibly survive, there must in every case be a struggle for existence, cither one individual with another of the same spe- cies, or -with the individuals of distinct species, or with the phys- ical conditions of life. It is the doctrine of Malthus applied with manifold force to the whole animal and vegetable king- doms ; for in this case there can be no artificial increase of food, and no prudential restraint for marriage. Although some species may be now increasing, more or less rapidly, in num- bers, all cannot do so, for the world would not hold them. There is no exception to the rule that every organic being naturally increases at so high a rate, that, if not destroyed, the earth would soon be covered by the progeny of a single pair. Even slow-breeding man has doubled in twenty-five years, and at this rate, in a few thousand years, there would literally not be standing-room for his progeny. Linnasus has calculated that if an annual plant produced only two seeds — and there is no plant nearly so unproductive as this — and their seedlings next year produced two, and so on, then in twenty years thei'C would be a million plants. The elephant is reckoned the slowest breeder of all known animals, and I have taken some pains to estimate its probable minimum rate of natural increase: it will be safest to assume that it begins breeding when thirty years old, and go'es on breeding till ninety years old, bringing forth six young in the interval, and surviving till one hundred years old ; if this be so, after a period of from seven hundred and forty to seven hundi'ed and fifty years, there would be alive nearly nineteen million elephants descended from the first pair. But we have better evidence on this subject than mere theoretical calculations, namely, the numerous recorded cases of the astonishingly ra\nd increase of various animals in a state of nature, "when circumstances have been favorable to them during two or three following seasons. Still more striking is the evidence from our domestic animals of many kinds which liave run wild in several parts of the world : if the statements of the rate of increase of slow-breeding cattle and horses in South America, and latterly in Australia, had not been well authenticated, they Avould have been incredible. So it is with plants : cases could be given of introduced plants which have b(>come common throughout whole islands in a period of less than ten years. Several of the plants, such as the caitloon, and a tall thistle, now most numerous over the wide plains of CiiAP. III. GEOMETRICAL RATIO OF' INCREASE. 73 La Plata, clothing square leagues of surface almost to the ex- clusion of all other plants, have been introduced from Europe ; and there are plants which now range in India, as I hear from Dr. Falconer, from Cape Comorin to tlie Himalaya, which have been imported from xVmerica since its discovery. In such cases, and endless instances could be given, no one supposes that the fertility of these animals or phuits has been suddenly and temporarily increased in any sensil)le degree. The obvious explanation is that the conditions of lifeliave been very favor- able, and that there has consequently been less destruction of the old and 3'oung, and that nearly all the young have been enabled to breed. In such cases the geometrical ratio of in- crease, the result of which never fails to be surprising, simply explains the extraordinarily rapid increase and wide diffusion of naturalized productions in their new homes. In a state of nature almost every plant produces seed, and among animals there are very few which do not annually pair. Hence we may confidently assert, that all plants and animals are tending to increase at a geometrical ratio — that all would most rapidly stock every station in which they could anjdiow exist — and that the geometrical tendency to increase must be checked by destruction at some period of life. Our familiarity with the larger domestic animals tends, I think, to mislead us : Ave see no great destruction falling on them, and we forget that thousands are annually slaughtered for food, and that in a state of nature an equal number would have somehow to be disposed of. The only difference between organisms which annually pro- duce eggs or seeds by the thousand, and those which produce extremely few, is, that the slow-breeders would require a few more years to people, under favorable conditions, a Avliole dis- trict, let it be ever so large. The condor lays a couple of eggs and tlic ostrich a score, and yet in the same country the condor may be the more numerous of the two: the Fulmar petrel lays but one oral, and this would certainly greatly alter (as indeed I have obseiTod in parts of South America) the vegetation : this again would largely aflect the insects; and this, as Ave have just seen in Staffordshire, the insectivorous birds, and so onward in ever-increasing circles of complexity. We began this scries CflAr. III. STRUGGLE FOR EXISTENCE. 79 by insectivorous birds, and -we have ended with tlieni. Not tliat in nature the rehitions can ever be as simple as this. Bat- tle within battle must ever be recurring' with varying' success ; and yet in the long-run the forces are so nicely balanced, that the lace of Nature remains uniform for long periods of time, tliough assuredly the merest trifle would often give the victory to one organic being over another. Nevertheless, so profound is our ignorance, and so high our presumption, that we marvel when we hear of the extinction of an organic being ; and, as we do not see the cause, we invoke cataclysms to desolate the world, or invent laws on the duration of the forms of life ! I am tempted to give one more instance showing how plants and animals, most remote in the scale of Nature, are bound together by a web of complex relations. I shall here- after have occasion to show that the exotic Lobelia fulgens, in this part of England, is never visited by insects, and conse- fiucntly, from its peculiar structure, never sets a seed. Near- ly all our orchidaceous plants absolutely require the visits of insects to remove their pollen-masses and thus to fertilize them. I find from experiments that humble-bees are almost indispen- sable to the fertilization of the heart's-ease (Viola tricolor), for other bees do not visit this flower. I have also found that the visits of bees are necessary for the fertilization of some kinds of clover : for instance, 20 heads of Dutch clover (Trifolium repens) yielded 2,290 seeds, but 20 other heads protected from bees produced not one. Again, 100 heads of red clover (T. pratense) produced 2,700 seeds, but the same number of pro- tected heads produced not a single seed. Humble-bees alone visit red clover, as other bees cannot reach the nectar. It has been suggested that moths may fertilize the clovers ; but I doubt whether they could do so in the case of the red clover, from their weight not being sufficient to depress the wing- petals. Hence we may infer as highly probable that, if the whole genus of humljle-bces became extinct or very rare in England, ihe heart's-ease and red clover would become very rare, or wholly disappear. The number of humble-bees in any district depends in a great degree on the number of field-mice, which destroy their combs and nests ; and Colonel Newman, who has long attended lo the habits of humble-bees, believes that " more than two-thirds of them are thus destroyed all over England." Now the number of mice is hirgely depend- ent, as every one knows, on tlic niunber of cats : and Colonel Newman says, " Near villages and small towns I have found 80 STRUGGLE FOK EXISTENCE. Chap. ^ly, the nests of humble-bees more numerous than elsewhere, Mhich I attribute to the number of cats that destroy the mice." Hence it is quite credible that the presence of a fefine animal in large numbers in a district might determine, through the interven- tion first of mice and then of bees, the frequency of certain flowers in that district ! In the case of every species, many different checks, acting at diflerent periods of life, and during different seasons or years, probably come into play ; some one check or some few being generally the most potent, but all concur in determining the average number or even the existence of the species. In some cases it can be shown that widely-different checks act on the same species in different districts. When we look at the jilants and bushes clothing an entangled bank, Ave are tempted to attribute their proportional numbers and kinds to what we call chance. But how false a view is this ! Every one has heard that when an American forest is cut down, a very differ- ent vegetation springs up ; but it has been observed that an- cient Indian ruins in the Southern United States, which must formerly haA'e been cleared of trees, now display the same beavitiful diversity and proportion of kinds as in the surround- ing virgin forest. What a struggle must have gone on during long centuries betAveen the scA'cral kinds of trees, each annual- ly scattering its seeds by the thousand ; Avhat Avar betAvcen insect and insect — between insects, snails, and other animals, Avith birds and beasts of pre}^ — all striving to increase, ail fcx'ding on each other, or on the trees, their seeds and seed- lings, or on the other plants Avhich first clothed the groimd and thus checked the groAvth of the trees ! ThroAV vip a handful of feathers, and all must fall to the ground according to defi- nite laAvs ; but Iioav simple is the jiroblem Avhere each shall fall compared to that of the action and reaction of the innumerable plants and animals which haA'C determined, in the course of centuries, the proportional numbers and kinds of trees noAV ffroAving on the old Indian ruins ! The dependency of one organic being on another, as of a parasite on its prey, lies generally betAveen beings remote in the scale of nature. This is likcAvise sometimes the case Avith those Avhich may strictly be said to struggle Avith each other for existence, as in the case of locusts and grass-feeding quad- rupeds. But the struggle Avill almost invariably be most severe betAveen the individuals of the same species, for they frequent the same districts, require the same food, and are CiiAr. 111. STRUGGLE FOlt EXISTENCE. 81 exposed to the same dangers. lu the case of vai-ietics of the same species, the struggle will generally be almost equally severe, and we sometimes see the contest soon decided : for instance, if several varieties of wheat be sown together, and the mixed seed be resown, some of the varieties Avhich best suit the soil or climate, or are naturally the most fertile, Avill beat the others and so yield more seed, and will consequently in a few years quite su^iplant the other varieties. To keep vip a mixed stock of even such extremely-close varieties as the variously-colored sweet-peas, they must be each year harvested separately, and the seed then mixed in due proportion, other- wise the weaker kinds Avill steadily decrease in number and disappear. So, again, with the varieties of sheep : it has licen asserted that certain mountain-varieties will starve out other mountain-varieties, so that they cannot be kept together. The same result has followed from keeping together different varie- ties of the medicinal leech. It may even be doubted whether the varieties of any of our domestic plants or animals have so exactly the same strength, habits, and constitution, that the original proportions of a mixed stock could be kept up for half a dozen generations, if they were allowed to struggle together, like beings in a stUte of nature, and if the seed or young were not annually sorted. ' Struggle for Life most severe between Individuals and Vari- eties of the same Sjyecles. As species of the same genus have usually, though by no means invariably, much similarity in habits and constitution, and always in structure, the struggle will generally be more severe between species of the same genus, when they come into competition Avith each other, than between species of dis- tinct genera. We see this in the recent extension over parts of the United States of one species of swallow having caused the decrease of another species. The recent increase of the missel-thrush in parts of Scotland has caused the decrease of the song-thrush. How frequently we hear of one. species of rat taking the place of another species under the most differ- ent climates ! In Russia the small Asiatic cockroach has every- where driven before it its great congener. In Australia the imported hivG^bce is rapidly exterminating the small, .stingless native bee. One species of charlock has been known to sup- plant another species ; and so in other cases. We can dimly 82 STRUGGLE FOR EXISTENCE. Cuap. III. see why the competition should be most severe between allied forms, which fill nearly the same place in the economy of Na- ture ; but probably in no one case could "vve precisely say why one species has been victorious over another in the great bat- tle of life. A corollary of the highest importance may be deduced from the foregoing remarks, namely, that the structure of every organic being is related, in the most essential yet often hidden manner, to that of all the other organic beings, with "which it comes into competition for food or residence, or from which it has to escape, or on which it preys. This is obvious in the structure of the teeth and talons of the tiger ; and in that of the legs and claws of the parasite which clings to the hair on the tiger's body. But in the beautifully-plumed seed of the dandehon, and in the flattened and fringed legs of the water-beetle, the relation seems at first confined to the ele- ments of air and Avater. Yet the advantage of plumed seeds no doubt stands in the closest relation to the land being already thickly clothed Avith other plants ; so that the seeds may be widely distributed and fall on iinoccupied ground. In the water-beetle, the structm-e of its legs, so well adapted for diving, allows it to compete with other a'quatic insects, to hunt for its own prey, and to escape ser\dng as prey to other animals. The store of nutriment laid up witliin the seeds of many plants seems at first sight to have no sort of relation to other jilants. But from the strong growth of young plants produced from such seeds (as peas and beans), when sown in the midst of long grass, it may be suspected that the chief use of the luitriment in the seed is to favor the growth of the young seed- ling, while struggling with other plants growing vigorously all around. Look at a plant in the midst of its range, why does it not double or qviadruplc its nimibers ? We know that it can per- fectly well withstand a little more heat or cold, dampness or dryness, for elsewhere it ranges into slightly hotter or colder, damper or drier districts. In this case we can clearly sec that if we wished in imagination to give the plant the power of in- creasing in number, we should have to give it some advantage over its competitors, or over the aTiimals which jireyed on it. ( )n the confines of its geographical range, a change of consti- tution Avith respect to climate would clearly be an advan- tage to our plant ; but we have reason to believe that only a Chap. III. STKUGGLE FOR EXISTENCE. 83 few plants or aniniiils rani]fc so far, that tbcy are destroyed by the rigor of the climate alone. Not until we reach the extreme confines of life, in the Arctic regions or on the borders of an utter desert, will competition cease. The land may be ex- tremely cold or dry, yet there will be competition between some few species, or between the individuals of the same spe- cies, for the Avarmest or dampest spots. Hence, also, we can see that when a plant or animal is placed in a new country among new competitors, tliough the climate may be exactly the same as in its former home, yet the conditions of its life will generally be changed in an essen- tial manner. If we wished to increase its average numbers in its new home, we should have to modify it in a diflerent way to what we should have to do in its native country; for we should have to give it some advantage over a different set of convpetitors or enemies. It is good thus to try in imagination to give any form some advantage over another. Probably in no single instance should we know what to do, so as to succeed. It will con- A-ince us of our ignorance on the mutual relations of all organic beings ; a conviction as necessar}', as it seems difficult to ac- quire. All that we can do is, to keep steadily in mind that each organic being is striving to increase in a geometrical ratio ; that each at some period of its life, during some season of the year, during each generation or at intervals, has to struggle for life, and to suffer great destruction. When we reflect on this struggle, we may console ourselves with the full belief that the war of Nature is not incessant, that no fear is felt, that death is generally jirompt, and that the vigorous, the healthy, and the haj^py, survive and multiply. 84 NATUEAL SELECTION. Ciiap. IV. • CHAPTER IV. NATURAL SELECTION, OK THE SURVIVAL OF THE FITTEST. Natural Selection— its Power compared with jMan's Selection— its Power on Char- acters of trifling Importance— its Power at all A<;e8 and on both Soxes— Sexual Selection— On the CJenenility of Intercrosses between Individuals of the same Species — Circumstances favorable and unfavorable to the Results of Natural Se- lection, namely. Intercrossing:, Isolation, Nnnibor of Individuals — Slow Action — Extinction caused by Natural Selection— Divergence of Character related to the Diversity of Inhabitants of any Small Area, and to Naturalization— Action of Nat- ural Selection, through Divergence of Character and Extinction, on the Descend- ants from a C( be borne in mind how inlinitely complex and close-fitting are the mutual relations of all organic beings to each other and to tlieir physical conditions of life ; and consequently what in- finitely-varied diversities of structure may be of use to each l)cing under changing conditions of life. Can it, then, be thought improbable, seeing that variations usefid to man have undoubtedly occurred, that other variations useful in some way CiiAP. IV. NATURAL SELECTION. 85 to each beinp^ in the frrcat and complex battle of life, sliouUl sometimes occur in the course of thousands of generations? If such do occur, can we doubt (remembering' that many more individuals are born than can possibly survive) that individuals having any advantage, however slight, over others, would have the best chance of surviving and of procreating their kind ? On the other hand, we may feel sure that any variation in the least degree injurious would be rigidly destroyed. This pres- ervation of favorable variations, and the destruction of injuri- ous variations, I call Natural Selection, or the Survival of the Fittest. Variations neither useful nor injurious would not be affected by natural selection, and would be left either a fluc- tuating element, as perhaps we see in certain polymorphic species, or would ultimately become fixed, owing to the nature of the organism and the nature of the conditions. Several writers have misapprehended or objected to the tenn Natural Selection. Some have even imagined that nat- ural selection induces variability, whereas it imphes only the preservation of such variations as occur and are beneficial to the being under its conditions of life. No one objects to agri- culturists speaking of the potent effects of man's selection ; and in this case the individual differences given by Nature, which man for some object selects, must of necessity first occur. Others have objected that the term selection implies conscious choice in the animals Avhich become modified; and it has even been urged that, as plants have no volition, natural selection is not applicable to them ! In the literal sense of the word, no doubt, natural selection is a false term ; but who ever objected to chemists speaking of the elective affinities of the various elements ? — and yet an acid cannot strictl}^ be said to elect the base with which it in preference combines. It has been said that I speak of natural selection as an active power or Deity ; but who objects to an author speaking of the attraction of gravity as ruling the movements of the planets ? Every one knows what is meant and is implied by such metaphorical ex- ])ressions ; and they are almost necessary for brevity. So, again, it is difficult to avoid personifying the word Nature ; but I mean by Nature, only the aggregate action and product of many natural laws, and by laws the sequence of events as as- certained by us. With a little familiarity such superficial ob- jections Avill be forgotten. We shall best understand the probable course of natural selection by taking the case of a country undergoing some SG NATURAL SELECTION. Chap. IV. slii^ht iihysical chanire, for instance, of climate. The propor- tional numbers of its inhabitants -would almost immediately undergo a change, and some species might become extinct. AVe may conclude, from what avc have seen of the intimate and complex manner in which the inhabitants of each country arc boinid together, that any change in the numerical jiropor- tions of some of the inhabitants, independently of the change of climate itself, would seriously affect the others. If the country were open on its borders, new fonns would certainly innnigrate, and this also would often seriously disturb the re- lations of some of the former inhabitants. Let it be remem- bered how powerful the influence of a single introduced tree or mammal has been sliOAvn to be. But in the case of an island, or of a country partly surrounded by barriers, into which new and better-adapted forms could not freely enter, we should then have places in the economy of Natvire Avhich would assuredly be better fdled up, if some of the original inhabitants were in some manner modified ; for, had the area been ojien to immigration, these same places would have been seized by in- truders. In such cases, slight modifications, which in any way favored the individuals of any species, by better adajiting them to their altered conditions, would tend to be prcsen-ed ; and natural selection would have free scope for the Avork of im- provement. We have reason to believe, as stated in the first chapter, that changes in the conditions of life cause or excite a ten- dency to vary ; and in the foregoing case the conditions are supposed to have changed, and this would manifestly be favor- able to natural selection, by giving a better chance of profit- able variations occurring; and unless such do occur, natural selection can do nothing. Under the term of "variations," it must never be forgotten that mere individual diflerences are always included. As man can certainly ]noducc a great result with his domestic animals and plants by adding up in any given direction individual differences, so could natural selection, but far more easily, from having incomparably longer time for action. Nor do I believe that any great physical change, as of climate, or any imusual degree of isolation to check immi- gration, is actually necessary to ])roduce new and unoccujiied places for natural selection to fill up by modifying and im])rf)v- ing some of the A-arying inhabitants. For as all the inhabit- ants of each country are struggling together Avith nic(^ly-bal- anccd forces, extremely-slight modifications in the structure oi CnAi-. IV. NATURAL SELECTION. 87 habits of one si)ecics would often give it an advantugc over others ; and still further modifications of the same liind Avould often still further increase the advantage, as long as the species continued under the same conditions of life and profited by similar means of sul)sistence and defence. No country can be named, in which all the native inhabitants are now so perfectly adapted to each other and to the physical conditions under which they live, that none of them could be still better adapted or improved ; for, in all countries, the natives have been so far conquered by naturalized productions, tliat they have allowed foreigners to take iirin ])ossession of the land. And, as for- eigners have thus in every country beaten some of the natives, Ave may safely conclude that the natives might have been mod- ified with advantage, so as to have better resisted the in- truders. As man can produce and certainly has produced a great result by his methodical and unconscious means of selection, what may not natural selection effect ? Man can act only on external and visible characters : Nature, if I may be allowed to j)ersonify the natural preservation or survival of the fittest, cares nothing for appearances, except in so far as they are use- ful to any being. She can act on every internal organ, on every shade of constitutional difference, on the whole machinery of life. Man selects only for his own good ; Nature only for that of the being which she tends. Every selected character is fully exercised by her, as is implied by the fact of their selection. Man keeps the natives of many climates in the same country ; he seldom exercises each selected character in some peculiac and fitting manner ; he feeds a long and a short beaked pigeon on the same food ; he does not exercise a long-backed or long- legged quadrup(^(l in any peculiar manner ; he exposes sheep with long and short wool to the same climate. lie does not allow the most vigorous males to struggle for the females. He does not rigidly destroy all inferior animals, but protects during each varying season, as far as lies in his power, all his produc- tions. He often begins his selection by some half-monstrous form; or at least by some modification prominent enough to catch the eye or to be plainly useful to him. Under Nature, the slightest differences of structure or constitution may well turn the nicely-balanceil scale in the struggle for life, and so be preserved. How fleet uig are the wishes and eflbrts of man! how short his time ! and consequently how poor will be his results, comjiared with those accumulated l)y Nature during 88 NATUKAL SELECTION. Chap. IV. whole geological periods ! Can we wonder, then, that Nature's productions should be far " truer " in character than man's pro- ductions ; tliat they should be infinitely better adapted to the most complex conditions of life, and should plainly bear the stamp of far higher workmanship? It may metaphorically l)e said that natural selection ig daily and hourly scrutinizing, throughout the world, the slightest vari- ations ; rejecting those that are bad, preserving and adding up all that are good ; silently and insensibly working, whenever and wherever opportunity oft'ers, at the improvement of each organic being in relation to its organic and inorganic conditions of life. We see nothing of these slow changes in progress, until the hand of time has marked tlie lapse of ages, and then so imperfect is our view into long-past geological ages, that we see only that the forms of life are now different from what they formerly were. In order that any great amount of modification in any part should be eflectcd, a variety when once formed must again, perliaps after a long interval of time, vary or present individ- ual difl"erences of the same favorable nature, and these must be again preserved, and so onward step by step. Seeing that individual differences of all kinds perpetually recur, this can hardly be considered as an unwarrantable assumption. But whether all this has actually taken place must be judged by how far the h}q5othesis accords with and explains tlie general phenomena of Nature. On the other liand, the ordinary belief that the amount of possible variation is a strictly-limited quan- tity is a simple assumption. Although natural selection can act only through and for the good of each being, yet characters and structures, which we 'are apt to consider as of very trifling importance, may thus be acted on. When we see leaf-eating insects green, and bark- feeders mottled-gray ; the alpine ptarmigan white in winter, the red-grouse the color of heather, avc must believe that these tints are of service to these birds and insects in preserving them from danger. Grouse, if not destroyed at some period of their lives, would increase in countless numbers ; they are known to suffer largely from bii'ds of prey ; and hawks are guided by eye- sight to their prey — so much so, that on parts of the Continent persons are warned not to keep white pigeons, as being the most liable to destruction. Hence natural selection miglit be most eflective in giving the proper color to each kind of grouse, and in keeping that color, when once acquired, true and con CiiM-. IV. NATURAL SELECTION. 89 stant. Nor ougbt we to think that the occasional destruction of an animal of any particular color would produce little effect : we should remember how essential it is in a flock of white slicep to destroy every lamb with the faintest trace of black. A\'e have seen how the color of the ho<^s, when fcedino: on the " paint-root " in Florida, determines whether they shall live or die. In plants the down on the fruit and the color of the flesh are considered by botanists as characters of the most trifling importance : yet we hear from an excellent horticulturist, Down- ing, that in the United States smooth-skinned fruits sullcr far more from a beetle, a curculio, than those Avith down ; that pur- ple plums suffer far more from a certain disease than 3X'llow plums ; whereas another disease attacks yellow-fleshed peaches far more than those with other colored flesh. If, with all the aids of art, these slight differences make a great difl'crence in cultivating the several varieties, assuredly, in a state of nature, where the trees would have to struggle with other trees and with a host of enemies, such differences would effectually settle whicli variety, whether a smooth or downy, a yellow or purple fleshed fruit, should succeed. In looking at many small points of difference between spe- cies, which, as far as our ignorance permits us to judge, seem quite unimportant, we must not forget that climate, food, etc., may have produced some direct eflect. It is also necessary to bear in mind that, owing to the law of correlation, Avhen one part varies, and the A\ariations are accumulated through natural selection, other modifications, often of the most unexpected nature, avUI ensue. As we see that those variations Avhicli under domestication appear at any particular })eriod of life, tend to rcapjwar in the offspring at the same period — for instance, in the shape, size, and flavor of the seeds of the many varieties of our culinary and agricultural plants ; in the caterpillar and cocoon stages of the varieties of the silk-worm ; in the eggs of poultry, and in the color of the down of tlieir chickens; in the horns of our sheep and cattle when nearly adult — so, in a state of nature, natural selection Avill be enabled to act on and modify organic beings at any age, by the accumulation of variations profitable j-t that age, and by their inheritance at a corresponding age. If it profit a plant to have its seeds more and more Avidely dis- seminated by the wind, I can sec no greater difliculty in this being effected tlirough natural selection than in the cotton- planter increasing and improving by selection the down in the DO SEXUAL SELECTION. CriAr. IV. pods on his cotton-trces. Natural selection may modify and adapt the larva of an insect to a score of continovncics, wholly tlillcrent from tliose which concern the mature insect; and these modilicalions may affect, throug'h correlation, the stnio ture of the adult. So, conversely, modifications in the adult may affect tlie structure of the larva; but in all cases natural selection will insure that they shall not be injurious: for, if they were so, the species would become extinct. Natural selection will modify the structure of the young in relation to the parent, and of the parent in relation to the young. In social animals it will adapt the structure of each indivitlual for the benefit of the Avhole community ; if this in consequence profits by the selected change. What natural selection cannot do, is to modify the structure of one species, without giving it any advantage, for the good of another spe- cies ; and, though statements to this effect may be found in works of natural history, I cannot find one case which will bear investigation. A structure used only once in an animal's life, if of high importance to it, might be modified to any ex- tent by natural selection ; for instance, the great jaws pos- sessed by certain insects, used exclusively for opening the cocoon — or the hard tip to the beak of nestling birds, used for breaking th(^ eir'^. It has been asserted that, of the best short- lieaked tumbler-pigeons, a greater number 2:)erish in the egg than are al^le to get out of it ; so that fanciers assist in the act of hatching. Now, if Nature had to make the beak of a full- j^rown pigeon very short for the bird's own advantage, the pro- cess of modification Avould be very slow, and there would be simultaneously the most rigorous selection of all the young birds within the egg, which had the most powerful and hardest beaks, for all with weak beaks would inevitably perish ; or, more delicate and more easily-l)rokcn shells might be se- lected, the thickness of the shell being knoAvn to vary like every other structure. Sexual Selection. Inasmuch as peculiarities often appear under domestication in one sex and become hereditarily attached to that sex, the same fact no doubt occurs under Nature, and if so, natural se- lection will be able to modify one sex in its functional n'lations to the otlier sex, or in relation to Avholly-ditrerent liabits of life in the two sexes, as is sometimes the case with insects. And Chap. IV. SEXUAL SELECTION. 91 this leads ine to saj a few words on what I call Sexual Selec- tion. This depends, not on a sti'ugf^le for existence, l)ut on a struo^g'lc between the males for possession of tlie females; the result is not death to the inisuccessful competitor, but few or HO oifsprino-. Sexual selection is, therefore, less rit^orous than natural selection. Generally, the most vigorous males, those \vhich are best fitted for their places in Nature, will leave most progeny. But, in many cases, victory depends not on general vigor, but on having special weapons, confined to the male sex. A hornless stag or spurless cock would have a poor chance of leaving numerous ollspring. Sexual selection, by always allowing the victor to breed, might surely give indom- itable courage, length to the spur, and strength to the wing to strike in the spurred leg, as in the case of the brutal cock- fighter, who knows well how to improve his breed hy the care- ful selection of the best cocks. How low in the scale of Nature the law of battle descends, I know not ; male alligators have been described as fighting, bellowing, and Avhirling round, like Indians in a war-dance, for the possession of the females ; male salmons have been seen fighting all day long ; male stag-])eetles sometimes bear wounds from the huge mandibles of other mules ; tlie males of certain hymcnopterous insects have been fivcjuently seen by that inimitable observer M. Fabre, fighting for a particular female, Avho sits by, an apparently imcon- cerned beholder of the struggle, and then retires with the con- queror. The war is, perhaps, severest between the males of l)olygamous animals, and these seem oftenest provided with special weapons. The males of carnivorous animals arc already well armed ; though to them and to others, special means of de- fence may be given through means of sexual selection, as the mane to the lion, and the hooked jaw to the male salmon; for the shield may be as important for victory as the sword or spear. Among birds, the contest is often of a more peaceful char- acter. All those who have attended to the subject, believe that there is the severest rivalry between the males of many species to attract l)y singing the females. The rock-thrush of Guiana, birds of Paradise, and some others, congregate; and successive males display tlieir gorgeous plumage and perform strange antics l)efore the females, whieli, standing bv as spec- tators, at last choose the most attractive partner. 1'hose who have closely attended to birds in confinement well know that they often take individual preferences and dislikes: thus Sir 02 ILLUSTRATIONS OF THE ACTION OF Chap. IV. Iv. Iloroii has described how one pied peacock was eminently attractive to all his hen-birds. I cannot here enter on the ne- cessary details ; but if man can in a short time jjive elegant carriage and beauty to his bantams according to liis standard of beauty, I can see no good reason to doubt that female birds, by selecting, during thousands of generations, the most melodious or beautiful males, according to their standard of beauty, might jiroduce a marked effect. Some well-known laws, with respect to the plumage of male and female birds, in comparison with the plumage of the young, can be explained through the action of sexual selection on variations occurring at diiierent ages, and being transmitted to the males alone or to both sexes at a corresponding age ; but I have not space here to enter on this "subject. Thus it is, as I believe, that when the males and females of any animal have the same general habits of life, but diller in structure, color, or ornament, such differences have been main- ly caused by sexual selection ; that is, by individual males hav- ing had, in successive generations, some slight advantage over other males, in their weapons, means of defence, or charms ; and having transmitted these advantages to their male off- spring. Yet, I would not Avish to attribute all such sexual dif- ferences to this agency : for we see peculiarities arising and becoming attached to the male sex in our domestic animals (as the greater development of the Avattle in male carrier-pigeons, horn-like protuberances in certain fowls, etc.), Avhich are in no way useful. We see analogous cases under Nature — for in- stance, the tuft of hair on the breast of the turkej'-cock, Avliich cannot be useful, and can hardly be ornamental ; indeed, had the tuft appeared under domestication, it would have been called a monstrosity. Illustratio7is of the Action of Natural Selection, or the Sur- vival of the Fittest. In order to make it clear how, as I believe, natural selec- tion acts, I must beg permission to give one or two imaginary illustrations. Let us take the case of a wolf, which preys on various animals, securing some by craft, some by strength, and some by (leetness ; and let us suppose that tlie fleetest prey, a deer for instance, had from any change in the country in- creased in numbers, or that other prey had decreased in num- bers, during that season of the year when the wolf was hardest CiiAP. IV. NATUKAL SELECTION. 93 pressed for food. Under such cLrcumstances the swiftest and slimmest wolves would have the best chance of survdving-, and so be preserved or selected — provided always that they retained strength to master their prey at this or at some other period of the year, when they might bo compelled to prey on other animals. I can see no more reason to doubt tliis, than that man can improve the fleetness of his greyhounds by careful and methodical selection, or by unconscious selection which results from each man trying to keep the best dogs without any thought of modifying the breed. I may add, that, according to Mr. Pierce, there are two varieties of the wolf inhabiting the Catskill Mountains in the United States, one with a light, greyhound-like form, which pursues deer, and the other more bulky, with shorter legs, which more frequently attacks the shepherd's flocks. It should be observed that, in the above illustration, I speak of the slimmest individual wolves, and not of any single strong- Ij'-marked variation having been preserved. In former editions of this work I sometimes spoke as if this latter alternative had frequently occurred. I saw the great importance of individual differences, and this led me fully to discuss the results of un- conscious selection by man, which depends on the preservation of the better-adapted or more valuable individuals, and on the destruction of the worst. I saw, also, that the preservation in a state of nature of any occasional deviation of structure, such as a monstrosity, would be a rare event ; and that, if preserved, it would generally be lost by subsequent intercrossing with ordinary individuals. Nevertheless, until reading an able and valuable article in the N'orth Uritish lievieio (18G7), I did not appreciate how rarely single variations, whether slight or strongly-marked, could be perpetuated. The author takes the case of a pair of animals, whicli produce during their lifetime two hundred offspring, of which, from various causes of de- struction, only two on an average survive to procreate their kind. Tliis is rather an extreme estimate for most of the higher animals, but by no means so for many of the lower organisms. He then shows that if a single individual were born, whicli varied in souk* maimer, giving it twice as g'ood a chance of life as that of the other individuals, yet the chances would be strongly against its survival. Supposing it to sur- vive and to breed, and that half its young inherited the favor- able variation ; still, as the Reviewer goes on to show, the young would liavo only a slightly-better chance of survaving 3 1 ILLUSTEATIOKS OF TUE ACTION OF Cuap. IV. and breeding- ; and this chance would go on decreasing' in the succeeding generations. The justice of these remarks cannot, I think, be disputed. If, for instance, a bird of some kind couhl procure its food more easily by having its beak curved, and if one Avere born with its beak strongly carved, and wliich consequently flourished, nevertheless there would be a very poor chance of this one individual perpetuating its kind to the exclusion of the common form ; but there can hardly be a doubt, judging by what we see taking place imder domesti- cation, that this result Avould follow from the preservation dur- ing many generations of a large number of indiWduals with more or less curved beaks, and from the destruction of a still larger numlier Avith the straightest beaks. It should not, however, be overlooked that certain varia- tions, which no one would rank as mere individual differences, frequently recur, owing to a similar organization being simi- larly acted on — of which fact numerous instances could be given with our domestic productions. In such cases, if a vary- ing individual did not actually transmit to its offspring its newly-acquired character, it would imdoubtedly transmit, as long as the existing conditions remained the same, a still stronger tendency to vary in the same manner. The condi- tions might indeed act in so energetic and definite a manner as to lead to the same modification in all the individuals of the species without the aid of selection. But we may suppose that the conditions sufficed to effect only a third, or fourth, or tenth part of the individuals ; and several such cases could be given ; for instance, it has been estimated by Graba that in the Faroe Islands about one-fiftli of the guillemots, Avhich all breed together, consist of a well-marked variety ; and this Avas for- meily ranked as a distinct species under the name of Uria lacrymans. Now, in such cases, if the variation were of a ben- eficial nature, the original form would soon be sup})lantcd by the modified form, through the survival of the fittest. With reference to the eftects of intercrossing and of com- petition, it should be borne in mind that most animals antl plants keep to their proper homes, and do not needlessly Avander aljout ; Ave see this CA'cn Avith migratory birds, Avhich almost always return to the same district. Consequently each newly- formed variety Avould generally be at lirst local, as seems to 1)0 the common rule Avith A'arieties in a state of nature ; so that siinilarlA'-modified individuals Avould soon exist in a small l)ody together, and would often breed together. If the v.vw variety Chap. IV. NATURAL SELECTION. 95 was successful in its battle for life, it would slowly spread from a central spot, competinp^ Avitli and conf[U(>rinof the uncluuifred individuals on the niar<>-ins of an ever-increasing circle. But to the subject of intercrossing we shall have to return. It may be objected by those who have not attended to natural history, that tlie long-continued accumulation of individual differences could not give rise to parts or organs which seem to us, and arc often called, new. But, as we shall hereafter find, it is dif- ficult to advance any good instance of a really new organ ; even so complex and perfect an organ as the eye can be showa t(< graduate downward into mere tissue sensitive to diffused light It may be worth Avhile to give another and more complex illustration of tlie action of natural selection. Certain plants excrete sweet juice, apparently for the sake of eliminating something injurious from their sap: this is effected, for in- stance, by glands at the base of the stipules in some Legunii- nosa^, and at the backs of the leaves of the common laurel. This juice, though small in quantity, is greedily sought by in- sects ; but their visits do not in any way benefit the plant. Now, let us suppose that the juice or nectar was excreted from the inside of the flowers of a certain number of plants of anv species. Insects in seeking the nectar would get dusted with pollen, and would certainly often transport it from one flower to another. The flowers of two distinct individuals of the same species would thus get crossed ; and the act of cross- ing, as Ave have good reason to believe, would produce vigor- ous seedlings, which consequently would have the best chance of flourishing and surviving. The plants which produced flow- ers with the largest glands or nectaries, excreting most nectar, would oftencst be visited by insects, and would oftenest be crossed ; and so in the long-run would gain the upper hand and form a local variet}'. The flowers, also, wliich had their sta- mens and pistils placed, in relation to the size and habits of the particular insect which visited them, so as to favor in any dcgr(>e the transportal of the pollen, would likewise be favored. AVe might have taken the case of insects visiting flowers for Ihc sake of collecting pollen instead of nectar; and as pollen is formed for the sole purpose of fertilization, its destruction ap- pears to be a simple loss to the plant ; 3-et if a little pollen "were canied, at first occasionally and then habitually, by the pollen-devouring insects from flower to flower, and a cross thus ellected, although nine-tenths of the pollen were destroyed, it might still be a great gain to the plant; and the individuals 96 ILLUSTRATIONS OF THE ACTION OF Chap. IV. which produced more and more pollen, and had larger anthers, would be selected. "When our plant, by the above process long continued, had been rendered highly attractive to insects, they would, unin- tentionally on their part, regularly carry pollen from flower to flower; and that they do this cfl'ectually, I could easily show l)y many striking facts. I will give only one, as likewise illus- trating one step in the separation of the sexes of plants. Some liolly-trees bear. only male flowers, which have four stamens ])roducing a rather small quantity of pollen, and a rudimentary l)istil ; other holly-trees bear only female flowers ; these have a full-sized pistil, and four stamens with shrivelled anthers, in which not a grain of pollen can be detected. Having found a female tree exactly sixty yards from a male tree, I put the stigmas of twenty flowers, taken from difierent branches, under the microscope, and on all, without exception, there were a few pollen-grains, and on some a profusion. As the wind had set for several days from the female to the male tree, the pollen could not thus have been carried. The weather had been cold and boisterous, and therefore not favorable to bees, neverthe- less every female flower which I examined had been effectually fertilized by the bees, which had flown from tree to tree in search of nectar. But to return to our imaginary case : as soon as the plant had been rendered so highly attractive to insects that pollen was regularly carried from flower to flower, another process might commence. No naturalist doubts the advantage of what has been called the " physiological division of labor ; " hence we may believe that it would be advanta- geous to a plant to produce stamens alone in one flower or on one whole plant, and pistils alone in another flower or on another plant. In plants under culture and placed under new conditions of life, sometimes the male organs and sometimes the female organs become more or less impotent ; now if we suppose this to occur in ever so slight a degree under Nature, tlien, as pollen is already carried regularly from flower to flower, and as a more complete separation of the sexes of our plant would be advantageous on the principle of the di\'ision of labor, individuals with this tendency more and more increased, would be continually favored or selected, until at last a eoin- phite separation of the sexes might be effected. It would take up too much space to show the various steps, through diinorpliism and other means, by which the separation of the sexes in plants of various kinds is aiiparently now in progress ; Chap. IV. NATUKAL SELECTION. 97 but I may add that .some of the species of holly in North America are, accordiiifj to Asa Gray, in an intermediate con- dition, or, as he cxpixvsses it, arc more or less diceciously po- lygamous. Let us now turn to the nectar-feeding insects. We may suppose the plant, of which we have been slowly increasing the nectar hy continued selection, to be a common plant ; and that certain insects depended in main part on its nectar for fooil. I could give many facts, showing how anxious bees are to save time: for instance, their habit of cutting holes and sucking the nectar at the bases of certain flowers, which they can, with a very little more troul)le, enter by the mouth. Bearing such facts in mind, it may be believed that, under cer- tain circumstances, individual differences in the curvature or length of the proboscis, etc., too slight to be appreciated by us, might profit a bee or other insect, so that certain individ- uals would be able to obtain their food more quickly than others ; and thus the communities to which they belonged would ilourish and throw off many swarms inheriting the same peculiarities. The tulles of the corolla of the common red and incarnate clovers (Trifolium pratense and incarnatum) do not on a hasty glance appear to differ in length ; yet the hive-bee ran easily suck the nectar out of the incarnate clover, but not out of the common red clover, Avhich is visited by humble-bees alone ; so that whole fields of the red clover in vain offer an abundant supply of precious nectar to the hive-bee. Tliat this nectar is much liked by the hive-bee is certain ; for I have re- ])eatedly seen, but only in the autumn, many hive-bees sucking the (lowei-s through holes bitten in the base of the tube by humble-bees. The difference in the length of the corolla in the two kinds of clover, which determines the visits of the hive-bee, must be very trifling ; for I have been assured that when red clover has been mown, the flowers of the second crop nve somewhat smaller, and that these are visited by many hive-bees. I do not know whether this statement is accurate; nor whether another published statement can be trusted, namely, that the Ligurian bee, which is generally considered a mere variety and which freely crosses with the common hive-bee, is able to. reach and suck the nectar of the connnon red clover. Thus, in a country where this kind of clover abounded, it might be a great advantage to the hive-bee to have a slightly-longer or differently-constructed proboscis.. On the other hand, as the fertility of this clover absolutely depends on bees visiting 98 INTEECEOSSING OF INDIVIDUALS. Chap. IV. tlio llowiTri, if humble-bees were to become rare in any country, it miglit be a great advantage to the plant to have a shorter or more deeply-divided corolhi, so that the hive-bees should be induced to suck its flowers. Thus I can understand how a flower and a bee might slowly become, either simultaneously or one after the other, modified and adapted to each other in the most perfect manner, by the continued preservation of all the individuals M'hich presented slight deviations of structure mutually favorable to each other. I am Avcll aware that this doctrine of natural selection, exemplified in the above imaginary instances, is open to the same objections which were at first urged against Sir Charles Lycll's noble \aews on " the modern changes of the earth, as illustrative of geology ; " but Ave now seldom hear the agencies, still at work, spoken of as trifling or insignificant, when applied to the excavation of the deepest valleys or to the formation of long lines of inland cliffs. Natural selection acts only by the preservation and accumulation of small inherited modifications, each profitable to the preserved being ; and as modern geology- has almost banished such views as the excavation of a great valley by a single diluvial wave, so will natural selection, if it be a true principle, banish the belief of the continued crea- tion of new organic beings, or of any great and sudden modifi- cation in their structure. On the Infererossmff of Individuals. I must here introduce a short digression. In the case of animals and plants with separated sexes, it is of course obvi- ous that two individuals must always (with the exception of the curious and not well-understood cases of parthenogenesis) unite for each birth ; but in the case of hermaphrodites this is far from obvious. Nevertheless tliere is reason to believe that with all hermaphrodites two individuals, either occasionally or habitually, concur for the reproduction of their kind. This vicAV was first suggested by Andrew Knight. "SVe shall pres- enth' see its importance : but I must here treat the subject with extreme 1)revity, thougli I have the materials prepared for an anijile discussion. All vertebrate animals, all insects, and some other large groups of animals, pair for each birth. Modern research has nuich diminished the number of supjiosed hermajihrodites, and of real hermaphrodites a large number pair; that is, two individuals regularly unite for reproduction, CiiAr. IV. INTERCROSSING OF INDIVIDUALS. 99 Aviiieh is all that concerns us. But still there arc many lier- maphrodito animals wliicli certainly do not habitually pair, and a \-ast majority of plants are hermaphrodites. What reason, it may be asked, is there for supposing in these cases that two individuals ever concur in reproduction? As it is impossible here to enter on details, I nuist trust to some general consid- erations alone. In the first place, I have collected so large a body of facts, showing, in accordance with the almost universal belief of breeders, that Avith animals and plants a cross between differ- ent varieties, or between individuals of the same variety but of another strain, gives vigor and fertility to the offspring ; and on the other hand, that dose interbreeding diminishes vig- or and fertility ; tliat these facts alone incline me to believe that it is a general law of Nature that no organic being fertil- izes itself for a perpetuity of generations ; but that a cross with another individual is occasionally — perhaps at long inter- vals of time — indispensable. On the belief that this is a law of Nature, we can, I think, miderstand several large classes of facts, such as the follow- ing, which on any other view are inexplicable. Every hybrid- izer knows how unfavorable exposure to wet is to the fertili- zation of a flower, yet Avhat a multitude of flowers have their anthers and stigmas fully exposed to the weather ! If an occasional cross be indispensable, notwithstanding that the plant's own anthers and pistil stand so near each other as almost to insure self-fertilization, the fullest freedom for the entrance of pollen from another individual Avill explain the above state of exposure of the organs. Many flowers, on the other hand, have their organs of fructification closely enclosed, as in the great papilionaceous or pea-fainily ; but in most of these flowers there is a curious adaptation between their struc- ture and the manner in which bees suck the nectar; for, in doing this, they either push the flower's own pollen on the stigma, or bring pollen from another flower. So necessary are the visits of bees to many ]iapiIionaceous flowers, that I have found, by experiments published elsewhere, that their fertility is greatly diminished if these visits be prevented. Now, it is scarcely possible that bees should fly from flower to flower, and not carry pollen from one to the other, to the great good, as I believe, of the plant. IJees will act lik(^ a camel-hair pen- cil, and it is quite sufficient just to touch the anthers of one flower and then the stigma of another with the same brush to 100 INTEKCEOSSING OF INDIVIDUALS. Ca.vp. IV. insure fertilization ; but it must not be supposed that bees would thus produce a multitude of hybrids between distinct species ; for if you brinf^ on the same brush a plant's own pol- len and pollen from another species, the former will have such a prepotent effect, that it will invariably and completely de- stroy, as has been shown by Gartner, any influence from the foreig'n pollen, ^V'hen the stamens of a flower suddenly spring toward the pistil, or slowly move one after the other toward it, the con- trivance seems adapted solely to insure self-fertilization ; and no doubt it is useful for this end : but the agency of insects is often required to cause the stamens to spring forward, as Kol- reutcr has shown to be the case with the barberry ; and in this very genus, which seems to have a special contrivance for self- fertilization, it is well known that, if closely-allied forms or varieties are planted near each other, it is hardly possible to raise pure seedlings, so largely do they naturally cross. In many other cases, far from there being any aids for self-fertili- zation, there are special contrivances, as I could show from the writings of C C. Sprengel and from my own observations, which effectually prevent the stigma receiving pollen from its own flower : for instance, in Lobelia fulgens, there is a really beautiful and elaborate contrivance by which all the infinitely numerous pollen-granules are swept out of the conjoined an- thers of each flower, before the stigma of that individual flower is ready to receive them; and as this flower is never visited, at least in my garden, b}^ insects, it never sets a seed, though by placing pollen from one flower on the stigma of another, I raised plenty of seedlings ; and while another species of Lobe- lia growing close by, which is visited by bees, seeds freely. In very many other cases, though there be no special mechani- cal contrivance to prevent the stigma of a flower receiving its own pollen, yet, as C. C. Sprengel has shown, and as I can confirm, either the anthers burst before the stigma is ready for fertilization, or the stigma is ready before the pollen of that flower is ready, so tliat these plants have in fact separated sexes, and nnist habitually be crossed. So it is with the recip- rocally dimorphic and trimorphic plants pre\dously alluded to. How strange are these facts ! How strange that the pollen and stigmatic surface of the same flower, though placed so close together, as if for the very purpose of self-fertilization, should in so many cases be nuitually useless to each other! Hov>' simjily are these facts explained on the view of an oeca- OiAP. IV. INTERCROSSING OF INDIVIDUALS. 101 sional cross Avilli a distinct individual being advantageous or indispensable ! If several varieties of tlie cabbage, radish, onion, and of some otlicr plants, be allowed to seed near eacli other, a large majority, as I have found, of the seedlings thus raised will turn out mongrels : for instance, I raised 233 seedling cab- Ixigcs from some jilants of different varieties growing near each other, and of these only 78 were true to their kind, and some even of these were not perfectly true. Yet the pistil of each cabbage-flower is surrounded not only l)y its own six sta- mens, but by those of the many other flowers on the same plant; and the pollen of each flower readily gets on its own stigma without insect-agency ; for I have found that a plant carefully protected produced the full number of pods. How, then, comes it that sucli a vast number of the seedlings are mongrel- ized ? • I suspect that it must arise from the pollen of a distinct varii'tj/ having a prepotent effect over a flower's own pollen ; and that this is part of the general law of good being derived from the intercrossing of distinct individuals of the same spe- cies. When distinct species are crossed the case is directly the reverse, for a jilant's own pollen is almost always prepotent over foreign pollen ; but to this subject Ave shall return in a future chapter. In the case of a large tree covered with innumerable flow- ers, it may be objected that pollen could seldom be carried from tree to tree, and at most only from flower to flower on the same tre*, and that flowers on the same tree can be con- sidered as distinct individuals only in a limited sense. I be- lieve this objection to be valid, but that Nature has largely pro- vided against it by giving to trees a strong tendency to bear flowers with separated sexes. When the sexes are separated, although the male and female flowers may be produced on the same tree, we can see that pollen must be regularly carried from flower to flower; and this will give a better chance of ]i()llen being occasionally carried from tree to tree. That trees belonging to all Ord<»rs have their sexes more often separated than other plants, I And to be the case in this country ; and at my request Dr. Hooker tabulated the trees of New Zealand, and Dr. Asa Gray those of the United States, and the result was as I anticipated. On the other hand. Dr. Hooker has re- cently informc(l me that he flnds that the rule does not hold in Australia ; and I have made these few remarks on the sexes of trees simply to call attention to the subject. 102 INTERCROSSING OF INDIVIDUALS. Cii\r. IV. Turning- for a very brief space to animals : on ilic land there are some hcrmaplirodites, as land-moUusca and eartli- worms ; but these all pair. As yet I have not found a single case of a terrestrial animal Avhich fertilizes itself. Wc can un- derstand this remarkable fact, Avhich offers so strong a contrast Avith terrestrial plants, on the view of an occasional cross being indispensable, by considering the medium in which terrestrial animals live, and the nature of the fertilizing element ; for we know of no means, analogous to the action of insects and of the wind in the case of plants, by which an occasional cross could be effected "with terrestrial animals, "without the concur- rence of two individuals. Of aquatic animals, there are many self-fertilizing hermaphrodites ; but here currents in the water offer an obvious means for an occasional cross. And, as in the case of flowers, I have as yet failed, after consultation Avith one of the highest authorities, namely, Prof. Huxley, to dis- cover a single case of an hermaphrodite anim:il with the organs of reproduction so perfectly enclosed within the body, that ac- cess from without and the occasional influence of a distinct in- dividual can be shown to be physically impossible. Cirripedes long appeared to me to present a case of very great difficulty under this point of view ; but I have been enabled, by a fortunate chance, elsewhere to prove that two individuals, though both are self-fertilizing hermaphrodites, do sonietimcs cross. It must have struck most naturalists as a strange anomaly that, in the case of both animals and plants, species of the same family and even of the same genus, though agreeing closely with each other in almost their whole organization, yet are not rarely, some of them hermaphrodites, and some of them uni- sexual. But if, in fact, all hermaphrodites do occasionally in- tercross with other individuals, the difference between hermaph- rodites and unisexual species, as far as function is concerned, becomes very small. From these several considerations and from the many spe- cial facts which I have collected, but which I am not here able to give, I am strongly inclined to suspect that, both in the vegetable and animal kingdoms, an occasional intercross with a distinct individual is a law of Nature. I am well aware that there are, on this view, many cases of difficulty, some of which I am trying to investigate. Finally, then, we may conclude that, in many organic beings, a cross between two individuals is an ()l)vious necessity for each l)irth ; in many others it occurs Chai-. IV. niODUCTION OF NEW FORMS. 103 perhaps only at long' intervals ; but in none, as I suspect, can self-fertilization go on for perpetuity. Clrcnmstanccs favorable for the Production of Neio Forms through Natural Selection. Tliis is an extremely intricate subject. A great amount of variability, under which term individual differences are always included, will evidently be favorable. A large number of in- dividuals, by giving a better chance for the appearance of profitable variations within any given period, will compensate for a lesser amount of variability in each individual, and is, I believe, an extremely important element of success. Though Nature grants long periods of time for the work of natural se- lection, she does not grant an indefinite period ; for, as all or- ganic -beings arc striving to seize on each place in the economy of Nature, if any one species docs not become modified and im- proved in a corresponding degree with its competitors, it will be exterminated. Unless favorable variations be inherited by some at least of the offspring, nothing can be effected by nat- ural sclecticjn. Tlie tendency to reversion may. often check or prevent the work ; but as tliis tendency has not prevented man from forming by selection nimierous domestic races, why .sliould it prevail against natural selection ? In the case of methodical selection, a breeder selects for some definite object, and free intercrossing Avill wholly stop his work. But when many men, without intending to alter the breed, have a nearly common standard of perfection, and all try to procure and breed from the best animals, much improve- ment surely but slowly follows from this unconscious process of selection, notwithstanding a large amount of crossing with inferior animals. Thus it Avill be in Nature; for within a con- Ihied area, with some place in its polity not perfectly occu]iicd, natural selection will always tend to preserve all the individ- uals varying in the right direction, though in different degrees, so as better to fill up the imoccupied place. But if the area be very large, its several districts will almost certainly present different conditions of life ; and then, if the same species under- g()(^s modification in different parts, the newly-formed varieties will intercross on the confines of each district. But we shall see in the seventh chapter that intermediate varieties, inhabit- ing an intermediate district, whether the result of the crossing of other varieties, or originally formed with an intermediate 104 CIRCUMSTANCES TAYOKABLE TO THE CnAr. IV. character, will in llie lonrr-run generally be supplanted by one of the varieties on either hand. Intercrossing will aflect those animals most which unite for each birth and Avauder much, and which do not breed at a very quick rate. Hence with animals of this nature, for instance birds, varieties will generally be confined to separated countries ; and this I find to be the case. With hennaphrodite organisms which cross only occasionally, and likewise with animals which unite for each l)irth,but which wander little and can increase at a very rapid rate, a new and iinpro\'ed variety might be quickly formed on any one spot, and might there maintain itself in a body and afterward spread, so that the crossing would be chiefly between the indi- viduals of the new variety living together in the same place. On this principle, nurserymen always prefer saving seed from a large body of plants, as the chance of intercrossing is thus lessened. Even in the case of animals which breed slowly and xmite for each birtli, Ave must not assume that the effects of natural selection Avill always be immediately oveq:)OAvered by free inter- crossing ; for I can bring a considerable body of facts, showing that, Avitljin the- same area, varieties of the same animal may long remain distinct, from haunting different stations, from breeding at slightly-tlifferent seasons, or from varieties of the same kind preferring to pair together. Intercrossing plays a very important part in Nature in keep- ing the individuals of the same species, or of the same \'ariety, true and xmifonn in character. It will obviously thus act far more elViciently with those animals Avhich unite for each birth ; but as already stated we have reason to believe that occasional intercrosses take place with all animals and with all plants. Even if these take ]ilace only at long intcr\-als of time, the young thus produced Avill gain so much in vigor and fertihty over the offspring from long-continued self-fertilization, that the}' Avill have a better chance of surviving and propagat- ing their kind ; and thus, in the long-run, the influence of inter- crosses, even at rare intervals, will be great. If there exist organic beings which never intercross, uniformity of character ran be retained among them, as long as their conditions of life remain tho same, only through the principle of inheritance and through natural selection destroying any which depart from tlic proper type ; but if their conditions of life change and they undergo modification, xmiformity of character can be given to their modified offspring, solely b^' natural selection prescn-ing similar favorable variations. Cbap. IV. KESULTS OF NATURAL SELECTION. IQH Isolation, also, is an important clement in the changes effected tlirout^h natural selection. In a confined or isolated area, if not very large, the organic and inorganic conditions of life will generally be almost uniform ; so that natural selection will tend to modify all the varying individuals of the same spe- cies in the same manner. Intercrossing with the inhal)itants of the surrounding districts ^vill, also, be prevented. Moritz Wagner has lately published an interesting essay on this sub- ject, and has shown that the service rendered by isolation in preventing crosses between newly-formed varieties is probably greater even than I have supposed. But from reasons already assigned I can by no means agree with this naturalist, that migration and isolation are necessary for the formation of new species. The importance of isolation is likewise great in pre- venting, after any physical change in the conditions, such as of climate, elevation of the land, etc., the immigration of better- adapted organisms ; and thus new places in the natural econ- cmy of the district are left open for the old inhabitants to strug- gle for and become ada])ted to. Lastly, isolation will give time for a new variety to be slowly improved ; and this may some- times be of importance in the productioTi of new species. If, however, an isolated area be very small, either from being sur- rounded by jjarriers, or from having very peculiar physical con- ditions, the total number of the inlialiitants will be small ; and this will retard the production of new species through natural selection, by decreasing the chances of the appearance of favor- able individual diflerences. The mere lapse of time by itself does nothing either for or against natural selection. I state this because it has been erroneously asserted that the element of time has been assumed by me to play an all-imjiortant part in modifying species, as if all ^vere necessarily undergoing change through the action of some iimate law. Lapse of time is only so fur injportant, and its importance in this respect is great, that it gives a better chaiKMj of beneficial variations arising, being selected, increased, and fixed, in relation to the; slowly-clianging organic and in- organic (•f)iiditions of life. It likewise favors the definite action of the conditions of life. If \\c turn to Nature to test the truth of these remarks, and look at any small, isolated area, such as an oceanic island, al- though the number of the dilTt^rent species inhabiting it is small, as we shall see in our chapter on Geographical Distribution; yet of the species a very large proportion are endemic — that is, lOO CinCU.MSTANCES FAVORABLE TO THE Ciiap. IV, have l.ccn produced llicre, and nowhere else in the world. Hencn^ :m oceanic ishxnd at first sie^ht seems to have been hig'hly favorable for the production of new species. But we may thus deceive ourselves, for, to ascertain whether a small, isolated area, or a large open area like a continent, has been most favorable for the production of new organic forms, we ought to make the comparison -within equal times ; and this wc are incajiahlo of doing. Although isolation is of great importance in the production of new species, on the Avhole I am inclined to believe that large- ness of area is still more important, especially for the produc- tion of species which shall prove capable of enduring for a long period, and of spreading Avidcly. Throughout a great and open area, not only Avill there be a better chance of favorable varia- tions arising from the large number of individuals of the same species there supported, but the conditions of life are much more complex from the large number of already existing spe- cies ; and if some of these many species become modified and improved, others will have to be improved in a corresponding degree, or they will be exterminated. Each new form, also, as soon as it has been much improved, will be able to spread over the open and continuous area, and will thus come into compe- tition with many others. Moreover, great areas, though now continuous, owing to former oscillations of level, will often have existed in a broken condition, so that the good effects of isolation will generally, to a certain extent, have concuiTed. Finally, I conclude that, although small isolated areas probably have beeii in some respects highly favorable for the production of new species, yet that the course of modification will gener- ally have been more rapid on large areas ; and, what is more important, that the new forms j)roduced on large areas, which already have been victorious over many competitors, will be tliose that will s])rea(l most widely, will give rise to most new varit^ties and species, and will thus play the most important part in the changing history of the organic Avorld. We can, perhaps, on these Aiews, imderstand some facts wliich will be again alluded to in our chapter on Geographical Distribution ; for instance, that the ]iroductions of the smaller continent of Australia are now yielding before those of the larger Europa'o- Asiatic area. Thus, also, it is that continental pniductions have everywhere become so largely naturalized on islands. On a small island, the race for life will have been less severe, and there will have been less modification and less ex- Chap. IV. KESULTS OF NATURAL SELECTION. iQ>j termination. Ilcncc, perhaps, it is liiat the flora of Madeira, accordin<^ to Oswald Heer, resembles the extinct tertiary flora of Europe. ^Vll fresh-water basins, taken together, make a small area compared with that of the sea or of the land ; and, consequently, the competition between fresh-water productions will have been less severe than elsewhere ; new forms will have been more slowly formed, and old forms more slowly extermi- nated. And it is in fresh water that Ave find seven genera of Ganoid fishes, remnants of a once preponderant order : and in ii?sh water we find some of the most anomalous forms now known in the world, as the Ornithorhynchus and Lepidosiren, which, like fossils, connect to a certain extent orders at present widely separated in the natural scale. These anomalous forms may be called living fossils ; they have endured to the present day, from having inhabited a confined area, and having been exposed to less varied and therefore less severe competition. To sum up the circumstances favorable and unfavorable for the production of new species through natural selection, as far as the extreme intricacy of the svibject permits. I conclude that fur terrestrial productions a large continental area, which has imdergone many oscillations of level, will have been the most favorable for the production of many ncAV forms of life, fitted to endure for a long time and to spread widely. While the area existed as a continent, the inhabitants will have been numerous in individuals and kinds, and Avill have been subjected to severe competition. When converted by subsidence into large separate islands, there will still have existed many indi- viduals of the same species on each island: intercrossing on the confines of the range of each new species will have been checked : after physical changes of any kind, immigration will have been prevented, so that new places in the polity of each island will have had to be filled up by modifications of the old inhabitants; and time will have been allowed for the varieties in each to become well modified and perfected. When, by renewed elevation, the islands were reconverted into a conti- nental area, there will again have been severe competition : the most faxorcd or improved varieties will have been enabled to spread : there will have been much extinction of the less improved forms, and the relative proportional ninnbcrs of the various inhabitants of tin; reunited continent will again have been changed ; and, again, there will have been a fair lield for natural selection to improve still further the inhabitants, and thus to produce new species. 108 EXTIXCTIOX BY NATURAL SELECTION. CiiAP. IV. That natural sc^lcction acts "vvith extreme slowness I fully iidniit. The result depends on there beings places in the polity of Nature, wliich can be better filled through some of the in- habitants of the country undergoinf^ modifications of some kind. The existence of such places will often depend on physical changes, which are generally very slow, and on the immigra- tion of better-adapted forms being checked. But the effects of natural selection will probably still oftener depend on some few of the inhabitants becoming slowly modified ; the mutual relations of the other inhabitants being thus disturbed. Al- though all the individuals of the same species differ more or less from each other, differences of the right nature, better adapted to the then existing conditions, may not soon occur. The results will often be greatly retarded by free intercrossing. Many will exclaim that these several causes are amply suffi- cient to neutralize the power of natural selection. I do not believe so. But I do believe that natural selection generally acts very slowly in effecting changes, at long intervals of time, and only on a few of the inhabitants of the same region. I further believe that these slow, intermittent results of natural selection accord perfectly with what geology tells us of the rate and manner at which the inhabitants of the world have changed. Slow though the process of selection may be, if feeble man can do much by artificial selection, I can see no limit to the amount of change, to the beauty and infinite complexity of the coadaptations between all organic beings, one with another and with their physical conditions of life, "which may be effected in the long course of time by Nature's power of selection, or the survival of the fittest. Extinction caused by Natural Selection, This subject will be more fully discussed in our chapter on Geology ; but it must be here alluded to from being inti- mately connected with natural selection. Natural selection acts solely through the preservation of variations in some way advantageous, which consequently endure. Owing to the high geometrical ratio of increase of all organic beings, each area is already stocked with the full number of its existing inhabit- ants, and as most areas are already stocked with a great diver sity of forms, it follows that, as each selected and favored form increases in number, so generally will the less favored forms CriAP. IV. EXTINCTION BY NATURAL SELECTION. 109 decrease and l^eooinc rare. Rarity, as geoloy few individuals will, during fluctuations in the sea- sons or in the numlxir of its enemies, run a good chance of utter extinction. But we may go further than this ; for, as new forms are continually and slowly being produced, unless we believe that tlie mnnber of specific forms goes on perpet- ually and almost indetinitely increasing, many inevitably must become extinct. That the number of speciiic forms has not indcHnitely increased, geology tells us plainly ; and we shall presently attempt to show why it is that the number of species throughout the world has not become immeasurably great. AW; have seen tliat the species which are most numerous in individuals have the best chance of producing favorable varia- tions within any given period. We have evidence of this, in the facts stated in the second chapter, showing that it is the common species which offer the greatest number of recorded varieties, or incipitnit species. Hence, rare species will be less quickly moditieil or improved within any given period, and they will consequently be beaten in the race for life by the modified descendants of the commoner species. From the sesevcral considerations I think it inevitably fol- lows, that, as new species in the course of time are formed through natural selection, others will become rarer and rarer, and finally extinct. The forms Avhich stand in closest compe- tition with those imderg^oing modification and improvement, will naturally suffer most. And we have seen in the chapter nn the Struggle for Existence that it is the most closely-allied forms — varieties of the same species, and species of the same genus or of related genera — which, from having nearly the same structure, constitution, and habits, generally come into tlie severest competition with each other. Consequently, eacli new variety or species, during the progress of its formation, will generally press hardest on its nearest kindred, and tcTid to exterminate them. We see tlie same process of extermination among our domesticated productions, through the selection of improved forms by man. Many curious instances could l)e given showing how quickly new breeds of cattle, sheep, and other animals, and varieties of flowers, take the place of older and inferior kinds. In Yorkshire, it is historically known that the ancient black cattle were displaced by the long-horns, and that these " were swept away by the short-horns " (I quote the words of an agricultural Avriter) "as if by some murderous pes- tilence." 110 DIVERGENCE OF CHARACTER. Chap. IV. Divergence of Character, The principle, which I liavc dcsip^nated by this term, is of high importance, and cxphiins, as I believe, several important facts, in tlie first place, varieties, even strongly-marked ones, though having somewhat of the character of species — as is shown by the hopeless doubts in many cases how to rank them — yet certainly differ from each other far less than do good and distinct species. Nevertheless, according to my view, varieties are species in the process of formation, or are, as I have called them, incipient species. How, then, does the lesser difference between varieties become augmented into the greater difference between species ? That this does habitually happen, we must infer from most of the innumerable species throughout Nature presenting well-marked differences ; where- as varieties, the supposed protot}-pes and parents of future well-marked species, jiresent slight and ill-defined differences. Mere chance, as we may call it, might cause one variety to differ in some character from its parents, and the offspring of this variety again to differ from its parent in the very same character and in a greater degree ; but this alone would never account for so habitual and large a degree of difference as that between the species of the same genus. As has always been my practice, I have sought light on this head from our domestic productions. We shall here find something analogous. It will be admitted that the produc- tion of races so different as short-horn and Hereford cattle, race and cart horses, the several breeds of pigeons, etc., could never have been effected by the mere chance accumulation of varia- tions of a similar character during many successive generations. In ])ractice, a fancier is, for instance, struck by a pigeon having a sliglitly shorter beak ; another fancier is struck by a pigeon having a rather longer beak ; and, on the acknowledged prin- ciple that " fanciers do not and will not admire a medium stand- ard, but like extremes," they both go on (as has actually oc- curred with the sub-breeds of the tumbler-pigeon) choosing and l)rceding from birds with longer and longer beaks, or with shorter and shorter beaks. Again, we may suppose that at an early period one man preferred swifter horses ; another stronger and more bulky horses. The early differences would be very Blight; in the course of time, from the continued selection of swifter horses by some breeders, and of stronger ones by others, the differences would become greater, and would be noted as rnxr. IV. DIVERGENCE or CHARACTER. HI Ibnnin;^ two sub-breeds; finally, after the lapse of centuries the sub-breods would become converted into two ■well-estab- lished and distinct breeds. As the dilVerenccs slowly became ,e:reater, the inferior animals with intermediate characters, bein;i^ neither very swift nor very strong, would have been neglected, and will have disappeared. Here, then, we see in man's pro- ductions the action of what may be called the principle of divergence, causing differences, at first barely appreciable, steadily to increase, and the breeds to diverge in character both from eacli other and from their common pax-ent. But 1»()W, it may be asked, can any analogous principle apply in Nature ? t believe it can and does apply most effi- ciently (though it Avas a long time before I saw how), from tlie simple circumstance that the more diversified the descendants from any one species become in structure, constitution, and habrts, by so much will they be better enabled to seize on many and widely-diversified places in the polity of Nature, and so be enabled to increase in numbers. We can clearly discern this in the case of animals with simple habits. Take the case of a carnivorous quadruped, of which the number that can be supported in an}?- country has long ago arrived at its full average. If its natural powers of increase be allowed to act, it can succeed in increasing (the country not imdergoing any change in its conditions) only by its varying descendants seizing on places at present occupied by other animals: some of them, for instance, being enabled to feed on new kinds of prey, either dead or alive ; some inhabit- ing new stations, climbing trees, frequenting water, and some [lerliaps becoming less carnivorous. The more diversified in lialjits and structure the descendants of our carnivorous animal lir'came, the more places they would bo enabled to occupy. AViiat applies to one animal will apply throughout all time to all animals — that is, if they vary — for otherwise natural selec- tion can effect nothing. So it will be witli j^lants. It has been experimentally proved that, if a plot of ground be sown witli one species of grass, and a similar plot be sown with several distinct genera of grasses, a greater number of phmts and a greater weight of dry herbage can be raised by the latter jjro- ccss. Tlie same has been found to hold good when one va- riety and several mixed varieties of wlieat liave been sown on equal spaces of ground. Hence, if any one species of grass were to go on varying, and those varieties were continually selected which differed from each other in at all the same man- 1 ] 2 DIVERGENCE OF CHARACTER. Chap. IV, lUT as distiiKl species and gwiora of grasses differ from each other, a greater miinber of individual phmts of this species of grass, inchiding its modified descendants, ■would succeed in living on the same piece of ground. And we well know that each species and each variety of grass is aimually sowing al- most countless seeds : and thus, as it may be said, is striving its utmost to increase its numbers. Consequently, in the course of many thousand generations, the most distinct varieties of any one species of gTass would always have the best chance of succeeding and of increasing in numbers, and thus of supplant- ing the less distinct varieties ; and varieties, when rendered very distinct from each other, take the rank of species. The truth of the principle that the greatest amount of life can be supported by great diversification of structure, is seen iinder many natural circumstances. In an extremely small area, especially if freely open to immigration, and where the contest between indivitkial and individual must be severe, we always find groat diversity in its inhabitants. For instance, I found that a piece of turf, three feet by four in size, which had been exposed for many years to exactly the same conditions, supported twenty species of plants, and these belonged to eighteen genera and to eight orders, which shoAvs how much these plants differ from each other. So it is with tlie plants and insects on small and imiform islets ; also in small ponds of fresh water. Farmers find that they can raise most food by a rotation of plants belonging to the most different orders : Nature follows what may be called a simultaneous rotation. Most of the animals and plants which live close round any small piece of ground, could live on it (supposing it not to be in any way peculiar in its nature), and may be said to be striving to the utmost to live there ; but, it is seen, that where tliey come into the closest competition with each other, the advantages of diversification of structure, with the accom])any- ing difVerenccs of habit and constitution, determine that the inhabitants, which thus jostle each other most closely, shall, as a general rule, belong to what we call different genera and orders. The same principle is seen in the naturalization of plants through man's agency in foreign lands. It might have been expected that the plants which would succeed in becoming naturalized in any land would generally have been closely aUied to tlie indigenes ; for these are commonly looked at as specially created and adapted for their own country. It might Chap. IV. DIVERGENCE OF CnARACTER. 113 also, pcrliaps, liave been expected that naturalized plants would have beloufred to a few groups more especially adapted to certain stations in their new homes. But the case is very dif- ferent ; and Alpli. dc Candolle has well remarked, in his great and admirable work, that floras gain by naturalization, propor- tionally with the number of the native genera and species, far more in new genera than in new species. To give a single instance : in tiie last edition of Dr. Asa Gray's " Manual of the Flora of the Northern United States," 260 naturalized plants are enumerated, and these belong to 162 genera. Wc thus see that these naturalized plants are of a highly-diversified nature. The}'" difler, moreover, to a large extent, from the in- digenes, for, out of the 162 naturalized genera, no less than 100 genera arc not there indigenous, and thus a large proportional addition is made to the genera now living in the United States. By considering tlie nature of the plants or animals which have struggled successfully with the indigenes of any country, and have there become naturalized, we may gain some crude idea in what manner some of the natives would have to be modilied, in order to gain an advantage over the other natives ; and we may at least safely infer that diversification of struc- ture, amounting to new generic differences, would be profit- able to them. The advantage of diversification in the inhabitants of the same region is, in fact, the same as that of the physiological division of labor in the organs of the same individual body — a subject so well elucidated by Milne Edwards. Xo physiolo- gist doubts that a stomach adapted to digest vegetable matter alone, or llesh alone, draws most nutriment from these sub- stances. So in the general economy of an}'' land, the more widely and perfectly the animals and plants are diversified for dilferent haljits of life, so will a greater number of inchviduals be capable of there sujiporting themselves. A set of animals, with their organization but little diversified, coulti hardly com- pete with a set more perfectly diversified in structure. It may be doubt(M], for instance, whether the Australian marsupials, which are divided into groups differing but little from each other, and feebly representing, as Mr, AVaterhouse antl others have remarked, our carnivorous, ruminant, and rodent mam- mals, could successfully compete with these well-pronounced orders. In the Australian manmials, we see the process of diversification in an early and incomplete stage of develop- ment. I ] t RESULTS OF THE ACTION OF Chap. IV, Tlie Pi'ohdhle Results of the Action of Katural Selection through Diverrjence of Character and JSxtinctiony vi the Descendants of a Common Ancestor. After tlie forcg'oing discussion, which has been much com- pressed, we may assume that the modified descendants of any one species Avill succeed by so much the better as they become more diversified in structure, and are thus enabled to encroach on places occupied by other beings. Now let us see how this principle of benefit being derived from divergence of character, combined with the principles of i>atural selection and of ex- tinction, tends to act. The accompanying diagram will aid us in understanding this rather perplexing subject. Let A to L represent the spe- cies of a genus large in its own country ; these species are supposed to resemble each other in unequal degrees, as is so generally the case in Nature, and as is represented in the cliagram by the letters standing at unequal distances. I have said a large genus, because Ave have seen in the second chap- ter that, on an average, more of the species of large genera vary than of small genera ; and the varying species of the large genera present a greater number of varieties. We have, also, seen that the species, which are the commonest and the most widely-diilused, vary more than do the rare and restricted species. Let (A) be a common, widely-diffused, and varying species, belonging to a genus large in its own country. The branching and diverging dotted lines of unequal lengths pro- ceeding from (A), may represent its varying offspring. The variations are supposed to be extremely slight, but of the most diversified nature ; they are not supposed all to appear simul- taneously, but often after long intervals of time ; nor are they all supposed to endure for equal periods. Only those varia- tions which are in some Avay profitable will be preserved or naturally selected. And here the importance of the principle of benefit being derived from divergence of character comes in ; for this will generally lead to the most different or diver- gent variations (represented by the outer dotted lines) being preserved and accumulated by natural selection. When a dot- ted line reaches one of the horizontal lines, and is there marked by a small numbered letter, a sufficient ainount of variation is supjiosed to have been accunuilated to have formed a fairly well-marked variety, such as would l)e thought worthy of rec- ord in a systematic work. -xLxrv' -A— xm '. XII XI a" V VLj"" ']:*«/" i 1 9\ ^- -#^ DC iy h' !• "'".■ .• a' ■■. f '.U' ■■::W '^ ^, i ; u' ■ ,. '. ;.' a"i v/" M*'^ \,.^« ^^e . ■ ■ ^ a-' it'^i'^ ■.]:■:. ■?« •^ "' 1 / -J. l'.' id* ■■* \ ■■ * ■■' ''iA'' =* j;: .' \;,'-^, ■.;,; J ; 1 , 1 l'- ■ Ls m \ i^W :;■.' TT <':,■ 2 1 ■ ^ T ABCD EF GHIKL /Jjippleton & C NcwYofk iBl*JIf^^l>tltl«]ll!n)nil»7.^-, Chap. IV. NATURAL SELECTION. 1 ] 5 The intervals between the horizontal lines in the dia^ara, may ix'prescnt each a thousand generations, or ten thousand. After a thousand generations, species (A) is supposed to have produced two fairly well-marked varieties, namely a} and rn). These two varieties will generally continue to be exposed to the same conditions Avhich made their parents variable, and the tendency to variability is in itself hereditary, consequently tliey will tend to vary, and generally to vary in nearl}' the same manner as their j)arents varied. Moreover, these two varieties, being only slightl^'-niodilied forms, will tend to in- herit those advantages which made their parent (A) more numerous than most of the other inhabitants of the same coun- try ; they will likewise partake of those more general advan- tages v/hich made the genus to which the parent-species be- longed a large genus in its ov,n country. And these circinn- stanccs we know to bo favoral)le to tiic production of new varieties. If, then, these two varieties be variable, the most divergent of their variations will generally be ])reserved during the next thousand generations. And after this interval, variety 0} is supposed in the diagram to have pi'oduccd variety a^, which will, owing to the principle of divergence, difler more from (A) than did variety a*. Variety «^' is supposed to have produced two varieties, namely Wi" and s", diflering from each other, and more considerably from their common parent (A). Wc may continue the process by similar steps for any length of time ; some of the varieties, after each thousand generations, produ- cing only a single variety, but in a more and more modi lied con- dition, some producing two or three varieties, and some failing to produce any. Thus the varieties, or modified descendants, proceeding from the common ])arent (A), will generally go on increasing in numljcr, and diverging in character. In the dia- gram tlie process is represented up to the ten-thousandth gen- eration, and under a condensed and siniplilled form up to the fourteen-thousandth generation, IJut I must hen; remark that I do not suppose that the process ever goes on so regularly as is represented in the dia- gram, though in itself made somewhat irregular, nor that it goes on continuously ; it is far more ])robable that each form remains for long periods unaltered, and tlien again imdcr- gocs modification. Nor do I suppose that the most di- vergent varieties are invariably preserved: a medium form may often long endure, and may or may not produce more IIG RESULTS OF THE ACTION OF C'kap. TV. than one modified descendant; for nalural selection Avill al- ways act according to the nature of the places which arc either unoccupied or not perfectly occupied by other beings ; and this will depend on infinitely-complex relations. But, as a general ruh>, the more diversified in structure tlic descendants from any one species can be rendered, the more places they will be enabled to seize on, and the more their modified pro- geny will increase. In our diagram the line of succession is liroken at regular intervals by small numbered letters marking the succ(>ssivc forms which have become sufficiently distinct to be recorded as varieties. But these breaks are imaginary, and might have been inserted an^^vhere, after intervals long enough to have allowed the accumulation of a considerable amount of divergent variation. As all the modified descendants from a common and wide- ly-diffused species, belonging to a large genus, Avill tend to partake of the same advantages which made their parent suc- cessful in life, they will generally go on multiplying in number as well as diverging in character: this is represented in the diagram by the several divergent branches proceeding from (A). The modified offspring from the latter and more highly- improved branches in the lines of descent, will, it is probable, often take the place of, and so destroy, the earlier and less im- j)i(ived branches : this is represented in the diagram by some of the lower branches not reaching to the upper horizontal lines. In some cases I do not doubt that the jirocess of modi- fication will be confined to a single line of descent, and the ntunber of the descendants Mill not be increased; althouglithe amount of divergent modification may have been increased in the successive generations. This case would be represented in tlic diagram, if all the lines proceeding from (A) Avere removed, excepting that from «* to rt'°. In the same way, for instance, the English race-horse and English pointer have apparently both gone on slowly diverging in character from their original stocks, Mithout either having given oft' any fresh branches or races. After ten thousand generations, species (A) is sujiposed to liave produced three forms, «",/'", and wi", which, from hav- ing diverged in character during the successive generations, will have come to differ largely, but jicrhaps unequally, from each other and from their common jvarcnt. If we suppose the amount of change between each horizontid line in our diagram to be excessively small, these three forms may still be only w('ll-mark,Ml varieties ; but we have only to suppose the steps Chap. IV. NATURAL SELECTION. 117 ill tiie process of modification to be more numerous or j^rcatcr in amount, to convert these three forms into doubtful or at last wcU-denncd species : thus the diao^ram iUustratcs the steps by wliich the small diflerences distinguishing varieties are increased into the larger differences distinguishing species. By continuing the same process for a greater number of gener- ations (as shown in the diagram in a condensed and simplified manner), we get eight species, marked by the letters between «" and m^\ all descended from (A). Thus, as I believe, spe- cies are multiplied and genera are formed. In a large genus it is probable that more than one species would vary. In the diagram I have assumed that a second species (I) has produced, by analogous steps, after ten thou- sand generations, either two well-marked varieties (w'° and z'") or two species, according to the amount of change supposed to be'rcjircsented between the horizontal lines. After fourteen thousand generations, six new species, marked by the letters w'* to z'*, are supposed to have been produced. In any genus, the species which are already very different in character from each other will generally tend to produce the greatest num- ber of modified descendants ; for they will have the best chance of filling new and widely-different places in the polity of Nature : hence in the diagram I have chosen the extreme species (A), and the nearly extreme species (I), as those which have largely varied, and have given rise to new varieties and species. The other nine species (marked by capital letters) of our original genus, may for long but unequal periods con- tinue to transmit unaltered descendants ; -and this is shown in tlu; diagram by the dotted lines unequally prolonged upward. But during the process of modification, represented in the diagram, another of our principles, namely, that of extinction, will have played an important part. As in each fully-stocked country natural selecti(jn necessarily acts by the selected form having some advantage in the struggle for life over other forms, there will be a constant tondcMicy in the improved descendants of any one species to supplant and exterminate in each stage of descent their pred(X"essors and their original progenitor. For it should be remembered that the competition will gen- erally be most severe between those forms which are most nearly related to each other in habits, constitution, and struc- ture. Hence all the intermediate forms between the earlier and later states, that is, between the less and more improved states of a species, as well as the original parent-species itself. lis RESULTS OF THE ACTION OF Chap. IV. will generally tend to l^ecome extinct. So it probably Avill be ^vith many -whole collateral lines of descent, Avhich ^vill be conquered by later and improved lines of descent. If, liow- ever, the modified offspring of a species get into some distinct country, or become quickly adapted to some cjuite new station, in whic-h offspring and progenitor do not come into compe- tition, both may continue to exist. If, then, our diagram be assumed to represent a consider- able amount of modification, species (A) and all the earlier va- rieties Avill have become extinct, having" been replaced by eight new species (a" to wi") ; and (I) will have been replaced by six (?i" to 2'*) new species. But we may go further than this. The original species of our genus were supposed to resemble eacli other in unequal degrees, as is so generally the case in Nature ; species (A) being more nearly related to B, C, and D, than to the other species ; and species (I) more to G, H, K, L, than to the others. These two species (A) and (I) were also supposed to be very common and widely-dilfuscd species, so that they must originally have had some advantage over most of the other species of the genus. Their modified descendants, fourteen in number at the fourteen-thousandth generation, will probably have inherited some of the same advantages : they have also been modified and improved in a diversified manner at each stage of descent, so as to have become adapted to many related places .in the natural economy of their country. It seems, therefore, extremely probable that they will have taken the places of, and thus exterminatod, not only their parents (A) and (I), but likewise some of the original species which were most nearly related to their parents. Hence very few of the original species will have transmitted offspring to the fourteen- tliousandth generation. AVe may suppose that only one (F), of the two species (E and F) which were least closely related to the other nine original species, has transmitted descendants to this late stage of descent. The new species in our diagram, descended from the origi- nal eleven species, will now be lifteen in number. Owing to the divergent tendency of natural selection, the extreme amoimt of difference in character betAveen species «'* and 2'* will be much greater than that between the most distinct of the original eleven species. The new species, moreover, wiK be allied to each other in a Avidely-differcnt manner. Of the eight descendants from (A) the three marked a", ", those marked i'* and/"'\ and those marked o'* to ni^*, will form three very distinct genera. We shall also have two very distinct genera descended from (1), differing widely from the descendcnts of (A). These two groups of genera will thus form two distinct families, or orders, accord- ing to the amount of divergent modification supposed to be represented in the diagram. And the two new families, or orders, are descended from two species of the original genus, and these are suj^posed to be descended from some still more ancient and unknown form. We have seen that in each countrj^ it is the species belong- ing to the larger genera which oftcnest present varieties or in- cipient species. This, indeed, might have been expected ; for as natural selection acts throvigh one form having some advan- tage over other forms in the struggle for existence, it will chiefly act on those which already have some advantage ; and the largeness of any grouji shows that its species have inherited from a connnon ancestor some advantage in common. Hence, the struggle for the production of new and modified descend- ants will mainly lie between the larger groups which are all trying to increase in number. One large grouj) will slowl}' ct)nfjuer another large group, reduce its numbers, and thus les- sen its chance of further variation and improveuKMit. Within the same large group, the later and more highly -perfected su]> Caw. IV. ADVANCE OF ORGANIZATION. 121 j^roups, from hraneliiiig" out and seizinc^ on many new plar-cs in the polity of Nature, will constantly tend to sujiplant and de- stroy the earlier and Icss-iinp.roved svib-<^oups. Small and broken ""roups and sul>irroups will hnally disappear, Lookinnf to the future, Ave can ])redict that the groups of organic beings which are now large and triumphant, and Avbich.are least broken u]i, that is, which as yet have sulTered least extinction, will for a long period continue to increase. But which groups will ultimately prevail, no man can predict ; for we well know that many groups formerly most extensively develojied, have now become extinct. Looking still more remotely to the future, we may predict that, owing to the continued and steady increase of the larger groups, a multitude of smaller groups will become utterly extinct, and leave no modified descendants; and consequently that, of the species living at any one period, extremely (gw will transmit descendants to a remote futurity. I shall have to return to this subject in the chapter on Classifi- cation, but I may add that as according to this view extremely few of the more ancient species have transmitted descendants, and as :ill the descendants of the same species form a class, we can imdcrstand how it is that there exist so few classes in each main division of the animal and vegetable kingdoms. Although few of the most ancient species have left motlified descendants, yet, at remote geological periods, the earth may have been almost as well jjcopled with species of many genera, families, oiders, and classes, as at the; present day. O71 the Degree to which Organization tends to advance. Natural Selection exclusively acts by the preservation and accunmlation of variations, which are beneficial under the or- ganic and inorganic conditions of life to which each creature is exposed at each successive period of life. The ultimate result is, that each creature tends to become more and more improved in relation to its conditions. This improvement inevitably leads to the gradual advancement of the organization of the greater number of living bi'ings throughout the world. But here we enter on a very intricate subject, for naturalists have not defined to each other's satisfaction what is meant by an advance in org-anization. Among the vertcbrata the degree of intellect and approac-h in structure to man clearly come into play. It might be thought that the amount of change Avhich the various ])arts and organs pass through in their develop- G ['^2 vc arc not able to perceive their use ; and these will have been acted on by natural selection, A still larger number of mor}:)hological dif- ferences may certainly be looked at as the necessary result — through pressure, the withdrawal or excess of nutriment, an early-formed jiart affecting a part subsequently developed, cor- relation, etc. — of other adaptive changes, through wliich all species must have j)assed during their long course of descent and modilication. No one will maintain that we as yet know the uses of all the pai-ts of any one plant, or the functions of each cell in any one organ. Five or six years ago, endless peculiarities of structure in the flowers of orchids, great ridges and crests, and the relative positions of the various parts AVould have been con sidered as useless moiphological difl'erences ; but now we know that they are of great service, and must have been under the dominion of natural selection. No one at present can explain Avhy the leaves in a spire diverge from each other at certain angles ; but we can see that their arrangement is related to their standing at equal distances from the leaves on all sides ; and we may reasonably expect that the angles will hereafter be shown to follow from some such cause, as the addition of new leaves to the crowded spire in the bud, as inevitably as the angles of a bee"'s cell follow from the manner in which the insects work together. In certain Avhole groups of ])lants the ovules stand erect, and in others they are suspended ; and in some few plants within the same ovarium one ovule holds the former and a sec- ond ovule the latter position. These positions seem at first purely moi-jihological and of no physiological signification ; but Dr. Hooker informs me that, of the ovules within the same ovarium, in some cases the upper ones alone and in other cases the lower ones alone are fertilized ; and he suggests that this probably depcn^ls on the direction in which the pollen-tubes enter. If so, the position of the ovules, even when one is erect and the other suspended, would follow from the selection of any slight deviation in position which might favor their fertilization and the production of seed. Several plants Ix'longing to distinct orders habitually pro- duce flowers of two kinds — the one open and of the orthnary structure, the other closed and impM'fect. In the latter the l)etals are almost alwaj-s reductxl to the merest rudiments ; the pollen-grains are reduced in diameter ; five of the alternate CnAP. IV. VARIOUS OBJECTIONS. log stamens are ruilimcntury in Ononis columnas ; and in some spe- (•i(^s of Viola three stamens arc in this state, two retaining their proper fimction, but being' of very small size. In six out of lliirtv of tlie closed flowers in an Indian violet (name unknown, for the plants have not as yet jiroduced perfect flowers), the sepals were reduced from the normal lunnber of live to three. In one section of the Malpighiacea3 the closed flowers, according to A. de Jussieu, are still further modified, for the five stamens which stand opposite to tlie sepals are all aborted, a sixth stamen standhig opposite to a j)etal being alone developed; and this stamen is not present in the ordinary flowers of these species ; the style is aborted ; and the ovaria are reduced froni three to two. In all the foregoing plants the minute closed flowers are of high service, for they yield with perfect seciu-ity, !Uid with the expenditure of extremely little pollen, or other organized matter, a large supply of seed ; while the perfect flowers permit occasional crosses with distinct individuals. Therefore, these changes may have been, and no doubt have been, efTi.'cted through natural selection ; and I may add that nearly all the gradations between the perfect and imperfect flowers may sometimes be observed on the same plant. With respect to modifications which necessarily follow from other cljangcs — through the withdrawal or excess of nutriment — througli pressiu-e and other unknown influences — there is space here only for a few brief illustrations. In the Spanish chestnut, and in certain fir-trees, the angles of divergence of the leaves differ, according to Schacht, in the nearly horizontal and in the upright branches. In the common rue and some other plants, one flower, usually the central or terminal one, ojicns first, and has five sepals and petals, and five divisions to the ovarium ; while all the other flowers on the plant are te- tramerous. In the British Adoxa the uppermost flower gener- ally has two cjilyx-lobes with the other organs tetramerous, while the surrounding flowers generally have three cal^'x-lobes with the other organs pentamerous ; and this difference appears to follow from the manner in which the flowers are closely ]> icked together. In many Compositic and Umbellifera?, and in some other plants, the circumferential flowers have their corollas much more developed tlian those of the centre; and this is probably the result of natural selection, for all the flowers are thus rendered nuich more conspicuous to those in- sects which are useful or even neccs.sary for their fertilization. In connection with the greater development of the corolla, the 130 VARIOUS OBJECTIONS. Chap. IV. reproductive organs are frequently more or less aborted. It is a more curious fact that the achenes or seeds of the circum- ference and of the centre sometimes differ greatly in form, color, and other characters. In Carthamus and some othoi Composita^ the central achenes alone are furnished with a pap- pus ; and in llyoseris the same head yields achenes of three different forms. In certain Umbcllifeni:; the exterior seeds, ac- cording to Tausch, are orthospermous, and the central one coelospermous, and this difference has been considered by De Candolle as of the highest systematic importance in the family. If in such cases as the foregoing all the leaves, flowers, fruits, etc., on the same plant had been subjected to precisely the same external and internal conditions, all no doubt would have presented the same morjohological characters ; and there clearly woiild have been no need to call in the aid of the principle of progressive development. With the minute closed flowers, as well as ^^^tll many degraded parasitic animals, if it be assumed that any such aid is requisite, we sliould have to call in an in- nate tendency to retrogi-essive development. ]\Iany instances could be given of morphological characters varying greatly in plants of the same species growing close together, or even on the same individual plant ; and some of these characters are considered as systematically important. I will specify only a few cases Avhich have first occurred to me. It is not necessary to give instances of flowers on the same plant being indifferently tctramcrous, pentamcrous, etc. ; but as when the parts are few, numerical variations are in all cases comparatively rare, I may mention that, according to De Can- dolle, the flowers of Papavcr bracteatum offer two sepals with four petals (and this is the common type with jioj^pies), or three se}>als with six petals. The manner in which the petals are folded in the bud is in most groups a constant morpho- logical character ; but Prof. Asa Gray states that with some species of Mimulus, the aestivation is almost as frequently that of the Rhinanthidea3 as c>f the Antirrhinidea?, to which tribe the genus belongs. Aug. St.-Hilaire gives the following cases ; the genus Zanthoxylon belongs to a di\-ision of the Kutacea3 with a single ovary, but in some of the species flowers may bo found on the same plant, and even in the same panicle, with cither one or two ovaries. In Helianthennnn the capsule has been described as unilocular or 3-locular ; and in H. nuitabih^ " Une lame, plus ou moins large, sY'tend entre le pericarpe et Ic placenta." In the flowers of Saponaria ollicinalis, Dr. Mas- CiiAr. IV. VARIOUS OBJECTlOiNS. 131 tcrs also observed instances of botli mar<^inal and free central placentation. Lastl}', St.-Hilaire foinid toward the southern extreme of the ranfjc of Goniphia oletvforniis two forms which he did not at first doubt were distinct species, but he subse- quently saw them g-rowinp^ on the same bush ; and he then adds, " A'oihl done dans uu memo individu des lof^es et un style qui sc rattachent tantOt il un axe verticale et tantot ;l un gynobase." In the case of these plants, will it be said that they have been detected in the act of progTessinom thid, whereas the production of new spe- cies must alwa3'S be slow. Imac^inc the extreme case of as many species as individuals in England, and the first severe winter or A-ery dry summer would exterminate thousands on thousands of species. Kare species, and each species will be- come rare if the number of species in any country becomes indefinitely increased, will, on the principle often explained, j)resent within a given period few favorable variations; conse- <|uently, the process of giving birth to new specific forms will thus be retarded. When any species becomes very rare, close interbreeding will help to exterminate it ; authors have thought that this comes into play in accounting for the deterioration of the Aurochs in Lithuania, of Ked Deer in Scotland, and of liears in Norway, etc. Lastly, and this I am inclined to think is the most important element, a dominant species, Avhich has nlnMdy beaten many competitors in its own home, Avill tend to spread and supplant many others. Alph. de Candolle has shown that those species which spread widely tend generally to spread vcnj widely ; and, consequently, they will tend to sup- |)lMnt and exterminate several species in several areas, and thus check the inordinate increase of specific forms throughout the world. Dr. Hooker has recently shown that in the south- east corner of Australia, where, apparently, there are manv in- vaders from difTerent quarters of the world, the endemic Aus- tralian species have been greatly reduced in number. ITow nuich weight to attribute to these several considerations I do not jiretend to assign ; but ronjointly they must limit in each country the tendency to an indefinite augmentation of specific fonns. 134 SUMMARY. Chap. IV. Sum)) tart/ of Chcfpter. If iiiulcr changing conditions of life organic beings present individual difiercnces in all parts of their structure, and this cannot be disputed ; if there bo, owing to the high geometrical ratio of increase of each species, a severe struggle for life at some age, season, or year, and tliis certainly cannot be disputed ; then, considering the infinite complexity of the relations of all organic beings to each other and to their conditions of life, causing an infinite diversity in structure, constitution, and habits, to be advantageous to them, it would be a most extraor- dinary fact if no variations ever occurred useful to each be- ing's OAvn welfare, in the same inanner as so many variations have occurred useful to man. /But if variations useful to any organic being do ever occur, assuredly individuals thus charac- terized will have the best chance of being preserved in the struggle for life ; and from the strong principle of inheritance, they will tend to produce ofi'spring similarly characterized. This principle of preservation, or the survival of the fittest, I have called Natural Selection^ It leads to the improvement of each creature in relation to its organic and in organic condi- tions of life ; and consequent]}', in most cases, to what must be regarded as an advance in organization. Nevertheless, low and simple forms will long endure if well fitted for their simple conditions of life. Natural selection, on the principle of qualities being in- herited at corresponding ages, can modify the egg, seed, or young, as easily as the adult. Among many animals, sexual selection Avill give its aid to ordinary selection, by assuring to the most vigorous and best adapted males the greatest num- ber of offspring. Sexual selection will also give characters useful only to the males, in their struggles with other males ; and these characters Avill be transmitted to one sex or to both sexes, according to the form of inheritance which prevails. Whether natural selection has really thus acted in adapting the various forms of life to their several conditions and stations, must be judged of by the general tenor and balance of the evi- dence given in the following chapters. But we already see how it entails extinction ; and how largely extinction has acted in the world's history, geology plainly tleclarcs. Natural selec- tion, also, leads to divergence of character ; for the more organic beings diverge in structure, habits, and constitution, by so much can a greater number be supported on the same area — of which CiiAF. IV. SUMMARY. I35 we see proof by looking to the inhabitants of any small spot or (o naturalized productions. Therefore, durin> the modifica- tion of the descendants of anj- one species, and dm'in<>* the in- cessant stru^f^le of all species to increase in numbers, the more diversified these descendants become, tlic better will be their chance of succccdinrr in the battle for life. Thus, the small dill'erences distin<]^uishint^ varieties of the same species, steadily tend to increase, till they come to equal the greater diflfereuces between species of tlie same genus, or even of distinct genera. We have seen that it is the common, the widely-dilfused, and widely-ranging species, belonging to the larger genera within each class, which vary most; and these tend to transmit ((> their modified offspring that superiority which now makes them dominant in their own countries. Natural selection, as has just been remarked, leads to divergence of character and to much Extinction of the less improved and intermediate forms of life. On these principles, the nature of the affinities, and the generally AvoU-dellned distinctions of the innumerable organic beings in each class througliout the Avorld, may be explained. It is a truly wonderful fact — the wonder of M'hich we are apt to overlook from familiarity — that all animals and all plants tliroughout all time and space should be related to each other in natural groups subordinate to groups, in the manner which Ave everywhere behold — namely, varieties of the same species most closely related together ; species of the same genus less closely and unequally related together, forming sections and sub-genera ; species of distinct genera much less closely related, and genera related m difierent degrees, forming sub- families, families, orders, sub-classes, and classes. The several subordinate groups in any class cannot be ranked in a single lile, but seem rather to be clustered round points, and these round other points, and so on in almost endless cycles. If each species has been independently created, no explanation rm be given of this great fact in the classification of all organic beings; but it is explained through inheritance and tlie com- plex action of natural selection, entailing extinction and diver- gence of character, as we have seen illustrated in the diagram. The affinities of all the beings of the same class have some- limes been repres(Mited by a great tree. I believe this simile largely speaks (lie trulli. The green and budding twigs may represent existing species ; and those produced during former years may represent the long succession of extinct species. At each period of growth all the growing twigs have tried to 133 SUMMARY. CuAr. IV. brancli out on all siili^s, and to overtop and kill the surrounding t\vii>s and l)ranchcs, in the same manner as species and groups of speeies have at all times overmastered other species in the great battle for life. The limbs, di\'ided into great branches, and those into lesser and lesser branches, were themselves once, when the tree Avas small, budding twigs ; and this connec- tioTi of the former and present Inids by ramifying branches may well represent the classification of all extinct and living species in groups subordinate to groups. Of the many twigs which nourished when the tree was a mere bush, only two or three, now grown into great branches, yet survive and bear all the other branches ; so with the species which lived during long- ])ast geological periods, very few now have living and modified descendants. From the first growth of the tree, many a limb and branch has decayed and dropped off; and these lost branches of various sizes may represent those whole orders, families, and genera, which have now no living representatives, and whicli are known to us only from being found in a fossil state. As we here and there see a thin, straggling branch sj)ringing from a fork low down in a tree, and which by some chance has been favored and is still alive on its summit, so we occasionally see an animal like the Ornithorh^mchus or Lepi- dosiren, which in some small degree connects by its affinities two large branches of life, and which has apparently been saved from fatal competition by having inhabited a protected station. As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation I believe it has been Avith the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever-branch- inc: and beautiful ramifications. Cu.vr. V. LAWS OF VARIATION. IO7 ciiArxER y. LAWS OF VAUIATIOX. Effects of clmntrofl Coiuiillont'—T'iJo and Dis>nso, combined with Natural Selection; Orirtiiii (if Fliirlit and of Vision— Acclimatization— Correlated Variatiou— Com- pensation and Keonoiny of (imwtli— False C:orrclation» — Mnlti|)le, Uudinicntary, and l.iiwly-or^'anized Stnietuies< variable— Parts developed in an Unusual Man- ner are hij;ldy variable : ?>poeific Characters more variable ttwn Generic: Second- ary Sexual characters variable— Species of the t^anic Genus vary in au analoguua llauucr— Ueverbioua to long-lost Charactera — Sumaiary. . I HAVE hitherto sometimes spoken as if the variations — so comn'ion and multilorin with org'anic beings nnder domestication, and in a lesser degree with those in a state of nature — had l)een due to cliance. This, of course, is a wholly incorrect ex- pression, but it serves to acknowledge plainly our ignorance of the cause of each particular variation. Some authors be- lieve it to be as much the function of the reproductive system to produce individual difTerences, or slight deviations of struc- tuie, as to make the child like its parents. But the fact of variations and monstrosities occurring much more frequently under domestication than under Nature, and the greater varia- l>ility of species having wide ranges than of those having re- stricted ranges, lead to the conclusion that variability is directly related to the conditions of life to Avhich each species has been exposed during several successive generations. In the first cliapter I attempt(>d to show that changed conditions act in two ways, directly on the whole organization or on certain ])ar(s alone, jind indirectly through the reproductive sj'stcm. Jn all cases th(M-e ate two factors, the nature of the organism, which is nuich the most important of the two, and the nature of the conditions. The direct acticm of changed conditions, leads to (h'Tmite or indefinite results. In the latter case the organization seems to become plastic, and we have nnich fluc- tuating variability. In the former case the nature of the or- ganism is such that it yields readily, when subjected to cer- lain conditions, and all, or nearly all the individuals become modified in the same way. l.iS LAWS OF VARIATION. Chap. V. It is very difficult to decide how far clianii^cd conditions, such as of climate, food, etc., have acted in a definite manner. There is some reason to believe that in the course of time the effects have been greater than can be proved to be the case by any clear evidence. But we may safely conclude that the in- niunerable complex coadaptations of structure which we see throughout Nature between various organic beings, cannot be attriljuted simply to such action. In the following cases the conditions seem to have produced some slight definite effect : E. Forbes asserts that shells at their southern limit, and when living in shallow water, are more brightly colored than those of the same species from farther north or from a greater depth; but these statements have lately been disputed, Mr. Gould believes that birds of the same species are more brightly col- ored under a clear atmosjihere, than when living on islands or near the coast ; and WoUaston is convinced that residence near tlie sea affects the colors of insects. Moquin-Tandon gives a li.st of plants which when growing near the sea-shore have their leaves in some degree ileshy, though not elsewhere fleshj'. Other similar facts could be given. The fact of varieties of one species, when they range into the habitations of other species, often acquiring in a slight degree some of their characters, accords Avith the view that species arc only well-marked and permanent varieties. Thus the species of shells Avhich are confined to tropical and shal- low seas are generally brighter-colored than those confined to cold and deeper seas. The birds Avhich are confined to conti- nents are, according to Mr. Gould, brighter-colored than those of islands. The insect-species confined to sea-coasts, as every collector knows, are often brassy or lurid. Plants which live exclusively on the sea-side are very apt to have Ileshy leaves. He who believes in the creation of each species, will have to say that this insect, for instance, was created of a brassy color, because it was intended to live near the sea, but that this other insect became brassy through variation as soon as it reached the sea-coast. When a variation is of the slightest use to any being, we cannot tell how much to attribute to the accumulative action of natural selection, and how much to the definite action of the conditions of life. Tlius, it is avcU known to furriers that ani- mals of the same species have thicker and better fur tlie far- tlier north they live ; but who can tell liow much of this dif- ference may be due to the warmest-clad individuals having CiiAi-. V. EFFECTS OF USE AND DISUSE. in<) 1)ftM» favored and preserved during many generations, and how much lo the action of tlie severe climate ? for it would appear that climate has some direct action on the hair of our domestic (juachupeds. Instances could be given of similar varieties being produced from the same species under external conditions of life as dif- ferent as can well be conceived ; and, on the other hand, of dissimilar varieties being produced imder apparently the same external conditions. Again, innumerable instances are known to ev(;ry naturalist, of species keeping true, or not varying at all, although living und<'r the most opposite climates. Such considerations as these incline me not to lay much weight on the direct and definite action of the conditions of life; but I fully admit that strong arguments of a general nature may be advanced on the other side. In one sense the conditions of life may be said, not only to cause variability, but likewise to include natural selection ; for the conditions determine whether this or that variety shall sur- yi\i.\ ]3ut when man is the selecting agent, we clearly see that the two elements of change are distinct ; the conditions cause the variability; the will of man, acting either consciously or unconsciously, accumulates the variations in certain direc- tions, and tills answers to the survival of the fittest under nature. J^Jfects of Use and Disrise, as controlled hy Natural Selection. From the facts alluded to in the first chapter, I think there can be no doubt that use in our domestic animals strengthens and enlarges certain parts, and disuse diminishes them ; and that such modifications are inherited. Under free Nature, we liave no standard of comparison, by which to judge of the elfecls of long-continued use or disuse, for "we know not the parent-fonns; but many animals have structures which- can be (•xi)lain<'d by the efl'ects of disuse. As Prof. Owen has remarked, llierc is no greater anomaly in Nature than a bird that cannot l!y ; yet there are several in this state. The logger-headed duck of South America can only llap along the surface of the water, and has its wings in nearly the same condition as the domestic Aylesbury duck. As the larger ground-feeding birds seldom tak(> fiight except to escap<> danger, I believe that the nearly wingless condition of several birds, which now inhabit or have lately inhabited several oceanic islands, tenanted by no beast 140 EFFECTS OF USE AND DISL'SE. Chap. V. of prey, lias been caused by disuse. The ostricli, indeed, in- Imbils continents and is exposed to dann^er from -wliicli it cannot escape by llii^^lit, but by kickinp^ it can defend itself from ene- mies, as well as any of the smaller ciuadrupeds, "We may be- lieve that the progenitor of the ostrich fj^enns had habits like those of a bustard, and that, as natural selection increased in successive g-enerations the size and weight of its body, its legs were used more, and its wings less, until they became incapa- ble of flight. Kirby has remarked (and I have observed the same fact) that the anterior tarsi, or feet, of many male dung-feeding beetles are very often broken off ; he examined seventeen spe- cimens in his own collection, and not one had even a relic left. In the Onites apclles the tarsi are so habitually lost, that the insect has been described as not having them. In some other genera they are present, but in a rudimentary condition. In the Ateuchus or sacred beetle of the Egyptians, they are totally deficient. The evidence that accidental mutilations can be in- herited is at present not quite decisive ; but the remarkable case observed by Brown-Sequard of inherited epilepsy in guinea- pigs, caused by an operation performed on the spinal cord, should make us cautious in denying such power. Hence it Avill ]>ei]iaps be safest to look at the entire absence of the an- terior tarsi in Ateuchus, and their rudimentary condition in some other genera, as due to the effects of long-continued disuse; for, as many dung-feeding beetles are generally found with their tarsi lost, this nnist happen earlv in life; therefore tlie tarsi cannot be of much impoi'tance or be nnich used by these insects. In some cases we might easily put do"^^'n to disuse modifi- cations of structure which are wholly, or mainly, due to natural selection. Mr. AYollaston has discovered the remarkable fact that ::300 beetles, out of the 550 species (but more are now known) which inhabit Madeira, are so far deficient in wings that they cannot fly; and that, of the twent^'-nine endemic genera, no less than twenty-three genera have all their species in this condition ! Several facts — namely, that beetles in many parts of the world are frequently blown to sea and perish ; that the beetles in Madeira, as observed by Mr. AVollaston, lie much concealed, until the wind lulls and the sun shines ; that the ])roportion of wingless beetles is larger on the exposed Desertas than in Madeira itself; and especially the extraor- dinary fact, so strongly insisted on by Mr. AVoUaston, of the CiiAP. V. EFFECTS OF USE AND DISUSE. 141 alino.st entire abs'jnco of certain lar^jo groups of beetles, else- where excessively numerous, and which groups have ha])its of life almost necessitating frequent flight — tliese several con- siderations have made me believe that the wingless condition of so many Madeira beetles is mainly due to the action of natural selection, but combined probably with disuse. For during many successive generations each individual beetle which Hew least, cither from its wings having been ever so little less perfectly developed or from indolent habit, will have had the best chance of surviving from not being blown out to sea ; and, on the other hand, those beetles whicli most readily took to flight would oftenest have been blown to sea and thus have been destroyed. The insects in Madeira which arc not ground-feeders, and which, as the flower-feeding coleoptera and lepidoptera, must habilually use their wings to gain their subsistence, have, as Mr. Wuilaslon suspects, their wings not at all reduced, but even elilarged. This is quite compatible with the action of natural selection. For when a new insect first arrived on the ishind, the tendency of natural selection to enlarge or to reduce the wings, would depend on whether a greater number of in- dividuals were saved by successfully battling with the winds, or by giving up the attempt and rarely or never flying. As wilh mariners shipwrecked near a coast, it would liave been bcttiT for the good swimmers if they had been able to swim still farther, whereas it would have been better for the bad swinnners if they had not been able to swim at all and had stuck to the wreck. The eyes of moles and of some burrowing rodents arc rudimentary in size, and in some cases are quite covered up ])y skin and fur. Tliis state of the eyes is probably due to gradual reduction from disuse, but aided perhaps by natural selection. In Soutli America, a burrowing rodent, the tuco-tuco, or Ctcn- omys, is even more subt(«rranean in its habits than the mole; .and 1 was assured by a Spaniard, who had often caught them, that th(»y were frequently blind; one which I kept alive was certainly in this condition, the cause, as appeared on dissection, having been inllammation of the nictitating membrane. As frefjuent inflammation of the eyes nnist be injurious to any animal, and as eyes are certaiidy not necessary to aiiimals having su])terranean habits, a reduction in their size, with the adhesion of the eyelids and growth of fur over them, might in 142 EFFECTS OF USE AND DISUSE. Chap. V, such case be an advantage ; and if so, natural selection would conslanlly aid the efl'ects of disuse. It is well known that several animals, belonging to the most different classes, which inhabit the caves of Carniola and of Kentucky, are blind. In some of the crabs the foot-stalk for tlie eye remains, though the eye is gone ; the stand for the telescope is there, though the telescope with its glasses has been lost. As it is difficult to imagine that eyes, though use- less, could be in any way injurious to animals living in dark- ness, I attribute their loss wholly to disuse. In one of the bUnd animals, namely, the cave-rat (Ncotoma), two of which were cajotured by Prof. Silliman at above half a mile dis- tance fj-om the moutli of the cave, and therefore not in the profoundest depths, the eyes were lustrous and of large size ; and these animals, as I am informed by Prof. Silliman, after having been exposed for about a month to a graduated light, acquired a dim perception of objects. It is difficult to imagine conditions of life more similar tlian deep limestone caverns imdcr a nearly similar climate ; so that, on the common view of the blind animals haWng been sepa- rately created for the American and European caverns, very close similarity in their organization and affinities might have been expected. This is certainly not the case if we look at the two whole faunas ; and, with respect to the insects alone, Schiodte has remarked : " We are accordingly prevented from considering the entire phenomenon in any other light than something purely local, and the similarity which is exhibited in a few forms between the INIannnoth Cave (in Kentucky) and the caves in Carniola, otherwise than as a very plain expression of that analogy which subsists generally between the fauna of Europe and of North America." On my view we must suppose that American animals, having in most cases ordinary powers of vision, slowly migrated by successive generations from the outer world into the deeper and deeper recesses of the Kentucky caves, as did European animals into the caves of Europe. We have some evidence of this gradation of habit ; for, as Schiodte remarks, " We accordingly look upon the subterranean faunas as small ramifications which have penetrated into the earth from the geograjihically limited faunas of the adjacent tracts, and which, as they extended themselves into darkness, have been accommodated to surrounding circumstances. Animals not far remote from ordinary forms, proiwre the transition from light to darkness. Next follow those that are constructed for twi- Chap. V. EFFECTS OF USE AND DISUSE. I43 li^lit ; and, last of all, tlioso destined for total darkness, and Aviiose formation is (juitc peculiar." These remarks of Schiiidte's, it should be understood, apply not to the same, but to distinct species. ]Jy the time that an animal had reached, after num- berless generations, the deepest recesses, disuse will on this view have more or less perfectly obliterated its eyes, and natural selection will often have ell'ected other changes, such as an increase in the length of the antenmu or palpi, as a com- pensation for l)lindness. Notwithstanding such modifications, we might expect still to sec in the cave-animals of America, allinities to tlie other inhabitants of that continent, and in those of Europe to the inhabitants of the European Continent. And this is the case with some of the American cave-animals, as I hi^ar from Prof. Dana ; and some of the European cave-insects arc very closely allied to those of the surroimding countrv. It woulil be most dillicult to give any rational explanation of the allinities of the blind cave-animals to the other inhabitants of the two continents on the ordinary view of their independent creation. That several of the inhabitants of the caves of the Old and New Worlds should l)e closely related, we might ex- pect from the Avell-knowu relationship of most of their other jiroductions. As a blind species of Bathyscia is found in abun- dance on shady rocks far from caves, the loss of vision in the cave-species of this one genus has probably had no relation to its dark habitation ; and it is very natural that an insect al- ready deprived of vision should readily become adapted to dark caverns. Another blind genus (Anophthalmus) offers this remarkable peculiarity: the several distinct species, as Mr. MuiTay has remarked, iidiabit several distinct European caves and likewise those of Kentucky, and the genus is found no- v.here except in caves ; but it is possible that the progenitor or progenitors of these several spec-ies, while furnished with eyes, formerly may have ranged widely over both continents, and then have become extinct, excepting in their ])resent se- cluded abodes. Far from feeling surprise that some of the rave-animals should be very anomalous, as Agassis has remarked in regard to flu' blind fisli, the Amblyopsis, and as is the case with the blind I'rotcus with refertMice to the reptiles of Europe, 1 am only surprised that more wrecks of ancient HA; have not ]>ccn preserved, owing to tin; less severe competition to which the inhabitants of these daik abodes must have been exposed. 144 ACCLIMATIZATION. Chat. V. Acclimatization, Habit is hereditary with plants, as in the period of flower- iiirent temperatures ; that is, they become acclimatized: thus the pines and rhododendrons, raised from seed collected by Dr. Hooker from the same species gro^^^ng at different heights on the Himalaya, were found in this country to possess different constitutional powers of resisting cold. ^Ir. Thwaites informs me that he has observed similar facts in Ceylon ; analogous observations have been made by Mr. H. C. Watson on Euro- pean species of plants brought from the Azores to England ; and I could give other cases. In regard to animals, several autlientic instances could be adduced of species within histori- cal times having largely extended their range from warmer to cooler latitudes, and conversely ; but we do not positively know tliat these animals were strictly adapted to their native climate, though in all ordinary cases we assume such to be the case ; nor do we know that they have subsequently become specially acclimatized to their new homes, so as to be better fitted for lliiMU than they Avere at first. As we may infer that our domestic animals were originally CiiAP. V. ACCLIMATIZATION. Ii5 chosen by uncivilized nuin because they ■were useful^ and be- cause they bred readily inider confinement, and not because they were subsequently found cai)ablc of far-extended trans- portation, the common and extraordinary capacity in our do- mestic animals of not only withstandinf^ the most dillcrent climates, but of beiiitics arc of recent origin, they cannot owe their constitutional differences to hab- it. The case of the Jerusalem artichoke, which is never propa- gated in England by seed, and of which consequently new va- rieties have not been produced, has even been advanced — for it is now as tender as ever it was — as proving that acclimatiza- tion cannot be effected ! Tlie case, also, of the kidney-bean has been often cited for a similar purpose, and with much greater weight; but until some one will sow, during a score of generations, his kidney-beans so early that a very large pro- portion are destroyed by frost, and then collect seed from the few survivors, with care to prevent accidental crosses, and then again get seed from these seedlings, with the same precau- tions, the experiment cannot be said to have been tried. Nor let it be supposed that differences in the constitution of seed- ling kidney-beans never appear, for an account has been pub- lished how much more hardy some seedlings were than others; and of this fact I have myself observed striking instances. On the whole, I think we may conclude that habit, or use and disuse, have, in some cases, played a considerable part in the modification of the constitution, and of the structure of various organs ; but that the effects of use and disuse have often been largely combined with, and sometimes overmastered by, the natural selection of innate variations. Correlated 'Variation. I mean by this expression that the whole organization is so tied together during its growth and development, that when slight variations in any one part occur, and are accumulated through natural selection, other parts become modified. This is a very important subject, most imperfectly understood, and no doubt totally different classes of facts may be here easily confounded together : we shall presently see that simple in- heritance often gives the false appearance of correlation. The most obvious instance of real correlation is, that variations of structure arising in the young or in the larvae naturally tend to affect the structin-e of the mature animal ; in the same man- ner as ;my malconfonnation in the early embryo is knoAvn Chap. V. CORRELATED VAKIATION. I47 seriously to .ifFect the whole org'anization of the adult. The several parts of the body which are homolof^ous, and which, at an early embryonic period, are identical in structure, and which are necessarily exposed to similar conditions, seem emi- nently liable to vary in a like manner: we see this in the right and left sides of the body varying in the same manner; in the front and hind legs, and even in the jaws and limbs, varying together, for the lower jaw is believed by some anatomists to bs homologous with the limbs. These tendencies, I do not doubt, may be mastered more or less completely by natural selection : thus a family of stags once existed with an antler only on one side ; and if this had been of any great use to the breed, it might probably have been rendered permanent by selection. Homologous parts, as has been remarked by some authors, tend to cohere ; this is often seen in monstrous plants : and nothing is more common than the union of homologous parts in normal structures, as the imion of the petals of the corolla into a tube. Hard parts seem to affect the forms of adjoining soft parts ; it is believed by some authors that the diversity in the shape of the pelvis in birds causes the remarkable diver- sity in the shape of their kidneys. Others believe that the shape of the pelvis in the human mother influences by pressure the shape of the head of the child. In snakes, according to Schlegol, the shape of the body and the manner of swallowing determine the position and form of several of the most impor- tant viscera. The nature of the bond of correlation is frequently quite obscure. M. Isidore Geoffroy St.-Hilaire has forcibly remarked that certain malconformations very frequently, and that others rarely coexist, without our being able to assign any reason. ^\''hat can be more singular than in cats the relation between complete whiteness with blue eyes and deafness, or between the tortoise-shell color and the female sex? or in pigeons be- tween their feathered feet and skin betwixt the outer toes, or b;^tween the jiresence of more or less down on the young birtl when first hatched, with the future color of its plumage ; or, again, the relation between the hair and teeth in the naked Turkish dog, though here no doubt homology comes into play? With respect to tliis latter case of correlation, I think it can hardly be accidental, that, if we pick out the two orders of" mammals which are most abnormal in their dermal covering, viz., Cetacca (whales) and Edentata (amindilloes, scaly ant- 148 CORRELATED VARIATION. Cuap. V. eaters, etc.), these are likewise the most abnormal in tlieir teeth. I know of no case better adapted to show the importance of the laws of variation and correlation, independently of util- ity and therefore of natural selection, than that before referred to, of the difference between the outer and inner flowers in some Conipositous and Umbelliferous plants. Every one knows the dilTercnce in the ray and central florets of, for instiuice, th{^ daisy, and this difference is often accompanied with the partial or complete abortion of the rejoroductive organs. But in some of these plants, the seeds also differ in shape and sculpture. These differences have been attributed by some authors to the I^rcssure of the involucra on the florets, or to their mutual press- in-e, and the shape of the seeds in the ray-florets of some Com- positie countenances this idea; but with the Umbellifene, it is by no means, as Dr. Hooker informs me, the species with the densest heads Avhich most frequently differ in their inner and outer flowers. It might have been tliought that the de- velopment of the ray-pc^tals by drawing- nourishment from the r(>productivc organs had caused their abortion ; but this can hardly be the S(jle cause, for in some Compositir; the seeds of the outer and inner florets differ without any difference in the corolla. Possibly these several differences may be connected Avith the diflcrent flow of nutriment toward tlie central and external flowers : we know, at least, that, with normally iireg- ular flowers, those nearest to the axis are most subject to jK'loria, that is, they become S3'mmetrical. I may add, as an instance of this, and of a striking case of correlation, that I have recently observed in many pelargoniums, that in the cen- tral flower of the truss the two upjier petals often lose their ])atches of darker color; and when this occurs, the adherent nectary is qviite aborted. When the color is absent from only one of the two upper petals, the nectary is not quite aborted, but is much shortened. With respect to the developmoit of the corolla in the cen- tral and exterior flowers, Sprengel's idea that the ray-florets serve to attract insects, whose agency is highly advantageous or necessary for the fertilteation of these plants, is highly jirob- able ; and if so, natural selection may have come into i^lay. ]>ut, witli respect to the seeds, it seems impossible that their diiVcrences in shape, which ai-e not ahvaj's correlated with any difference in the corolla, can be in any way beneficial : yet in the Umbellifere these differences arc of such apparent impor- riiAP. V. COMPENSATION OF GKOWTII. 149 tiincc — the seeds bcinj^ sometimes orlliospermons in tlic exte- rior llowers and ca-lospennoiis in the central ilowors — that the elder iJe Caiidollo founded his main divisions in the order on such char;- -ters. Hence, as before remarked, wc see that modi- fications of structure, viewed by systematists as of hi^h value, may be AvlK)lly due to the laws of variation and correlation, without beiuf^, as far as we can judf^e, of the slightest service t(^ the species. We may often falsely attribute fo correlated variation structures which arc common to whole groups of species, and which in truth arc simply due to inheritance : for an ancient progenitor may have acquired through natural selection some one modification in structure, and, after thousands of genera- tions, some other and independent modilication ; and these two modiiications, liaving been transmitted to a whole group of descendants witli diverse habits, would naturally l^e tlumght _to be correlated in some necessary manner. Some correlations are apparently due to the manner in wliich natural selection acts. For instance, Al]ih. de Candollc has remarked that winged seeds are never found in fruits wliich do not open : I should ex})lainthis rule by the impossiljility of seeds gradually becoming winged through natural selection, unless the ca])- sules first opened themselves ; for in this case alone could the seeds, which were a little better adapted to be Avafted by the wind, gain an advantage over those less well fitted for wide dispersal. Compensation and Economy of Groxcth. llie elder GeoiTroy and Goethe propounded, at about the same period, their law of compensation or balanccment of growth ; or, as Goethe expressed it, " in order to spend on one side, Nature is forced to economize on the other side." I think this holds true to a certain extent with our domestic produc- tions: if nourishment flows to one part or organ in excess, it rarely flows, at least in excess, to another part; thus it is dif- ficult to get a cow to give nuicli milk and to fatten readily. The same varieties of tlic cabbage do not yield abundant and luitritious foliage and a copious supply of oil-bearing seeds. When the seeils in our fruits become atrophied, tlie fruit itself gains largely in size and quality. In our poultry, a large tuft of feathers on the head is generally accompanied by a dimin- ished comb, and a large beard by diminished wattles. With 150 COMPENSATION OF GROWTU. Chap. V. species in a state of nature it can hardly be maintained that the law is of universal application ; but many jrood observers, more especially botanists, believe in its truth. I will not, how- ever, here give any instances, for I see hardly any way of dis- tinguishing between the effects, on the one hand, of a part being largel}' developed through natural selection and another and adjoining part being reduced by this same process or by disuse, and, on the other hand, the actual willidrawal of nutri- ment from one part owing to the excess of growth in another and adjoining part. I suspect, also, that some cases of compensation which have been advanced, and likewise some other facts, may be merged inider a more general principle, namel}', that natural selection is continually trying to economize in every part of the organ- ization. If under changed conditions of life a structure before useful becomes less useful, any diminution, however slight, in its development, Avill be seized on by natural selection, for it will ]iroiit the individual not to have its nutriment wasted in building up a useless structure. I can thus only understand a fact with which I was much struck when examining cirri- pedes, and of which many other instances could be given : namely, that when a cirripede is parasitic within another and is thus protected, it loses more or less completely its own shell or carapace. This is the case with the male Ibla, and in a truly extraordinary manner with the Proteolepas ; for the car- apace in all other cirripedes consists of the three highly-impor- tant anterior segments of the head enormously developed, and furnished with great nerves and muscles ; but in the parasitic and protected Proteolepas, the whole anterior part of the head i.s reduced to the merest rudiment attached to the bases of the prehensile antenna?. Now the saving of a large and complex structure, when rendered superfluous by the parasitic habits of the Proteolepas, though effected by slow steps, would be a decided advantage to each successive individual of the species ; for in the struggle for life to which every animal is exposed, each individual Proteolepas would have a better chance of supporting itself, by less nutriment being wasted in develop- ing a structure now become useless. Thus, as I believe, natural selection will always succeed in the long-run in reducing and saving every part of the organi- zation, as soon as it is rendered by changed habits of life super- fluous, without by any means causing some other part to be largely develojicd in a corresponding degree; and, converse- Chap. V. MULTIPLE STKUCTUKES VAKIABLE. 1.51 ly, that natural selection may perfectly well succeed in largely developing any organ, without requiring as a necessary com- pensation the reduction of some adjoinhig part. Mullij)le, liudimentaryy and Loichj-organized Structures are variable. It seems to be a rule, as remarked by Is. Geoffroy St.-IIi- laire, both in varieties and in species, that, when any part or organ is repeated many times in the structure of the same individual (as the vertebnu in snakes, and the stamens in poly- androus ilowers), tlie number is variable; whereas the number of the same part or organ, when it occurs in lesser num])ers, is constant. The same author and some botanists have further remarked that multiple parts are also very liable to variation in structure. Inasmuch as this " vegetative repetition," to use Prof. Owen's expression, seems to be a sign of low organiza- tion, the foregoing remark seems connected with the very gen- eral opinion of Jiaturalists that beings low in the scale of Na- ture arc more variable than those which are liigher. I pre- sume that lowncss in this case means that the several parts of the organization have been but little specialized for particular functions ; and, as long as the same part has to perform diver- sified Avork, wo can perhaps see Avhy it should remain variable, that is, why natural selection should not have preserved or rejected eacli little deviation of form so carefully as when the part had to serve for one special purpose alone — in the same way that a knife which has to cut all sorts of things may be of almost any shape, while a tool for some particular j^urpose had better be of some particular shape. Natural selection, it should never be forgotten, can act on each part of each being, solely through and for its advantage. Kudimentary parts, it has been stated by some authors, and I believe with truth, are apt to be highly varialjle. AVe shall have to recur to tlie general subject of rudimentary and aborted organs; and I will here only add tliat tlieir variability seems to be owing to their uselessness, and therefore to natiu-al selec- tion having no power to check deviations in their structure. Thus rudimentary parts are left to the free play of the various laws of growth, to the effects of long-continued disuse, and to tlic tendency to reversion. 152 UNUSUALLY DEVELOPED PARTS Chap. V. A Part developed in any F^pccks in an extraordinary Degree or Manner, in comparison with the same Part in allied JS^)ccic^, tends to he highly variable. Several years ago I was much struck by a rcmaik, to the above effect, made by Mr, Waterliouse ; Prof. Owen, also, seems to have come to a nearly similar conclusion. It is hope- less to attempt to convince any one of the truth of the aljovc j)roposition witliout giving the long array of facts which I have collected, and which cannot possibly be here introduced. I can only state my conviction that it is a rule of high generality. I am aware of several causes of error, but I hope that I have made due allowance for them. It should be understood that the rule by no means applies to any part, however unusually developed, unless it be unusually developed in comparison with the same part in closely-allied species. Thus, the wing of the bat is a most abnormal structure in the class of mammalia ; but the rule would not apply here, because the whole group of bats possesses wings ; it would apply only if some one species had wings develojoed in a remarkable manner in comparison with other species of the same genus. The rule applies very strongly in the case of secondary sexual characters, when displayed in any unusual manner. The term, secondary sexual characters, used by Hunter, applies to characters which are attached to one sex, but are not directly connected Avith the act of repro- duction. The nde applies to males and females ; but as females more rarely offer remarkable secondary sexual characters, it ap- plies more rarely to them. The rule being so plainly applicable in the case of secondary sexual characters, may be due to the great variability of these characters, whether or not displayed in any unusual manner — of Avhich fact I think there can be little doubt. But that our rule is not confined to secondary sexual characters is clearly shown in the case of hermaphrodite cirripedes ; I particularly attended to ^Ir. \Vaterhouse's remark, while investigating this order, and I am fully convinced that the rule almost invariably holds good. I shall, in a future; work, give a list of all the more remarkable cases ; I will lierc only give one, as it illustrates the rule in its largest application. The opercular valves of sessile cirripedes (rock barnacles) are, in every sense of the word, very important structures, and they diller extremely little even in different genera ; but in the several species of one genus, Pyrgoma, these valves present a marvel- lous amount of diversification ; the homologous valves in the CnAi-. V. HIGHLY VARIABLE. I.53 different species beinp^ sometimes wlioUy unlike in shape; and the amount of variation in the individuals of the same speciey is so great, that it is no exaf^goration to state that the varieties differ more from each other in the characters derived from these important valves than do other species belonging to distinct genera. As birds within the same country vary in a remarkably small degree, I have jiarticuhu-ly attended to them, and the rule seems to me certainly to hold good in this class. I cannot make out that it applies to plants, and this would seriously have shaken my belief in its truth, had not the great variability in plants mad(; it jiarticularly dillicult to compare their relative degrees of variabilit}'. When we see any part or organ developed in a remarkable degree or maimer in any species, the fair presumption is, that it is of high importance to that species ; nevertheless it is in this case eminently liable to variation. Why should this be so ? On the view that each species has been independently created, with all its ])arts as we now see them, I can see no explanation. But on the view that groups of species have descended from other species, and have been modified through natural selection, T think we can obtain some light. First, let me remark that if any part in our domestic animals, or the whole animal, be neglected and no selection be applied, that I)art (for instance, the comb in the Dorking fowl) or the whole bref'd will cease to have a imiforin character. The breed will then be said to have degenerated. In rudimentary organs, and in tijose which have been but little specialized for an}'' partic- ular purpose, and perhaps in poljmiorphic groups, we see a nearly parallel natural case; for in such cases natural selection either has not or cannot have come into full play, and thus the organization is left in a fluctuating condition. But what here concerns us is, that those points in our domestic animals, whicli at the present time are undergoing rapid change by continued selection, are also eminently liable to variation. Look at the breeds of the pigeon ; see what a prodigious amount of differ- ence there is in tht; beaks of tumblers, in the beaks and wattle of carriers, in the carriage and tail of fantails, etc., these being the points now mainly attiMided to by Englisli fanciers. Even in sub-breeds, as in that of the short-faced tumbler, it is notori- ously dillicult to breed nearly perfect birds, some fre(|uently departing widely from the standard. There may truly be said to be a constant struggle going on between, on the one hand, 15.4 UNUSUALLY DEVELOPED PARTS Cnxr. V. the tendency to reversion to a less perfect state, as well as an innate tendency to fnrtlier variability, and, on the other hand, the power of steady selection to keep the breed true. In the long'-run, selection gains the day, and we do not expect to faU so far as to breed a bird as cf)arse as a common tumbler from a good short-faced strain. Bnt as long as selection is rapidly going on, nnich variability in the parts \mdergoing modification may always he, expected. It further deserves notice tliat char- acters, modified through selection by man, are sometimes trans- mitted, from causes quite unknown to us, more to one sex than to the other, generally to the male sex, as with the wattle of carriers and the enlarged crop of pouters. Now let us turn to Nature. AYlien a part has been devel- oped in an extraordinary manner in any one species, compared with the other species of the same genus, we may conclude that this part has undergone an extraordinary amount of modifi- cation since the period when the species branchetl off from the connnon progenitor of the genus. This period will seldom be remote in any extreme degree, as species rarely endure for more than one geological period. An extraordinary amovmt of modification implies an unusually large and long-continued amount of variability, which has continually been accumulated by natural selection for the benefit of the species. But as the variability of the extraordinarily developed part or organ has ■];)een so great and long-continued within a period not exces- sively remote, Ave might, as a general rule, still expect to find more A-ariability in such parts than in otlier parts of the organi- zation which have remained for a much longer period nearly constant. And this, I am convinced, is the case. Tliat the struggle between natural selection on the one hand, and the tendency to reversion and variability on the other hand, will in the course of time cease ; and that the most abnormally devel- oped organs may be made constant, I see no reason to doubt. Ilence, when an organ, however abnormal it may be, has been transmitted in approximately the same condition to many modi- fied descendants, as in the case of the wing of the bat, it must have existed, according to my theory, for an immense period in nearly the same state; and thus it comes to be no more vari- al)le than any other structure. It is only in those cases in wliich the modification has been comparatively recent and extraordinarily great that we ought to find i\\o generative vari- (ihllitij^ as it may be called, still present in a high degree. For in this case tlie variability will seldom as yet have been fixed Chap. V. HIGHLY VARIABLE. I55 by the continued selection of the individuals varj-ing in the •required manner and degree, and by the continued rejection of those tending to revert to a former and less-modified con- dition. Si'iecific Characters more variable than Generic Characters^ The principle included in the above remarks may be ex- tended. It is notorious that specific characters are more va- riable than generic. To exjilain by a simple example -what is meant : If in a large genus of jilants some species had blue flowers and some had red, the color Avould be only a specific character, and no one "would be surprised at one of the blue species varying into red, or conversely ; but if all the species had blue ilowers, the color would become a generic character, and its variation Avould be a more unusual circumstance. I liave chosen this example because an explanation is not in this case applicable, which most naturalists would advance, namely, that specific characters are more variable than generic, because they are taken from parts of less physiological importance than those commonly used for classing genera. I believe this ex- planation is partly, yet only indirectly, true ; I shall, however, have to return to this subject in the cliapter on Classification. It would be almost superfluous to adduce evidence in support of the a])ove statement, that specific characters are more va- riable than generic ; but I have repeatedly noticed in Avorks on natural history, that, when an author has remarked with sur- prise that some imjiortant organ or part, which is generally very constant throughout large groups of species, has differed considerably in closelj'-allied species, it has also been vari- able in the individuals of some of the species. And this fact shows that a character, Avhicli is generally of generic value, when it sinks in value and becomes only of specific value, often becomes variable, though its physiological importance may re- main the same. Something of the same kind applies to mon- strosities : at least Is. GeoflVoy St.-Hilaire seems to entertain no doubt that the more an organ normally differs in the differ- ent species of the same group, the more subject it is to indi- vidual anomalies. On the ordinary view of each species having been inde- pendently created, why should that part of the structure, which differs from the same part in other independently-created species of the same genus, be more variable than those parts 156 SECONDARY SEXUAL Chap. V. which are closely alike in the several species? I do not sec that any explanation can be given. But on the vie'.v that spe- cies are only stron2;ly marked and fixed varieties, Ave might ex- pect to find them still oil en continuing to vary in those parts of their structure which had varied witliin a moderately-recent period, and which had thus come to diCfer, Or, to state the case in another manner : The points in which all the species of a genus resemble each other, and in ■which they differ from allied genera, are called generic characters ; and these charac- ters in common I attribute to inheritance from a common pro- genitor, for it can rarely have happened that natural selection Avill have modified several species, fitted to more or less wide- ly-diflerent habits, in exactly the same manner : and as these so-called generic characters have been inherited from before tlie period when the different species first branched off from their common progenitor, and subsequently have not varied or come to differ in any degree, or only in a slight degree, it is not probaljle that tliey should vary at the present day. On the other hand, the points in which species differ from other spe- cies of the same genus are called specific characters ; and as these specific characters have varied and come to differ since tlie period when the species branched off from a common pro- genitor, it is probable that they should still often be in some degree variable — at least more variable than those parts of the organization which have for a very long period remained con- stant. Secondary Sexual Characters variable. In coimection with the present subject, I Avill make only two other remarks. I think it will be admitted, without my entering on details, that secondary sexual characters are very variable; I think it also will be admitted that species of the same group diller from each other more widely in their second- ary sexual characters tlian in other parts of their organization; (^ompare, for instance, tlie amount of difference between the males of gallinaceous birds, in which secondary sexual charac- ters are strongly displayed, with the amount of difference be- tween the females; and the tnith of tliis jiroposition will be granted. Tiie cause of the original variability of secondary S(\xual characters is not manifest ; but we can see why these characters should not have been rendered as constant and imi- form as other parts of the organization; for secondary sexual characters have been accumulated by sexual selection, which is Chap. V. CHARACTERS VARIABLE. 157 less rigud in its action tlian ordinary selection, as it does not entail death, bnt only gives fewer offspring to the less favored males. Whatever tlie cause may be of the variability of sec- ondary sexual characters, as they are highly variable, sexual se- lection, will have had a wide scope for action, and may thus readily have succeeded in giving to the species of the same group a greater amount of difference in their sexual characters tlian in other parts of their structure. It is a remarkable fact that the secosdary sexual differ- ences between the two sexes of the same sjiecies arc generally displayed in the very same parts of the organization in whicli the different species of the same genus dilfer from each other. Of this fact I will give two instances in illustration, the first which hap])en to stand on my list; and, as the differences in these cases are of a very unusual nature, the relation can hard- ly be accidental. The same number of joints in the tarsi is a character generally common to very large gi-oups of beetles, but in the Engid:e, as Westwood has remarked, the number varies greatly; and the number likewise differs in the two sexes of the same species : again, in fossorial liymenoptera, the manner of neuration of the wings is a character of the highest importance, because common to large groups ; but in certain genera the ncin-ation differs in the different species, and like- wise in the two sexes of the same species. Sir J. Lubbock has recently remarked that several minute crustaceans offer ex- cellent illustrations of this law. " In Pontella, for instance, the sexual characters arc afforded mainly l)y the anterior an- tenna) and by the fifth pair of legs : the specific differences also are principally given by these organs." This relation has a clear meaning on my view of the subject : I look at all th(! species of the same genus as ha^nng as certainly descended from the same progenitor as have the two sexes of any one of the species. Conseijuently, whatever part of the structure of the common progenitor, or of its early descendants, became A-ariable, variations of this part Avould, it is highly jiroljable, be taken advantage of by natural and sexual selection, in order to fit the several species to their several j)lac(\s in the economy of Nature, and likewise to fit the two sexes of the same S]iecies to each other, or to fit the males and females to different habits of life, or the males to struggle with other males for the jiosses- sion of the females. Finally, then, I conclude that the greater variability of specific characteis, or those which distinguish species from 158 DISTINCT SPECIES PRESENT Chap. V. species, ihan of generic characters, or those which the species possess in roinmon ; that the frequent extreme variability of any part which is developed in a species in an extraordinary manner in comparison with the same part in its congeners ; and the slight degree of variability in a part, however extraor- dinarily it may be developed, if it be common to a whole group of species ; that the great variability of secondarj' sexual char- acters, and the great amount of difference in these same char- acters between closely-allied species ; that secondary sexual and ordinary specific differences are generally displayed in the same parts of the organization — are all principles closely con- nected together. All being mainly due to the species of the same group having descended from a common progenitor, from whom they have inherited much in common — to parts whicli have recently and largely varied being more likely still to go on varying than parts which have long been inherited and have not varied — to natural selection having more or less com- pletely, according to the lapse of time, overmastered the ten- dency to reversion and to further variability — to sexual selec- tion being less rigid than orchnary selection — and to variations in the same parts having been accumulated by natural and sexual selection, and having been thus adapted for secondary sexual, and for ordinary purposes. Distinct Species present analogous Variations ; and a Variety of one Species often assumes some of the Characters of an allied Species, or reverts to soyne of the Characters of an early Progenitor. These propositions will be most readily understood by look- ing to our domestic races. The most distinct breeds of pigeons, in countries most widely apart, present sub-varieties with re- versed feathers on the head and feathers on the feet — charac- ters not possessed by the aboriginal rock-pigeon ; these, then, are analogous variations in two or more distinct races. The frequent presence of fourteen or even sixteen tail-feathers in the pouter may be considered as a variation representing the nor- mal structure of another race, the fantail. I presume that no one will doubt that all such analogous variations are due to the several races of the pigeon having inherited from a common ]>arent the same constitution and tendency to variation, when acted on bv similar unknown influences. In the vegetable kingdom we have a case of analogous variation in the enlarged Chap. V. ANALOGOUS VARIATIONS. I59 stems, or roots as commonly called, of the Swedish turnip and Ruta-ba^'a, })hmts wliich several botanists rank as varieties produced by cultivation from a common parent; if this be not so, the case will then be one of analogous variation in two so- called distinct species ; and to these a third may be added, namely, the common turnip. AccoriUng to the ordinary view of each species having be(?n independently created, we should have to attribute this similarity in the enlarged stems of these three plants, not to the vera causa of community of descent, and a consequent tendency to vary in a like manner, but to three separate yet closely-related acts of creation. Many simi- lar cases of analogous variation have been observed by Naudin in the great gourd-family, and by various authors in oiu* cereals. Similar cases occurring with insects under their natural con- ditions have lately been discussed with much al)ility by Mr. Walsh, who has grouped theni under his law of Equable Va- riability. With pigeons, however, we have another case, namely, the occasional appearance in all the breeds of slaty-blue birds with two black bars on the v/ings, white loins, a bar at the end of the tail, with the outer feathers externally edged near their bases with white. As all these marks are characteristic of the parent rock-pigeon, I presume that no one will doubt that this is a case of reversion, and not of a new yet analogous variation appearing in the several breeds. ^Ve may, I think, confidently come to tliis conclusion, because, as we have seen, these colored marks are eminently liable to appear in tlie crossed oflsining of two distinct and differently-colored breeds; and in this case there is nothing in the external conditions of life to cause tlie reappearance of the slaty-blue, with the several marks, beyond the influence of the mere act of crossing on the laws of inher- itance. No doubt it is a very surprising fact that characters should reappear after having lieen lost for many, probably for hun- dreds of generations. But when a ])reed has been crossed only once by some otlier breed, the offspring occasionally show a tendency to revert in character to the foreign breed for many generations — some say for a dozen or even a score of genera- tions. After twelve general i(ms, the proportion of blood, to use a common expression, of any one ancestor, is only 1 in 3,048 ; and yet, as we see, it is generally believed that a ten- dency to reversion is retained by tliis very small projiortion of foreign blood. In a breed which has not been crossed, but IGO DISTINCT SPECIES PRESENT Chap. V. in which both parents liave lost some character which ilieir progenitor possessed, the tendency, Avliether stronf^ or -weak, to reproduce the lost character might be, as was formerly re- marked, for all that wc can sec to the contrary, transmitted for almost any uuml:)cr of generations. When a character which h;is been lost in a lu'ced, reappears after a gTcat immber of generations, the most probable hypothesis is, not that the ofl- spring suddenly takes after an ancestor removed by some hun- dred generations, but that in each successive generation the character in question has been lying latent, and at last, luidei imknown favorable conditions, is developed. With the barb- pigeon, for instance, which very rarely produces a blue bird, it is probable that a latent tendency exists in each generation to produce blue plumage. The possibility of chai'acters long lying latent can be imderstood according to the hypothesis of })angenesis, which I have given in another work. The abstract improbability of a latent tendency being transmitted through a vast number of generations, is not greater tlian that of quite useless or rudimentary organs being thus transmitted. A mere tendency to produce a nuHment is indeed sometimes inherited. As all the species of the same genus are supposed, on our theory, to be descended from a connnon progenitor, it might be expected that they would occasionally vary in an analogous manner; so that the varieties of two or more species would re- semble each other, or that a variety of some one species would resemble in certain characters another and distinct species — this other species being, according to our view, only a well- marked and permanent variety. But characters thus gained would probably be of an unimportant nature, for the presence of all important characters will be governed by natural selec- tion, in accordance with tlic tlifTerent habits of the species, and Avill not be left to the mutual action of the nature of tlie organ- ism and of the conditions of life. It might further be expected that the species of the same genus Avould occasionally exhil^it reversions to long-lost ancestral characters. As, however, we never know tlic exact character of the common ancestor of a natural group, we could not distinguish these two cases : if, iV)r instance, we did not know that the rock-pigeon was not feather-footed or turn-crowned, we could not have told whether these characters in our domestic breeds were reversions or only analogous variations ; but wc might have inferred that the blue color was a case of reversion from llie number of the markings, whicli arc correlated with this tint, and which it docs not ap- CiiAr. V. ANALOGOUS VAKIATIONS. Id pear probable would all appear top^cthcr from simple ^•arialion. More especially we might have infen-ed this, from the blue color and the several marks so often appearinp^ when distinct breeds of distinct colors are crossed. Hence, although under Nature it must generally be left doubtful, what cases are re- versions to a formerly-existing character, and what are ncAV l)ut analogous variations, yet we ought, on oiu* theory, some- times to iind the varying offspring of a species assuming characters ((^ither from nivcrsion or from analogous variation) which already are jirescnt in other members of the same group and tliis undoubteclly is the case. A considcrabl(! jjart of the difficulty in recognizing in our systematic works a variable species, is due to its varieties mocking, as it were, other species of the same genus. A con- siderable catalogue, also, could be given of forms intermediate between two other forms, which themselves can only doubt- fully be ranked as species ; and this shows, unless all these forms be considered as independently-created species, that the one in varjing has assumed some of the characters of the other, so as to produce the intermediate forms. But the best evi- dence is afforded by parts or organs of an important and gen- erally uniform nature occasionally varying so as to acquire, in some degree, the character of the same part or organ in an allied species. I have collected a long list of such cases ; but here, as before, I lie mider the great disadvantage of not being able to give them. I can only repeat that such cases certainly do occur, and seem to me very remarkable. I will, however, give one curious and complex case, not in- deed as affecting any important character, but from occurring in several species of the same genus, partly under domestica- tion and partly under Nature. It is a case almost certainly of reversion. The ass sometimes has very distinct transverse bars on its legs, like those on the legs of the zebra : it has been asserted that these are jilainest in the foal, and, from in- qiiirics which I have made, I believe this to be true. The stripe on the shoulder is sometimes double and is very variable in length and outline. A white ass, but not an albino, has been described without either spinal or shoulder stripe : and these stripes are sometimes verv obscure, or actually (luitelost, in dark-colored asses. The koulan or Pallas is said to have been seen with a double shoulder-stripe. Mr. I31yth has seen a specimen of the hemionus with a distinct shoulder-stripe, tliough it properly has none ; and I have been informed by 1(32 DISTINCT SPECIES PRESENT Chap. V. Colonel Poole tliat the foals of this species arc generally striped on the legs, and faiiitly on the shoulder. The quagga, though so plainly barred like a zebra over the body, is without bars on the legs ; but ]3r. Gray has figured one specimen with very distinct zebra-like bars on the hocks. "With respect to the horse, I have collected cases in Eng- land of the spinal stripe in horses of the most distinct breeds, and of all colors : transverse bars on the legs are not rare in duns, mouse-duns, and in one instance in a chestnut: a faint shoulder-stripe may sometimes be seen in duns, and I have seen a trace in a bay horse. JNIy son made a careful examination and sketch for me of a dun Belgian cart-horse Avith a double stripe on each shoulder and with leg-stripes ; I have myself seen a dun Devonshire ponj^, and a small dun Welsh pony has been carefully described to me, both with three parallel stripes on each shoulder. In the northwest part of India the Kattywar breed of horses is so generally striped, that, as I hear from Colonel Poole, who examined the breed for the Indian Government, a horse with- out stripes is not considered as purely-bred. The spine is always striped; the legs are generally barred; and the shoul- der-stripe, which is sometimes double and sometimes treble, is common ; the side of the face, moreover, is sometimes striped. The stripes are often plainest in the foal ; and sometimes quite disappear in old horses. Colonel Poole has seen both gray and bay Kattywar horses striped Avhen first foaled. I have also reason to suspect, from information given me by Mr. AV. W. Edwards, that Avith the English race-horse the spinal strijie is much commoner in the foal than in the full-grown animal. I have myself recently bred a foal from a bay mare (offspring of a Turcoman horse and a Flemish mare) by a bay English race-horse ; this foal when a week old Avas marked on its hinder quarters and on its forehead Avith numerous, A'ery narroAV, dark, zebra-like bars, and its legs were feebly striped : all the stripes soon disappeared completely. Without here entering on fur- ther details, I may state that I have collected cases of leg and shoulder stripes in horses of A'ery different breeds, in A'arious countries from Britain to Eastern China ; and from NorAvay in the north to the ISIalay Archipelago in the south. In all jiarts of the Avorld these stripes occur far oftenest in duns and mouse- duns ; by the term dun a large range of color is included, from one between broAvn and black to a close approach to cream- color. CnAP. V. ANALOGOUS VAEIATIONS. 163 I am uwaro tliat Colonel Hamilton Smith, who lias written on this subject, believes that the several breeds of the horse are descended from several abori^^inal species — one of which, the dun, was striped ; and that the above-described appear- ances are all due to ancient crosses with the dun stock. But this view may be safely rejected ; for it is highly improbable that the heavy Belgian cart-horse, Welsh ponies, cobs, the lanky Kattywar race, etc., inhabiting the most distant parts of the world, should all have been crossed with one supposed aborigi- nal stock. Now let us turn to the effects of crossing the several species of the horse-genus. lloUin asserts that the common mule from the ass and horse is particularly apt to have bars on its legs ; according to Mr. Gosse, in certain parts of the United States about nine out of ten mules have striped legs. I once saw a mule with its legs so much striped that any one might have thought that it was a hybrid-zebra ; and Mr. W. C. Mar- tin, in his excellent treatise on the horse, has given a figure of a similar mule. In four colored draAvings, which I have seen, of hybrids between the ass and zebra, the legs were much more plainly barred tlian the rest of the body ; and in one of them there was a double shoulder-stripe. In Lord Morton's famous hybrid from a chestnut mare and male quagga, the hybrid, and even the pure offspring subsequently produced from the mare l)y a black Araljian sire, were much more plainly barred across the legs than is even the pure quagga. Lastly, and this is an- other most remarkable case, a hybrid has been figured by Dr. Gray (and he informs me tliat he knows of a second case) from the ass and the hemionus ; and this hybrid, though the ass only occasionally has stripes on his legs and the hemionus has none and has not even a shoulder-stripe, nevertheless had all four legs barred, and had throe short shoulder-stripes, like those on tlie dun Devonshire and Welsh ponies, and even had some zebra-like stripes on the sides of its face. With respect to this last fact, I was so convinced that not even a stripe of color appears from what is commonly called chance, that I was led solel}' from the occurrence of the face-stripes on this hybrid from the ass and hemionus to ask Colonel Poole whether such face-stripes ever occurred in the eminently striped Kattywar l)reed of horses, and was, as we have seen, answered in the adirmative. AVhat now arc we to say to these several facts ? We sec several very distinct species of the horse-genus becoming, by 164 DISTINCT SPECIES PRESENT Cuap. V, siin])le vaiiiition, slripod on tlio legs like a zebra, or stri})cd on the sliouUler.s like an ass. In the liorsc we see this tendency strong^ whenever a dun tint appears — a tint Avhidi approaches to that of the general coloring' of the other species of the ge- nus. The appearance of the stripes is not accompanied by any cliange of form or by any other new character. AVe see this tendency to become striped most strongly displayed in hybrids from between several of the most distinct species. Now ob- serve the case of the several breeds of pigeons : they are de- scended from a pigeon (including two or three sub-species or geographical races) of a bluish color, with certain bars and other marlcs ; and Avhcn any breed assmnes by simple variation a l)luish tint, these bars and other marks invariably reappear ; but without any other change of form or character. When the oldest and truest breeds of various colors are crossed, wc sec a strong tendency for the blue tint and bars and marks to reap- pear in the mongrels. I have stated that the most probable hypothesis to account for the reappearance of xery ancient characters, is — that there is a tendency in the young of each successive generation to produce the long-lost character, and that this tendency, from unknown causes, sometimes prevails. And we have just seen that in several species of the horse- genus the stripes arc either plainer or appear more commonly in tlie young than in the old. Call the breeds of pigeons, some of which have bred true for centuries, species ; and how exact- ly parallel is the case with that of the species of the horse- genus ! For myself, I venture confidently to look back thou- sands on thousands of generations, and I see an animal striped like a zebra, but perhaps other^vise very differently constructed, the common parent of our domestic lu)rse (whether or not it be descended from one or more wild stocks), of the ass, the hemi- onus, quagga, and zebra. He who believes tliat each equine sjiccies was indepen- dently created, will, I presume, assert that each species has been created with a tendency t(i vary, botli under Nature and under domestication, in this particular manner, so as often to become striped like other species of tlie genus ; and that each has been created with a strong tendency, when crossed with species inhabiting distant quarters of tlie world, to produce hybrids resembling in tlieir stri})es, not their own parents, but other species of the genus. To admit this view is, as it seems to me, to reject a real for an imreal, or at least for an unknown, cause. It makes the works of God a mere mockery and decep CiiAr. V. ANALOGOUS VARIATIONS. 105 tioii; I would almost as soon believe ■with the old and if^no- rant cosinof>;'oiusts, that fossil shells had never lived, but had been created in stone so as to mock the shells living on the sea-shore. Suiiunari/. Our ignorance of the laws of variation is profound. Not in one case out of a hundred can we pretend to assign any reason why this or that part has varied, liut whenever we have the means of instituting a comparison, the same laws ajipear to have acted in jiroducing the lesser dillerences be- tween varieties of the same species, and the greater differences between species of the same genus. Changed conditions gen- erally induce mere fluctuating variability, but sometimes they cause direct and definite effects ; and these may become strpngly marked in the coiu'se of time, though we have not sulhcient evidence on this head. Habit in producing constitu- tional peculiarities and use in strengthening and disuse in weakening and diminishing organs, appear in many cases to have been potent in their effects. Homologous parts tend to vary in the same way, and homologous parts tend to cohere, ilodilications in hard parts and in external parts sometimes affect softer and internal parts. When one part is largely developed, perhaps it tends to draw nourishment from the ad- joining parts ; and every part of the structure which can be saved without detriment will be saved. Changes of structure at an early age may aflect parts subsefjuently developed ; and many cases of correlated variation, the nature of which we are unable to understand, undoubtedly occur. Multiple parts are variable in number and in structure, perhaps arising from such ])arts not having been closely specialized for any particular function, so that their modifications have not been closely checked by natural selection. It follows probably from this same cause, that organic beings low in the scale are more vari- able than those standing higher in the scale, and which have llieir whole organization more specialized. Kudiinentary or- gans, from being useless, are not regulated by natural selec- tion, and hence arc varial)le. Specific characters — that is, the characters which have come to differ since the several species of (he same genus branched off from a common parent — are more varia1)le than generics characters, or those which have long becii inherited, and have not differed within this same perioil. In these remarks we have referred to special parts or 1G6 SUMMARY. Chap. V. org-ans bcin,;^ still variable, because they have recently viiried and thus come lo differ; but vrc have also seen in the second chapter that the same principle applies to the whole individual ; for in a district Mhcre many species of any genus are found — that is, where there has been much former variation and differ- entiation, or where the manufactory of new specific forms has been actively at work — in that district and among these spe- cies, we now find, on an average, most varieties. Secondary sexual characters are highly variable, and such characters differ much in the species of the same group. Variability in the same parts of the organization has generally been taken ad- vantage of in giving secondary sexual differences to the sexes of the same species, and specific differences to the several spe- cies of the same genus. Any part or organ developed to an extraordinary size or in an extraordinary manner, in compari- son with the same part or organ in the allied species, must have gone through an extraordinary amount of modification since the genus arose ; and thus Ave can understand why it should often still be variable in a much higher degree than other parts ; for variation is a long-continued and slow process, and natiu"al selection will in such cases not as yet have had time to overcome the tendency to further variability and to reversion to a less modified state. But when a species with any extraordinarily-developed organ has become the parent of many modified descendants — which on our Wew must be a very slow process, requiring a long lapse of time — in this case, natural selection has succeeded in giving a fixed character to the organ, in however extraordinary a manner it may have been developed. Species inheriting nearly the same constitu- tion from a common parent and exposed to similar influences, natui-ally tend to present analogous variations, or these same species may occasionally revert to some of the characters of their ancient progenitors. Although new and important modi- fications may not arise from reversion and analogous variation, such modifications will add to the beautiful and harmonious diversity of Nature. Whatever the cause may be of each slight difference be- tween the offspring and their parents — and a cause for each must exist — it is the steady accumulation, through natural se- lection, of beneficial differences that has given rise to all those modifications of structure which are the most important for the welfare of each species. Chap. VI. DIFFICULTIES OF THE TIIEOKY 1(J7 CHAPTER YI. DIFFICULTIES OF THE TIIEOKY. Difflcnilies of the Theory of Descent with Modification— Transitions— Ahsence or Karlty of Transitional Varieties— Transitions in Habits of Life— Diversified Hab- its iu the same Specios— Species with Habits widely different from those of their Allies — Orj^iiis of Extreme Perfection— Modes of Tranh-ition— Cases of Ditflcully — Natura nou fucit saltnm— Ort'ans of small Importance— Organs not in all Cases absolutely perfect- The Law of XJniiy of Type aud of the Conditions of Existence embraced by the Theory of Natural Selection. Loxr, before havins; anivcd at this part of my ^-ork, a crowd of dilFiculties will have occurred to the reader. Some of them are so serious that to this day I can hardly reflect on them without bein ing from the difficulty ; for I believe that many perfcctlj^Ie- fmed sjiecies have been formed on strictly continuous areas ; though I do not doubt that the formerly broken condition of areas now continuous has played an important part in the for- mation of new species, more especially ^vith frcelj'-crossing and wandering animals. In looking at species as they are now distributed over a ^vide area, we generally find them tolerably numerous over a large territory, then becoming somewhat abruptly rarer and rarer on the confines, and finally disappearing. Hence the neutral territory between two representative species is gener- ally narrow in comparison with the territory projier to each. ^^'e see the same fact in ascending mountains, and sometimes it is quite remarkable how abruptly, as Alph. de Candollc has (il)served, a common alpine species disappears. The same fact has been noticed by E. Forbes in sounding the depths of the s(>a with the dredge. To those who look at climate and the ])]iysical conditions of life as the all-important elements of dis- tribution, these facts ought to cause surprise, as climate and height or depth graduate away insensibly. But when we bear in mind that almost every species, even in its metropolis, would increase immensely in numbeis, were it not for other com- peting species; that nearly aH either prey on or serve as prey s 170 ABSENCE OF RARITY Chap. VI. for otlicrs ; in short, that cadi organic beinf^ is cither directly or indirectly related in the most important manner to other organic beings, \vc must see that the range of the inhabitants of any country by no means exclusively depends on insensibly changing pliysical conditions, but in large part on the presence of other species, on which it lives, or by ■which it is destroyed, or with which it comes into comjietition ; and as these species arc already defmed objects, not blending one into another by insensil)le gradations, the range of any one species, depending as it does on the range of others, will tend to be sharply de- fined. Moreover, each species on the confines of its range, wherc it exists in lessened numbers, will, during fluctuations in the number of its enemies or of its prey, or in the seasons, be extremely liable to utter extermination ; and thus its geo- graphical range Avill come to be still more sharply defined. If I am right in believing that allied or representative spe- cies, when inhabiting a continuous area, are generally so dis- tributed that each has a wide range, with a comparatively narrow neutral territory between tliem, in which they become rather suddenly rarer and rarer ; then, as varieties do not essen- tially differ from species, the same rule will probably apply to both ; and if we take a varying species inhabiting a very large area, we shall have to adapt two varieties to two large areas, and a third variety to a narrow intermediate zone. The inter- mediate variety, consequently, will exist in lesser numbers from inhabiting a narrow and lesser area ; and practical!}', as fiir as I can make out, this rule holds good with varieties in a state of Tjature. I have met with striking instances of the rule in the case of varieties intermediate between well-marked varieties in the genus Balanus. And it would appear from information given me by ]\Ir. "Watson, Dr. Asa Gray, and Mr. Wollaston, that generally, when varieties intermediate between two other forms occur, they arc much rarer numerically than the forms which they connect. Now, if we may trust these facts and inferences, and therefore conclude that varieties link- ing two other varieties together hare generally existed in lesser numbers than the forms which they connect, then, I think, we can understand wh}' intermediate varieties should not en- dure for very long periods ; Avhy, as a general rule, they should be exterminated and disaiii)ear, sooner than the forms v>hich they originally linked together. "For any form existing in lesser numbers would, as already remarked, ran a greater chance of being exterminated tlian CuAi-. VI. OF TRANSITIOXAL VARIETIES. 171 one exist inn" in laro-c numbers; and in this particular case the interniedi:it(! form would be cminentl}"^ liable to the inroads of closely-allied forms existing on both sides of it. But a far more important consideration, as I believe, is that, during the process of further modification, by Avhich two varieties are supposed to be converted and perfected into two distinct spe- cies, the two which exist in larger numbers, from inhabiting larger areas, will have a great advantage over the intermediate variet}', which exists in smaller numbers in a narrow and inter- mediate zone. For forms existing in larger numbers will al- ways have a better chance, within any given period, of present- ing further favorable variations for natural selection to seize on, than will the rarer fcjrms which exist in lesser numbers. Hence, the more common forms, in the race for life, will tend to beat and supplant the less common forms, for these will be more slowly modified and improved. It is the same principle which, as I believe, accounts for the common species in each country, as shown in the second chapter, presenting on an average a greater number of well-marked varieties than do the rarer species. I may illustrate what I mean by supposing three varieties of sheep to be kept: one adapted to an extensive mountainous region ; a second to a comparatively narrow, hilly tract ; and a third to wide plains at the base ; and that the inhabitants are all trying with equal steadiness and skill to improve their stocks by selection ; the chances in this case will be strongly in favor of the great holders on the mountains or on the plains improving th.eir breeds more quicldy than the small holders on the intermediate narrow, hilly tract; and con- sequently the improved mountain or j^lain breed Avill soon take the place of the less improved hill-breed ; and thus the two breeds, which originally existed in greater numbers, will come into close contact with each other, without the interpo- sition of the supplanted, intermediate hill-variety. To sum up, I believe that species come to be tolerabh' well- dcfini^d objects, and do not at any one period present an inex- tricable chaos of varying and intermediate links : first, because new varieties are very slowly formed, for variation is a slow ])roccss, and natural selection can do nothing until favorable individual diflerences or variations occur, and until a place in the natural polity of the country can be better filled by some modification of some one or more of its inhabitants. And such «ncw places will depend on slow changer> of climate, or on the occasional innnigratinn of new inhabitants, and, ]m)bably, in a 172 ABSENCE OR IJARITY Chap. VI. still more important doffrco, on some of the old inhabitants be- coming slowly moditied, with the new forms thus produced and the old ones acting and reacting on each other. So that, in any one region and at any one time, we ought only to sec a few species j)resenting slight modiiications of structure in some degree permanent ; and this assuredly we do see. Secondly, areas now continuous must often have existed within the recent period as isolated portions, in which many forms, more especially among the classes which unite for each birth and wander much, may have separately been rendered suiKciently distinct to rank as representative species. In this case, intermediate varieties betw^een the several representative species and their common parent, must have existed formerly within each isolated portion of the land, but these links during the process of natural selection will have been supplanted and exterminated, so that they will no longer be found in a living state. Thirdly, wlien two or more varieties have been formed in diflerent j)ortions of a strictly continuous area, intermediate varieties Avill, it is probable, at first have been formed in the intermediate zones, but they Avill generally have had a short duration. For these intermediate varieties will, from reasons already assigned (namely, from what we know of the actual distribution of closely-allied or representative species, and like- Avise of acknowledged varieties), exist in the intermediate zones in lesser numbers than the varieties which they tend to con- nect. From this cause alone the intennediate varieties will be hable to accidental extemiination ; and during the process of further modification through natural selection, they will al- most certainly lie beaten and supplanted by the forms which they connect ; for these from existing in greater numbers will, in the aggregate, present more variation, and thus be further improved through natural selection and gain further advan- tages. Lastly, looking not to any one time, but to all time, if my theory be true, numberless intermediate varieties, linking closely together all the species of the same grouji, must assur- edly have existed; but the very process of natural si'leetion constantly tends, as has been so often remarked, to exterminate the parent-forms and the intermediate links. Consequently evidence of their former existence could be found only among fossil remains, Avhich are preserved, as we shall in a future* chapter attempt to show, in an extremely imperfect and inter- mittent record. CiiAi-. VI, OF TRANSITIONAL VARIETIES. 173 On the Origin and IW'nsifions of Orr/anlc Heings wifhjjecH' liar Habits and /Structure. It has been asked by the opponents of such views as I hold, how, for instance, a land carnivorous animal could have been converted into one with aquatic habits ; for how could tlu; ani- mal in its transitional state have subsisted ? It would be easy to show that within the same group carnivorous animals exist, having' every intermediate grade between truly aquatic and strictly terrestrial habits ; and as each exists by a struggle for life, it is clear that each is well adapted in its habits to its place in Nature. Look at the Mustela vison of North Amei-ica, which has webbed feet, and which resembles an otter in its fur, short legs, and form of tail; during summer this animal dives for and preys on fish, but during the long winter it leaves the frpzen waters, and iM'e3'S, like other polecats, on mice and land animals. If a different case had been taken, and it had 1)een asked how an insectivorous quadruped could possibly have been converted into a flying bat, the question would have been far more difficult to answer. Yet I think such difficulties have little weight. Here, as on other occasions, I lie imder a heavy disadvan- tage, for, out of the many striking cases which I have collected, I can give only one or two instances of transitional hal)its and structures in closely-allied species of the same genus; and of diversified habits, either constant or occasional, in the satiKj species. And it seems to me that nothing less than a long list of such cases is sufficient to lessen the difficulty in an}- ])ar- ticidar case like that of the bat. Ix)ok at the family of squirrels : here we have the finest gra- dation from animals with their tails only slightly flattened, and from others, as Hir J. IJichardson has remarked, with the pos- terior part of their bodies rather wide and with the skin on their flanks rather full, to the so-called flying-squirrels ; and fly- ing-squirrels have their limbs and even the base of the tail united l)y a broad expanse of skin, which serves as a parachute and allows them to glide through the air to an astonishing distance from tree to tree. We cannot doubt that each structure is of use to each kind of squirrel in its own country, by enabling it to escape birds or beasts of prey, or to coUect food moie quickly, or, as there is reason to believe, by lessming the dan- ger from occasional falls, lint it does not follow from this fact that the structure of each sfjuirrel is the best that it is possible l^.t TKANSITIOXS Chat. VI. lo conceive under all natural conditions. Lot the climate and vegetation change, let other coTnpcting rodents or new beasts of prey immigrate, or old ones become modified, and all anal- ogy would lead us to believe that some at least of the squirrels would decrease in numbers or become exterminated, unless they also became modified and improved in structure in a cor- responding manner. Therefore, I can see no difficulty, more especially under changing conditions of life, in the continued preservation of individuals with fuller and fuller flunk-mem- branes, each modification being useful, each being propagated, until, by the accumulated efl'ects of this process of natural selec- tion, a perfect so-called flying-squirrel was produced. Now look at the Galeopithecus, or flying lemur, which for- merly was ranked among bats. It has an extremely wide flank-membrane, stretching from the corners of the jaw to the tail, and including the limbs and the elongated fingers : the flank-membrane is, also, furnished with an extensor muscle. Although no graduated links of stracture, fitted for gliding through the air, now connect the Galeopithecus with the other LemuridcTJ, yet there is no difficulty in supposing that such links formerly existed, and that each had been formed by the same steps as in the case of the less perfectly gliding squurels ; and that each grade of structure was useful to its possessor. Nor can I see any insuperable difficulty in further belie\-ing it possi- ble that the membrane-connected fingers .and forearm of the Galeopithecus might be greatly lengthened by natural selec- tion ; and this, as far as the organs of flight 'are concerned, would convert it into a bat. In certain bats which have the wing-mem])rane extended from the top of the shoulder to the tail, including the hind legs, we perhaps yet see actual traces of an apparatus originally fitted for gliding through the air rather than for flight. If about a dozen genera of birds had become extinct or were unknown, who would have ventured to surmise that birds might have existed which used their wings solely as flappers, like the logger-headed duck (Micropterus of Eyton) ; as tins in the water and front legs on the land, like the penguin; as sails, like the ostrich ; and functionallj' for no purpose, like the Apteryx? Yet the structure of each of these birds is good for it, under the conditions of life to which it is exposed, for each has to live by a struggle ; but it is not necessarily the best possible under all possible conditions. It must not be inferred from these remarks tliat any of the grades of wing-structure Chap. VI. OF ORGANIC BEINGS. 175 here alluded to, ■\vliicli j)c*rliiips may all have resulted from dis- use, indicate the natural stops by whicli birds have acquired their perfect jK)wer of ilifj^ht ; but they serve, at least, to show what diversified means of transition are possible. Seeing that a few members of such water-breathing classes as tlie Crustacea and Mollusca are adapted to live on the land; and seeing that we have flying birds and mammals, lining in- sects of the most divei-sified types, and formerly had Hying rej)- tiles, it is conceivable that liying-flsh, Mhich now glide far through the air, slightly rising and turning by the aid of their fluttering llns, might liave been modified into perfectly-winged animals. K this had Ix^en effected, who would have ever im- agined that in an early ti-ansitional state they had been the in- habitants of the open ocean, and had used their incipientorgans of flight exclusively, as far as wc know, to escape being de- voured by other fish ? AVhen m'C see any structure liighlj' perfected for an}- par- ticular habit, as the wings of a bird for llight, we should bear in mind that animals displaying early transitional grades of the structure will seldom exist at the present day, lor they will have been supplanted by their successors, which Avere gradu- ally rendered more perfect through natural selection. Further- more, we may conclude that transitional states between struct- ures fitted for very difl'erent habits of life Avilkrarely have been developed at an early period in great numbers and under many subordinate forms. Thus, to return to our imaginary illustra- tion of the llying-fish, it does not seem probable that fishes capable of true flight would have been develoi^ed under many subordinate forms, for taking prey of many kinds in many ways, on the land and in the water, imtil their organs of flight had come to a high stage of perfection, so as to have given them a decided advantage over other animals in the battle for life. Hence the chance of discovering species with transition- al grades of structure in a fossil condition will always be less, from their having existed in lesser numbers, than in the case of species with fully-developed structures. I will now give two or three ihstances of diversified and of changed habits in the individuals of the same species. In either case it Avould be easy for natural selection to adapt the structure of the animal to its changed habits, or exclusively to one of its several habits. It is, however, dilhcult to decide, and immaterial for us, whether habits generally change first and structure afterward ; or Avhether slight modifications of struo 17G TRANSITIONS Chap. VI. turc lead to clian^ed habits ; botli probably often occurring- al- most sinuiltaneouslj. Of cases of champed habits it will sullice merely to allude to that of the many British insects which now- feed on exotic plants, or exclusively on artificial substances. Of diversified habits innumerable instances could Ixi given : I have often watched a tyrant flycatcher (Sauropha<^us sulphu- ratus) in South America, hovering over one spot and then proceeding to another, like a kestrel, and at other times stand- ing stationary on the margin of water, and then dashing into it like a kingfisher at a fish. In our own country the larger titmouse (Parus major) may be seen climbing branches, almost like a creeper ; it sometimes, like a shrike, kills small birds by blows on the head ; and I have many times seen and heard it hammering the seeds of the yew on a branch, and thus break- ing them like a nuthatch. In North xVmerica the black bear was seen by Hearne swimming for hours with widely-open mouth, thus catching, almost like a whale, insects in the water. As we sometimes see individuals of a species following habits widely different from those of their own species and of the other species of the same genus, "we might expect that such individuals Avould occasionally give rise to new sjiccies, having anomalous habits, and with their structure either slightly or considerably modified from that of their proper type. And such*nstancy natural selection, though insuperable by our imagination, cannot be considered real. How a nerve comes to be S(Misitive to light, hardly concerns us more than how life itself first originated ; but I may remark that, as some CuAP. VI. AND COMPLICATION. I79 of the lowest organisms, in Avbich nerves cannot be detected, are known to be sensitive to light, it does not seem impossible that certain elements in the sarcode, of which they are mainly composed, should become aggregated and developed into nerves endowed with this special sensibility. In searching for the gradations through which an organ in any species has been perfected, we ought to look exclusively to its lineal progenitors ; but this is scarcely ever possible, and we are forced to look to other species and genera of the same group, that is, to the collateral descendants from the same j)arent-form, in order to see what gradations arc possible, and for the cliance of some gradations liaving been transmitted in an unaltered or little altered condition. But the state of the organ even in distinct classes may incidentally throw light on the steps hy which it has been perfected in any one species. The simplest organ which can be called an eye consists of an optic nerve, surrounded by pigment-cells covered by trans- lucent skin, but without any lens or other refractive body. AVe may, however, according to M. Jourdain, descend even a step lower and find aggregates of pigment-cells, apparently serving as an organ of vision, but without any nerve, and resting merely on sarcodic tissue. Eyes of the above simple nature are not capable of distinct vision, and serve only to distinguish light from darkness. In certain starfishes, small depressions in the layer of pigment which surrounds tlie nerve are filled, as de- scribed by the author just quoted, Avith transparent gelatinous matter, projecting with a convex surface, like the cornea in the higher animals. He suggests that this serves not to form an image, but only to concentrate the luminous raj's and render their perception more easy. In this concentration of the rays we gain the first and by far the most important step toward the formation of a true, picture-forming eye ; for we have only to place the naked extremity of the optic nerve, which in some of the lower animals lies deeply buried in the body, and in some near the surface, at the right distance from the concentrat- ing ap]iaratus, and an image will be formed on it. In the great class of tlie Articulata, we may start from an optic nerve simply coatccl with pigment, the latter sometimes forming a sort of pupil, but destitute of a lens or other optical contrivance. "NVitii insects it is now known that the numer- ous facets on the cornea of the great comjiound eyes form true lenses, and that the cones include curiously-modified nervous filaments. But these organs in the Articulata are so ISO ORGANS OF EXTKEME PERFECTION Chap. VI. mucli diversified that Milller formerly made three main chxsses of compound eyes -with seven subdivisions, l^esidcs a fourth main chiss of ag-j^regated simple eyes. AVhcn we reflect on these facts, here f^ivcn too briefly, with respect to the wide, diversified, and graduated range of struct- in-e in the eyes of the lower animals ; and when we bear in mind how small the number of all the forms now living must he in comparison with those which have become extinct, the ditliculty ceases to be very great in believing that natural se- lection may have converted the simple apparatus of an optic nerve, coated with pigment and invested by transparent mem- brane, into an optical instrument as perfect as is possessed by an}' member of the great Articulate Class. He who will go thus far, ought not to hesitate to go one step further, if he finds on finishing this volume that large bod- ies of facts, otherwise inexplicable, can be explained by the theory of descent with modification ; he ought to admit that a structure even as perfect as an eagle's eye might be formed by natural selection, although in tliis case he does not know the transitional states. It has been objected that, in order to mod- ify the eye and still preserve it as a perfect instrument, many changes would have to be effected simultaneously, which, it is assumed, could not be done through natural selection ; but, as I have attempted to show in my work on the variation of do- mestic animals, it is not necessary to suppose that all the modi- fications were simultaneous, if tliey were extremely slight and gradual. Even in the most highly-organized division of the animal kingdom, namely, the Vcrtebrata, we can start from an eye so simple, that it consists, as in the lancelet, of a little sack of transparent skin, furnished with a nerve and lined with pigment, but destitute of any other apparatus. In both fishes and reptiles, as Owen has remarked, " the range of gradations of dioptric structures is very great." It is a significant fiict that even in man, according to tlie high authority of Virchow, tlic beautiful crj^stalline lens is formed in tlie embryo by an accumulation of epidermic cells, lying in a sack-like fold of the skin ; and tlie vitreous body is formed from embryonic subcu- taneous tissue?. It is indeed indispensable, in order to arrive at a just conclusion regarding the formation of the eye, with all its marvellously perfect characters, that the reason should con- quer the imagination ; but I have felt this difficulty far too keenly to be surprised at any degree of hesitation in extend ing the principle of natural selection to so startling a length. CiiAr. VI. AND COMPLICATION. 181 It is scarcely possible to avoid comparing tlic eye with a telescope. We know tliat tliis instrument lias been peri'eeted by the long-continued ell'orts of the highest human intellects ; and we natm-ally infer that the eye has been formed by a some- what analogous process. But may not this inference be pre- sumptuous ? Have we any right to assume that the Creator works by intellectual powers like those of man? If we must compare the e^'e to an ojjtical instrument, we ought in imagi- nation to take a thick layer of transparent tissue, with spaces tilled with fluid, and with a nerve sensitive to light beneatlt, and then suppose every ])art of this layer to be continually changing slowly in d(Misity, so as to separate into layers of dillerent densities and thicknesses, placed at different distances from each other, and with the surfaces of each layer slowly changing in form. Further, Ave must suppose that there is a power, r(>presented by natural selection or the survival of the Mttest, always intently watching each sliglit alteration in the transparent la^'ers ; and carefully preserving eaoli which, undc^' varied circumstances, in any way or in any degree, tends to jiroduce a distincter image. We must suppose each new state of the instrument to be multiplied by the million; each to be preserved until a better one is produced, and then the old ones to be all destroj-ed. In living bodies, variation will cause the slight alterations, generation will multiply them almost infi- nitely, and natural selection Avill pick out Avith unerring skill each improvement. Let this process go on for millions of years ; and during each year on millions of individuals of many kinds; and may we not believe that a living optical instrument might tluis be formed as superior to one of glass, as the works of the Creator are to those of man ? Modes of Transition. If it could be demonstrated that any com])lex organ existed, which could not possibly have been formed by numerous, suc- cessive, slight modifications, my theor}- would absolutely break down. But I can find out no such case. No doubt manv or- gans exist of which we do not know the transitional grades, more especially if we look to much-isolated spt^cies, roimd which, according to the theory, there has been nujch extinction. Or again, if we take an organ common to all the members of a large class, for in this latter case the organ must have been originally fonnod at an extremely remote period, since which 1S2 MODES OF TRANSITION. Chap. VI. all the mail)'- members of the class have been developed ; and, iu order to discover the early transitional grades through which the organ has passed, Ave should have to look to very ancient ancestral forms, long since become extinct. We should be extremely cautious in concluding that an or- gan could not have been formed by transitional gradations of some kind. Numerous cases could be given among the lower animals of the same organ performing at the same time wholly distinct functions ; thus the alimentary canal respires, digests, and excretes, in the larva of the dragon-fly and in the fish Co- bites. In the Hydra, the animal may be turned inside out, and the exterior surface will then digest and the stomach respire- In such cases natural selection might specialize, if any advan- tage were thus gained, the whole or part of an organ, Avhich had previously performed two functions, for one function alone, and thus by insensible steps greatly change its nature. Many plants are known which regularly produce at the same time differently-constructed flowers ; and if such plants were to pro- duce one kind alone, a great change would in some cases be effected in the character of the species. It can also be shown that the pi'oduction of the two sorts of flowers by the same plant has been effected by finely-graduated steps. Again, two distinct organs in the same individual may simultaneously per- form the same function, and this is a highly-imj^ortant means of transition : to give one instance — there are fish with gills or branchiae that breathe the air dissolved in the water, at the same time that they breathe free air in their swim-bladders, this latter organ being divided by highly-vascular partitions, and having a ductus pneumaticus for the supply of air. To give another instance from the vegetable kingdom: plants climb l^y three distinct means, by spirally twining, by clasping a support with their sensitive tendrils, and by the emission of aerial rootlets ; tliese three means are usually found in distinct groups, but some few plants exhibit two of the means, or even all three, combined in ttie same individual. In all such cases one of the two organs for performing the same function might be modified and perfected so as to ])erf()rm all the work, being aided during the progress of modification by the other organ ; and then this other organ might be modified for some other and quite distinct purpose, or be wholly obliterated. The illustration of the swim-l)ladder in fishes is a good one, becausi? it shows us clearly the higher important fact that an organ originally constructed for one purpose, namely flotation, CiiAr. VI. JIODES OF TRANSITION. 183 may be converted into one for a widcly-tliffercnt purpose, namely, respiration. Tlie swim-bladder has, also, been worked in as an accessory to the auditory org'ans of certain hsh. All physiolog^ists admit that the swim-bladder is homologous, or "ideally similar" in position and structure with the lungs of the higher vertebrate animals : hence there is no reason to doubt that the swim-bladder has actually been converted into lungs, or an organ used exclusively for respiration. According to this view it may be inferred that all vertebrate animals with true lungs have descended by ordinary generation from an ancient and unknown prototype, which was furnished witii a floating apparatus or swim-bladder. We can thus, as I infer from Owen's interesting description of these parts, under- sta,nd the strange fact that every particle of food and drink which we swallow has to pass over the orifice of the trachi^a, with some risk of falling into the lungs, notwithstanding the beautiful contrivance by which the glottis is closed. In the higlier Vertebrata the branchiaj have wholly disappeared — but in the embryo the slits on the sides of the neck and the loop- like course of the arteries still mark tlunr former position. But it is conceivable that the now utterly lost branchiae might have been gradually worked in by natural selection for some distinct purpose : for instance, the branchiir; and dorsal scales of Anne- lids are believed to be homologous with the wings and wing- covcrs of insects, and it is not improbable that with our existing insects organs, which at an ancient period served for respiration, have actually been converted into organs of flight. In considering transitions of organs, it is so important to bear in mind the probability of conversion from one function to another, that I will give another instance. Pedunculated cir- ripcdes have two minute folds of skin, called by me the ovigerous frena, Avhidi serve, through the means of a sticky secretion, to retain the eggs until they are hatched within the; sack. These cirri|)edes have no branclu;ii, the whole surface of the body and of the sack, together with the small frena, serving for respira- tion. Tiic 13alanid;\3 or sessih; cirripedes, on the other hand, have no ovigerous frena, the eggs lying loose at the bottom of the sack, within the well-enclosed shell ; but they iiave, in the same relative position with the frena, large, much-fokU'd mem- branes, which freely communicate with the circulatory lacunixj of the sack and body, and which have been considered to be branchiaj by Prof. Owen and by all other naturalists who have treated on the sul)ject. Now I think no one will dispute that 184 DIFFICULTIES OF THE THEORY Chap. VI. the ovigcrous frcna in the one family arc strictly homologous ■with the braiicliiiu of tlic other family; indeed, they graduate into each other. Therefore it need not be doubted that the two little folds of skin, -which originally served as ovigerous frena, but -which, likewise, very slightly aid in the act of res- piration, have been gradually con\erte(l by natural selection into branchije, simply through an increase in their size and the obliteration of their adhesive glands. If all pedunculated cir- rijiedes had become extinct, and they have already suffered far more extinction than have sessile cirripedes, who Avonld ever have imagined that the branchioi in this latter family had originally existed as organs for pre\'cnting the ova from being washed out of the sack ? iSj^ecial Difficulties of the, Theory of Natural Selection. Although -we must be extremely cautious in concluding that any organ could not have been produced by successive transitional gradations, yet vmdoubtedly serious cases of difli- culty occur, some of which will be discussed in my fiiture work. One of the most serious is that of neuter insects, which are often diiFcrently constructed from either the males or fertile females; but this case will be treated of in the next chapter. Tlie electric organs of fishes offer another case of special diffi- culty ; for it is impossible to conceive by what steps these Avondrous organs have been produced. As Owen has remarked, there is much analogy between them and ordinary muscles, in their manner of action, in the influence on them of the nervous power and of stimulants svich as strychnine, and as some be- lieve in their intimate structure. M^e do not even know of -what use these organs are ; though in the Gjinnotus and Tor- pedo they no doubt serve as powerful means of defence and perhaps for securing prey; 3'et in the Kay an analogous organ in the tail, even when greatly irritated, manifests, as lately observed by Matteuccu, but little electricity — so little that it can hardl}^ be of use for such purposes. ^Moreover, in the Kay, besides the organ just referred to, there is, as Dr. K. ^McDonnell has shown, another organ near the head, not known to be elec- trical, but which appears to be the real homologue of the elec- tric batter}' in the torpedo. Lastly, as we know nothing about the lineal progenitors of these fishes, it must be admitted that we are too ignorant to be enabled to aflirm that no transitions arc possible, through which the electric organs might have been developed. CiiAi-. YI. OF NATURAL SELECTION. 185 Tliesc same organs at first appear to offer anotncr and far more serious difficulty ; for they occur in about a dozen kinds of fisli, of Avliich. several arc Avidely remote in their affinities. Generally when the same organ is found in several niemhers of the same class, especially if in mc^mbers having very differ- ent habits of life, we may attribute its presence to inheritance from a common ancestor; and its absence in some of the mem- bers to loss through disuse or natural selection. So that, if the electric organs had been inherited from some one ancient progenitor, we might have expected that all electric fishes would have been specially related to each other; but this is far from the case. Nor does geology at all lead to the Ijclief that most fishes formerly possessed electric organs, which their modified descendants have now lost. But when we look at the subject more closely, wc find in the several fishes provided with electric organs that these are situated in diiferent parts of the body — that they differ in construction, as in the arrange- ment of the plates, and according to Pacini, in the process or means by which the electricity is excited — and lastly, in the requisite nervous power being supplied through different nerves from widely-diffi;rent sources, and this is perhaps the most im- ])ortant of all the diflerences. Hence in the several remotely- atlied fishes furnished with electric organs, these cannot be ccnisidered as homologous, but only as analogous in function. Consequently there is no reason to suppose that they have been inherited from a common progenitor ; for, had this been the case, they would have closely resembled each other in all respects. Thus tlic greater difficulty disappears, leaving only the lesser yet still great difliculty ; namely, by what graduated steps these organs have arisen and been develojied in each sepa- rate grouji of fishes. Tiic luminous organs which occur only in a few insects, belonging to widely-different fiunilies and orders, and v.hich arc situated in different jiarts of the body, offer a difficidty almost exactly jiaralhd witli tiiat of the electric organs. Other cases could be given; for instance, in plants, the very curious contrivance of a mass of pollen-grains, borne on a foot-stalk with an adhesive gland, is apparently the same in Orchis and Asclcpias — genera almost as remote as is possible among flowering ])lants. In all such cases of two species, fiir removed from each other in the scale of organization, being furnislied with similar anomalous organs, it should be observed that, al- though the general appearance and function of the organ may 180 DIFFICULTIES OF THE THEORY Ciur. VI. 1)0 iilcMitically the saiiie, yet some fundamental difference be- tween them can always, or almost always, be detected. I am inclined to believe that, in the same manner as two inen have sometimes independently hit on the same invention, so natural selection, working for tlie good of each being and taking ad- vantage of analogous variations, has sometimes modified in nearly the same way two organs in two distinct organic beings, which owe but little of tlieir structure in common to inheritance from a common progenitor. Fritz MllUcr, in a remarkable work recently published, has investigated a nearly parallel case, in order to test the views advanced in this volume. Several families of crustaceans in- clude a few species which possess an air-breathing apparatus, and are fitted to live out of the water. In two of these fam- ilies, which -were more especially examined by Miiller, and which are nearly related to each other, the species agree most closely in all important characters ; namely, in their sense-or- gans, circulating system, in the position of the tufts of hair with whicli their complex stomachs arc lined, and lastly in the v/hole structure of the Avater-breatliing Ijranchia^, even to the microscopical hooks by Avhich they arc cleansed. Hence it might have been expected that the equally important air- breathing apparatus would have been the same in the fevv^ species in both families Avhich live on the land ; and this might Iiavc been tlie more confidently expected by those wlio believe in distinct creations ; for why sliould this one apparatus, given for the same special piu^ose to these species, have been made to differ, Avhile all the other important organs are closely simi- lar or rather identical ? Fritz Mllller argues that this close similarity in so many points of structure must, in accordance with the views ad- vanced by me, be accounted for by inheritance from a common progenitor. But as the vast majority of the species in tlie above two families, as well as most crustaceans of all orders, are aquatic in their habits, it is improbable in the highest degree, that their common progenitor should have been adapted for breathing air. Miiller was thus led carefully to examine the apparatus in the air-breathing species; and in each he found it to diffor in several important points, as in the position of the orifices, in the manner in which they are opened and closed, and in some accessory details. Now such differences are intelli- gible, and might even have been anticipated, on the siip]iositi<-)n that species belonging to distinct families had slowly become Chap. VI. OF NATURAL SELECTION. 1S7 adapted to live more and more out of "water, and to breathe the air. For these species, from beloiif^ing to distinct families, ■would difler to a certain extent, and in accordance with the principle that the nature of each variation depends on two fac- tors, viz., the nature of the orp^anism and that of the condi- tions, the variability of these crustaceans assuredly would not have been exactly the same. Consequently natural selection would have had diiTerent materials or variations to work on, in order tu arrive at the same functional result; and the structures thus acquired would almost necessarily have dillered. On the hypotliesis of 6C]>arate acts of creation the whole case remains unintelli,Lrible. The above line of arn^mcnt, as advanced by Fritz Miiller, seems to have had j^reat weiglit in leadino^ this distin<^uislicd naturalist to accept the views maintained by me in this volume. • In the several cases just discussed, wc have seen that in beings more or less remotely allied, the same end is gained and the same function performed by organs in appearance, though not in truth, closely similar. But the common rule throughout Nature is that the same end is gained, even some- times in the case of beings closely related to each other, by the most diversified means. How differently constructed is the feathered wing of a bird and the membrane-covered yying of a l)at with all the digits largely developed ; and still more so the four Avings of a butterfly, the two wings of a fly, and the two wings of a beetle, together with the elytra ! 13ivalve shells are made to open and shut, but on what a immber of ])atterns is the hinge constructed, from the long row of neatly inter- locking teetli in a Nucula to the simple ligament of a Mussel ! tSeeds arc disseminated by their minuteness — by their capsule being converted into a light balloon-like envelope — by being- embedded in pulp or flcsli, formed of tlie most diverse parts, and rendered nutritious, as well as conspicuously colored, so as to attract and be devoured by birds — ])y having hooks and grapnels of many kinds and serrated awns, so as to adhere to the fur of quadmpeds — and by being furnish(>d Avith wings and plumes, as different in shape as elegant in structure, so as to be wafted by every breez(\ I will give one other instance ; for this subject of the same end being gained by the niost di- versified means well deserves attention. Some authors main- tain that organic beings have been formed in many ways for the sake of mere variety, almost like toys in a shop, but such a 188 DIFFICULTIES OF THE THEOKY Chap. VI. view of Nature is incrediljlo. Witli plants having' scjiarated Bcxcs, and witli those in whicli, tlious-li herniaplnodites, the pollen docs not spontaneously fall on tlie stig'ma, some aid is necessary for their fertilization. AVith several kinds this is cHected by the polli'U-yrains, which are lig'ht and incoherent, Ix'ino; blown 1)y the wind through mere chance on to the stigma ; and this is the simj)lest plan which can well be conceived. An almost equally simple, though very different, plan occurs in many plants in which a symmetrical flower secretes a few drops of nectar, and is consequently visited by insects ; and these carry the pollen from the anthers to the stigma. From this simple stage we may pass through an inexhaust- ible number of contrivances, all for the same purpose and cflected in essentially the same manner, but entailing changes in every part of the llower. The nectar may be stored in vari- ously-shaped receptacles, with the stamens and pistils modi- fied in many ways, sometimes forming traj)-like contrivances, and sometimes capable of neatly-adapted movements through irritability or elasticity. From such structures we may ad- vance till Ave come to such a case of extraordinary adaptation as that lately described by Dr. Crugcr in the Coryanthcs. Tliis orchid has jiart of its labellum or lower lip hollowed out into a great bucket, into which drops of almost pure water contin- ually fall from two secreting horns which stand above it ; and v.hen the bucket is half full, the water overflows by a spout on one side. The basal part of the labellum stands over the bucket, and is itself hollowed out into a sort of chamber with two lateral entrances ; Avithin tliis chamber there are curious fleshy ridges. The most ingenious man, if he had not Avit- nessed Avhat takes ])lace, could never liaA^e imagined Avhat pur- pose all these parts serve. But Dr. Crtiger saAV croAvds of large humble-bees A'isiting the gigantic flowers of this orchid, not in order to suck nectar, but to gnaAV ofi" the ridges Avithin the chamber aboA^e the bucket ; in doing this they frequently I)ushe(l each other into the bucket, and their wings being thus Avetted they could not fly away, liut had to craAvl out through the passage formed by tlie spout or OA-erflow. Dr. Crllger saw a "continual procession" of bees thus crawling out of their in- A'oluntary bath. The passage is narroAver, antl is roofed over by the column, so that a bee, in forcing its Avay out, first rubs its back against the viscid stigma and then against the viscid glands of the pollen-masses. The jiollen-masses are thus glued to the back of that bee Avhich first happens to craAvl out through CiiAP. VI. OF NATURAL SELECTION. 189 the passage of a litely-cxpandcd flower, and arc thus carried away. Dr. Cril<^('r sent ine a (lower in spirits of wine, with a bee which he liad killed before it liad ([uite crawled ont with a pollen-mass still fastened to its back. When the bee, thus provided, (lies to another (lower, or to the same flower a sec- ond time, and is jnished by its comrades into the bucket and then crawls out by the iiassag-e, the pollen-mass necessarily comes first into contact with the viscid stig'ma, and adheres to it, and the (lower is fertilized. Now at last we see the full use of every part of the flower, of the water-secreting horns, of the bucket half full of water, wliicli prevents the bees from flving away and forces them to crawl out through the spout, and lub against the properly-placed viscid pollen-masses and viscid stigma. The construction of tlie (lower in another closely-allied or- chid, namely, the Catasetum, is widely dilVerent, though serving the same end; and is equally curious. Bees visit these flow- ers, like those of the Coryanthes, in order to gnaw the label- lum ; in doing this they inevitably touch a long, tapering, sensitive projection, or, as I have called it, the antenna. This antenna, when touched, transmits a sensation or vibration to a certain membrane which is instantly ruptured ; this sets free a spring by which the pollen-mass is shot forth, like an arrow, in the right direction, and adheres by its viscid extremity'' to the back of the bee. The pollen-mass of a male plant is thus carried to the flower of a female plant, where it is brought into contact with the stigma, which is A'iscid enough to break cer- tain elastic threads, and, retaining the pollen, fertilization is elTected. How, it may be asked, in the foregoing and in innumerable other instances, can we imderstand the graduated scale of complexity and the multifarious means for gaining the same end V The answer no doubt is, as already remarked, that when two forms vary, which already diller from each other even in a slight degree, tht; variability will not be of the same exact na- ture, and conseciuently the results obtained through natural selection for the same general ])urpose will not be the same. We should also bear in mind that every highly-developed or- ganism has passed through a long course of modilication; and that each moditied structure tends to be inherited, so that it will not readily be wholly lost, but may be modified again and again. Hence the structure of each part of each species, for whatever purpose used, is the sum of the many inherited 100 OEGANS OF LITTLE IMPORTANCE Chap. VL changes, through which that species has passed during its suc- cessive adaptations to changed habits and conditions of life. Finally, then, although in many cases it is most difficult even to conjecture by what transitions many organs have ar- rived at their present state; yet, considering how small the proportion of living and known forms is to the extinct and un- known, I have been astonished how rarely an organ can be named, toward which no transitional grade is known to lead. It certainly is true that new organs, appearing as if specially created for some purjDOse, rarely or never appear suddenly in any class ; as indeed is sho^\'n by that old, but somewhat ex- aggerated, canon in natural history of " Natura non facit sal- tum." Vio meet with this admission in the writings of almost every experienced naturalist ; or, as Milne Edwards has well expressed it, Nature is prodigal in variety, but niggard in inno- " vation. Why, on the theory of Creation, should there be so much variety and so little novelty ? "Why should all the parts and organs of many independent beings, each supposed to have been separately created for its proper place in Nature, be so commonly linked together by graduated steps ? Why should not Nature take a sudden leap from structm-e to struct- ure ? On the theory of natural selection, we can clearly un- derstand why she should not; for natural selection acts only by taking advantage of slight successive variations ; she can never take a sudden leap, but must advance by short and sure though slow steps. Organs of little apparent Importance^ as affected by Natural Selection. As natural selection acts by life and death — by the survival of the fittest, and by the destruction of the less wcll-litted indi- viduals— I have sometimes felt great dilllculty in understand- ing the origin or formation of parts of little importance ; al- most as great, though of a very different kind, as in the case of the most perfect and complex organs. In the fii-st place, we are much too ignorant in regard to the whole economy of any one organic being, to say what slight modifications would be of importance or not. In a for- mer chapter I have given instances of very trifling characters, such as tlie down on fruit and the color of its ilesh, the color of the skin and hair of quailrupeds, which, from being corre- lated with constitutional differences or from determining the Chap. VI. AFFECTED BY NATURAL SELECTION. 191 attacks of insects, mif^lit assuredly he acted on by natural se- lection. Tlie tail of tiie g^iraffc looks like an artificially-con- structed fly-flapper; and it seems at first incredible that tliis could have been adapteil for its present purpose by successive slight niodiiications, each better and better lifted, for so triliino; an object as to drive away flies ; yet avc should pause before being too positive even in this case, for we know that the dis- tribution and existence of cattle and other animals in South America absolutely depend on their power of resisting the at- tacks of insects : so that individuals which could by any means defend themselves from these small enemies, would be able to range into new pastures and thus gain a great advantage. It is not that the larger (juadrupeds are actually destroyed (exceiDt in some rare cases) by flies, but they are incessantly harassed and their strength reduced, so that they are more subject to disease, or not so well enabled in a coming dearth to search for food, or to escape from beasts of pre}'. Organs now of trifling importance have probably in some cases been of high imjiortance to an early progenitor, and, after having been slowly perfected at a former period, have been transmitted to existing species in nearly the same state, although now of ver}' slight use ; but any actually injurious deviations in their structure will of course have been checked by natiu'al selection. Seeing how important an organ of loco- motion the tail is in most aquatic animals, its general prcsenco and use for many purposes in so many land animals, which in their lungs or modified swim-bladders betray their aquatic ori- gin, may perhaps be thus accounted for. A well-developed tail having been formed in an aquatic animal, it might subse- quently come to be worked in for all sorts of purposes, as a Ily- flapper, an organ of prehension, or as an aid in turning, as with the dog, though the aid in this latter respect nuist be slight, for the hare, with hardly any tail, can double cjuickly eTiough. In the second ])lace, ^ve may sometimes wrongly attribute im]X)rtance to characters which have originated from quiti; sec- ondary causes, independently of natural selection. We should remember that climate, food, etc., probably have had some, ])erhaps a considerable, direct influence on the organization ; that characters reajjpear from the law of reversion ; that ct>r- relation is an important element of change; and finallv, that sexual selection has often largely modified the external i-Iiarac- ters of the higher animals, so as to give one male an advantage in fighting witli other males, or in charming the female ; and 192 ORGA^'S OF LITTLE LMPOKTANCE Chap. VL characters gained through sexual selection may be transmitted to both sexes. Moreover, a modification, caused in any of tlic above specified ways, may at first have been of no direct ad- vantage to a species, ])ut may subsequently have been taken advantage of by its descendants under new conditions of life and newly-acquired luibits. If, for instance, green woodpeckers alone had existed, and "\ve did not know that there Avere many black and pied kinds, I dare say that we should have thought that the green color was a beautiful adaptation to hide this tree-frequenting bird from its enemies ; and consequently that it was a character of importance and had been acquired through natural selection ; as it is, the color is probably in chief part due to sexual selec- tion. A trailing palm in the Malay Archipelago climbs tlie loftiest trees by the aid of exquisitely-constructed hooks clus- tered around the ends of the branches, and this contrivance, no doubt, is of the highest service to the plant ; but as we see nearly similar hooks on many trees which are not climbers, and which there is reason to l:)elieve, from tlie distribution of the thorn-bearing species in Africa and South America, serves as a defence against browsing quadrupeds, so tlie hooks on the palm may first have been developed for this object, and subse- quently been taken advantage of by the plant as it imderwent further modification and became a climber. The naked skin on the head of a vulture is generally considered as a direct adaptation for wallowing in putridity ; and so it may be, or it may possibly be due to tlie direct action of putrid matter ; but we should be very cautious in drawing any such inference, when we see that the skin on the head of the clean-feeding male Turkey is likewise naked. The sutures in the skulls of young mammals have been advanced as a beautiful adapta- tion for aiding parturition, and no doubt they facilitate, or may be indispensable for this act ; but as sutures occur in the skulls of young birds and reptiles, Avliich have only to escape from a broken C(i;ji:, we may infer that this structure has arisen from the laws of growth, and has been taken advantage of in the parturition of the higher animals. We are profoundly ignorant of the cause of each slight va- riation or individual diflbrence ; and we are immediately made conscious of this by reflecting on the diiferences in the breeds of our domesticated animals in different countries — more espc- ially in the less civilized countries where there has been but littie methodical selection. Animals kept by savages in difler- Chap. YI. AFFECTED BY NATUKAL SELECTION. 193 ent countries often have to struggle for their own subsistence, and are exposed to a certain extent to natural selection, and individuals witli sliglitly-dillerent constitutions would succeed best under dilTerent climates. A good observer states tliat in cattle susceptibility to the attacks of flies is correlated with color, as is the liability to be poisoned by certain plants ; so that even color would be thus subjected to the action of nat- ural selection. Other observers are convinced that a damp climate affects the growth of the hair, and tliat with the hair the horns are correlated. Mountain-breeds alwaj-s differ from lowland-breeds ; and a mountainous country would probably affect the hind-limbs fmm exercising them more, and possibly even the form of the jielvis ; and then, by the law of homolo- gous variation, the fiont-liml)s and the head would probably be affected. Tlie shape, also, of the pelvis might affect by press- ure the shape of certain parts of the young in the womb. The laborious breathing necessary in high regions would, we have reason to believe, increase the size of the chest ; and again- correlation would come into play. The effects on the whole organization of lessened exercise, together with abun- dant food, is probably still more important ; and this, as H. von Nathusius has lately shown in his excellent Treatise, is appar- ently one chief cause of the great modilication which the breeds of swine have undergone. But Ave are far too ignorant to speculate on the relative importance of the several known and unknown causes of variation ; and I have made these remarks only to show that, if Ave are unable to account for the characteristic differences of our domestic breeds, which never- theless are generally admitted to have arisen through ordinary generation from one or a few parent-stocks, we ought not to lay too much stress on our ignorance of the precise cause of the sliglit analogous differences between species. I might have adduced for this same purpose the dilferences between the races of man, which are so strongly marked ; I may add that some light can apparently be thrown on these differences, through sexual selection of a particular kind, but without entering on full details my reasoning Avould appear frivolous. Uillliarian Doctrine howfac true: JJcauti/ /tow acquired. The foregoing remarks lead me to say a few Avords on the protest, lately made by some naturalists, against th«> utilitarian doctrine that CA'cry detail of structure has been produced for 9 191 • UTILITARIAN DOCTRINE IIOW FAR TRUE: Chap. VI. the good of its possessor. Tliey believe that many structures have been created for l)eauty in the eyes of man, or, as already discussed, for the sake of mere variety. Such doctrines, if true, Avould be aljsolulely fatal to my theory. Yet I fully admit that many structures are now of no direct use to their possessors, and may never have been of an}- use to their pro- genitors. No doubt, as recently remarked, the definite action of changed conditions, correlated variation, and reversion, have all produced their efTects. But the most important consider- ation is that the chief part of the organization of every liv- ing creature is simply due to inheritance; and consequently, though each assuredly is well fitted for its place in Nature, many structures now have no direct relation to existing habits of life. Thus, we can hardly believe that the webbed feet of the upland goose or of the frigate-bird are of special use to these birds ; we cannot believe that the similar bones in the arm of the monkey, in the fore-leg of the horse, in the wing of the bat, and in the flipper of the seal, are of special use to these animals. We may safely attribute these structures to inheritance. But to the progenitor of the upland goose and of the frigate-bird, webbed feet no doubt were as useful as they now are to the most aquatic of living birds. So we ma^' believe that the progenitor of the seal did not possess a flipper, but a foot with live toes fitted for walking or grasping; and we may further venture to believe that the several bones in the liml)s of the monkey, horse, and bat, which have been in- herited from some ancient progenitor, were formerly of more special use than they now are to these animals M'ith their Avidely-diversificd habits, and might consequently have been modilied through natural selection, M:>king due allowance for the definite action of changed conditions, correlation, rever- sion, etc., we may conclude that every detail of structure in every living creature is either now or was formerly of use — directly or indirectly through the comj^lex laws of growth. With respect to the belief that organic beings have been created beautiful for the delight of man — a view which it has been pronounced may safely be accepted as true, and as sub- versive of my whole theory — I may first remark that the idea of the beauty of any object obviously d(^pends on the mind of man, irrespective of any real quality in the a(hnircd object ; and that the idea is not an innate and imalterable element in the mind. We see this in men of ditlerent races admiring an en- tirely diirerent standard of beauty in their women ; neither the Cii-vr. VI. BEAUTY HOW ACQUU4ED. 195 Negro nor the Chinese admires the Caucasian bcau-idcal. The idea also of picturesque beauty in scenery has arisen only within modern times. On the view of beautiful objects havinj^ been created for man's gratification, it ought to be shown that there was less beauty on tlie face of the earth before man appeared than since he canae on the stage. Were the l^cautiful volute and cone shells of the Eocene epoch, and the gracefully-sculi> tured ammonites of the Secondary period, created that man might ages afterward admire them in his cabinet ? Few ob- jects are more beautiful than the minute siliceous cases of the diatomacea? : were these created that they might be examined and admired xmder the higher powers of the microscope? The beauty in this latter case, and in many others, is apparently wholly due to symmetry of growth. Flowers rank among the most beautiful productions of Nature ; and they have become through natural selection beautiful, or rather conspicuous in 'contrast with the green leaves, that they might easily be ob- served and visited by insects, so that their fertilization miglit b'3 favored. I have come to this conclusion from finding it an invariable rule that when a flower is fertilized by the wind it never has a gayly-colored corolla. Again, several plants habitu- ally produce two kinds of flowers : one kind open and colored so as to attract insects; the other closed and iiot colored, desti- tute of nectar, and never \-isited by insects. Hence we may conclude that, if insects had never existed on the face of the earth, the vegetation would not have been decked Avith beauti- ful flowers, but Avould have produced only such poor flowers as we now see on our firs, oaks, nut and ash trees, on grasses, spinach, docks, and nettles. A similar line of argument holds good with the many kinds of beautiful fruits ; that a ripe straw- berry or cherry is as pleasing to the eye as to the palat<% that the gayly-colored fruit of the spindle-wood-tree and the scarlet berries of the holly are beautiful objects, will be admitted by every one. But this beauty serves merely as a guide to birds and beasts, that the fruit may be devoured and the manured seeds thus disscminatod : I infer that this is the case from havhig as yet found in every instance that seeds, Avhich are embedded within a fruit of any king, tliat is witliin a fleshy or pulpy envel- ope, if it be colored of any brilliant tint, or merely rendered conspicuous by being white or black, are always disseminated by being first devoured. On the other hand, I willinglv admit that a great nuinlKT of male animals, as all our ir.< st jrorgcous birds, some f\^he3 190 UTILITARIAN DOCTRINE HOW FAR TRUE: Chap. VI. some maimnals, and a host of majTnificently-colored butterflies and some other insects, have been rendered beautiful for beauty's sake ; but this has Ijcen cfTected not for the dehf^ht of man, but throup^'h sexual selection, that is, from the more beautiful males liaving been continually preferred by their less ornamented females. So it is Avith the music of birds. We may infer from all this that a similar taste for lieautiful colors ami for musical sounds runs throug-h a large part of the animal kingilom. AVhen tlie female is as beautifully colored as the male, Avhich is not rarely the case with birds and butterflies, the cause simply lies in the colors acquired throuo-h sexual se- lection having been transmitted to both sexes, instead of to the males alone. In some instances, however, the acquirement of conspicuous colors by the female may have been checked through natural selection, on account of the danger to which she would thus have been exposed during incubation. Natural selection cannot possibly produce any modification in any one species exclusively for the good of another species ; though throughout Nature one species incessantly takes advan- tage of, and profits by, the structure of others. But natural selection can and does often produce structures for the direct injury of otlicr animals, as we see in the fang of the adder, and in the ovipositor of the ichneumon, by which its eggs are depos- ited in the living bodies of other insects. If it could be proved that any part of the structure of any one species had been formed for the exclusive good of another species, it would annihilate my theory, for such could not have been produced through natural selection. Although many statements may be found in Avorks on natural history to tliis eflcct, I cannot find even one which seems to me of any weight. It is admitted that the rattlesnake has a poison-fang for its own defence and for the destruction of its prey ; but some authors supjiose that at the same time this snake is furnished with a rattle for its own injury, namely, to warn its prey to escape. I would almost as soon believe that the cat curls the end of its tail wlicn ])reparing to spring, in order to warn the doomed mouse. But I have not space here to enter on this and other such cases. Natural selection will never produce in a being any thing injurious to itself, for natural selection acts solely by and for the good of each. No organ will be formed, as Palcy has remarked, for the purpose of causing pain or fordoing an injury to its possessor. If a fivir balance be struck between the good a-.id evil caused by each part, each will 1w found on the whole Chap. VI. BE.VUTY HOW ACQUIKKD. 197 advantagccous. After the lapse of time, under changing con- ditions of life, if any part comes to be injurious, it will be modi- fied; or, if it be not so, the being will become extinct, as myriads have become extinct. Natural selection tends only to make each organic being as perfect as, or sliglitly more perfect than, the other inliabitants of the same country with which it has to struggle for existence. And we see that this is the degree of perfection attained under nature. Tlie endemic productions of New Zealand, for instance, are perfect one compared with another; but tlicy are now rapidly yielding before the advancing legions of ])lants and animals introduced from Europe. Natural selection will nc»t produce absolute perfection, nor do we always meet, as far as we can judge, with tliis high standard under nature. The cor- rection for the al)erration of light is said by !Mliller not to be perfect even in that most perfect organ, the human eye. If our •reason leads us to admire with enthusiasm a multitude of in- imitable contrivances in nature, this same reason tells us, though we may easily err on both sides, that some other contrivances are less perfect. Can we consider the sting of the bee as per- fect, which, when used against many attacking animals, cannot be withdrawn, owing to the backward scrratures, and so inevi tably causes the death of the insect by tearing out its viscera ? if we look at the sting of the bee, as having originally ex- isted in a remote progenitor as a boring and serrated instru- ment, like that in so many members of the same great order, and wliich has been modified but not perfected for its present purjiose, with the poison originally adapted for some other pur[)ose, such as to produce galls, subsequently intensified, mo can ]ierhaps understand how it is that the use of the sting should so often cause the insect's own death : for if on the whole the power of stinging be useful to the social conununity, it will fulfil all the requirements of natural selection, though it may cause the death of some few members. If we admire the truly wonderful power of scent by which the males of many insects find their females, can we admire the production for this single purpose of thousands of drones, which are utterly useless to the community for any other purpose, and which are ultimately slaughtered by their industrious and sterile sisters ? It may be ditVicult, Init we ought to admire the .savage instinctive hatred of the queen-bee, which urges her to destroy the young queens her daughters as .soon as l)orn, or to perish herself in the combat ; for undoubtedly this is for the good of the community ; and 108 SUMMARY. Cuap. VI. inatcriKil love or maternal hatred, though the latter fortunately is most rare, is all the same to the inexr)rablc principle of natu- ral selection. If we admire the several ini^enious contrivances, by which orchids and many other plants are fertilized through insect agency, can we consider as equally perfect the elabora- tion of dense clouds of pollen by our fir-trees, so that a few granules may be wafted by a chance breeze on to the ovules ? Summary : tJie Law of Unity of Type and oftJie Conditions of Existence embraced by the Theory of JVcitural Se- lection. We have in this chapter discussed some of the difficulties and objections which may be urged against the theory. Many of them are serious ; but I think that in the discussion light has been thrown on several facts, which on the belief of indepen- dent acts of creation are uttercly obscure. We have seen that species at any one period are not indefinitely variable, and are not linked together by a multitude of intermediate gradations, partly because the process of natural selection will always be very slow, and will act, at any one time, only on a few forms ; and partly because the very process of natural selection implies the continual supplanting and extinction of preceding and in- termediate gradations. Closely-allied species, now living on a continuous area, must often have been formed when the area was not continuous, and when the conditions of life did not in- sensibly graduate away from one part to another. When two varieties are formed in two districts of a continuous area, an intermediate variety will often be formed, fitted for an interme- diate zone ; but, from reasons assigned, the intermediate variety will usually exist in lesser numbers than the two forms which it connects ; consequently the two latter, during the course of further modification, from existing in greater numbers, Avill have a great advantage over the less numerous intermediate variety, and Avill thus generally svicceed in sujiplanting and exterminat- ing it. AVe have seen in this chapter how cautious we should be in concluding that the most different habits of life could not graduate into each other ; that a bat, for instance, could not have been formed by natural selection from an animal which at first could only glide through the air. We have seen that a species may under new conditions of life change its habits, or have diversified habits, Avith some very Chap. VI. SUMMARY. 199 unlike those of its ne:u-est conveners. Hence, we can under- stand, bearing in mind that each organic being is trying to Hvc wliercver it can live, how it has arisen that there arc upland geese with Avebbcd feet, ground woodpeckers, diving thrushes, and jx'trels with the hal)its of auks. Although the belief that an organ so perfect as the eye could have been formed by natural selection, is more than enouiih to sta^rger any one ; yet in the case of any organ, if we know of a long series of gradations m complexity, each good for its possessor, then, under changing conditions of life, there is no logical impossibilit}'^ in the acquirement of any conceivable degree of perfection through natural selection. In the cases in which we know of no intermediate or transitional states, we sliouhl be very cautious in concluding that none have existed, for the homologies of man}' organs and their intermediate states show what wonderful metamorphoses in function are at least 'possible. For instance, a swim-bladder has apparently been converted into an air-breathing lung. The same organ having performed simultaneously very diflerent functions, and then liaving Ixicn in part or in whole specialized for one function ; and two distinct organs having performed at the same time the same function, the one having been perfected while aided by the other, must often have largely-facilitated transitions. We have seen in two beings, widely remote from each other in the natural scale, that an organ serving in both for the same purpose, and appearing closely similar, may have been sepa- rately and independently formed ; but when such organs are closely examined, essential dilTerences in their structure can almost always be detected; and this naturally follows from the principle of natural selection. On the other hand, the conunon rule throughout Nature is infinite diversity of structure for gaining the same end; and this again naturally, follows on the same great principle. In almost every case we are far too ignorant to be enaljled lo assert that any part or organ is so unimportant for the wel- fare of a species, that modifications in its structure could not have been slowly accumulated by means of natural selection. But Ave may confidently believe that many modifications, wholly due to the laws of growth, and at first in no way advanta- geous to a species, have been subsef|uently taken advantage of by the still furtlier modified descendants of this species. We may also believe tliat a part formerly of high importance has often been retained (as tlie tail of an aquatic animal by its ter- restrial descendants), though it has become of such small im- 200 SUMMAEY. CiiAr. VI. portancc that it could not, in its present state, have been ac- quired by natural selection — a power which acts solely through tlic sur^-ival of tlie best-fitted individuals in the strufrgle for life. Natural selection will produce nothing in one sjiecies foi the exclusive good or injury of anotlier; though it may well produce parts, organs, and excretions, highly useful or even in- dispensable, or highly injurious to another species, but in all cases at the same time useful to the owner, Natural selection in each well-stocked country must act chiefly through the com- pc^tition of the inhabitants one with another, and consequently will produce perfection, or strength in the battle for life, only according to the standard of that countrj^ Hence the inhab- itants of one countr}', generally the smaller one, will often yield to the inhabitants of another and generally larger country. For in the larger country there will have existed more individuals, and more diversified forms, and the competition will have been severer, and thus the standard of perfection will have been rendered higher. Natural selection will not nccessarilj' pro- duce absolute perfection ; nor, as far as Ave can judge by our limited faculties, can absolute perfection be evei-ywhcrc found. On the theory of natural selection we can clearly inider- stand the full meaning of that old canon in natural history, " Natura non facit saltum." This canon, if we look only to the jiresent inhabitants of the world, is not strictly correct ; but if we include all those of past times, whether known or not yet known, it must by my theory be strictly true. It is generally acknowledged that all organic beings have been formed on two groat laws — Unity of Type, and the Con- ditions of Existence. B}^ imity of type is meant that funda- mental agTcement in structure which we see in organic beings of the same class, and which is quite independent of their halnts of life. . On my theory, unity of type is explained by unity of descent. The expression of conditions of existence, so often insisted on by the illustrious Cuvier, is fully embraced by tlie principle of natural selection. For natural selection acts by either now adapting the varying parts of each being to its organic and inorganic conditions of life ; or by having adapted i\unn during long-past periods of time : the adaptations being aided in some cases by use and disuse, being affected by the direct action of the external conditions of life, and being in all crises subjected to the several laws of growth. Hence, in fact, the law of the Conditions of Existence is the higher law ; as it iiuhidcs, thrnugli the inheritance of former adaptations, that of Unity of Type, Cii.i.Vll. INSTINCT. 201 CHAPTER VII. INSTINCT, Instincts coniparnl)lo with Ilabiis, but difTcront in their Orii^in— Inptincls £;radu.ited — Aphides and Ants— Instincts vnriable— DomcBtic Instincts, tlicir Origin— Nat- ural luHtincta of tlic Cuckoo. Ostrich, and Parasitic Bees— Slave-niakin:,' Ants— Hive-bee, its cell-inakini; instinct— Changes of Instinct and Structure not neces- sarily simultaneous- DilDculties of the Theory of the Natural tjelectiou of lu- btincta— Neuter or Sterile Insects — Summary. Instincts might have been worked into the previous chap- ters ; but I thought that it would be more convenient to treat the subject separately, especially as an instinct so wonderful as that of the construction of the comb by the hive-bee will prob- ably have occurred to many readers, :is a difficulty sufficit-nt to overthrow the whole theory. I must premise that I have notliing to do with the origin of tlic primary mental powers, any more than I have with that of life itself. We are con- cerned only with the diversities of instinct and of the other mental qualities of animals Avithin the same class. I will not attempt any definition of instinct. It would be easy to show that several distinct mental actions are commoidy embraced by this term ; but every one underst;uids what is meant, when it is said that instinct impels the cuckoo to mi- grate and to lay her eggs in other birds' nests. An action, which we ourselves should require experience to enable us to perform, when performed by an animal, more especially by a very young one, without any experience, and when performed by many individuals in the same way, without their knowing for what purpose it is performed, is usutilly said to he instinc- tive. But I could show that none of these cliaracters of in- stinct are universal. A little dose, as Pierre Hubcr expresses it, of judgmtMit or rea.son, often comes into play, even in ani- mals low in the scale of nature. Frederick Cuvier and several of the older m('tai)hysicians have compared instinct with habit. This comparLsou gives, I think, an accurate notion of the frame of mind under which an instinctive action is performed, but not necessarily of its 202 INSTINCT. Chap. VII. origin. How unconsciously many habitual actions are per* fornieil, indeed not rarely in direct opposition to our conscious Avill ! yet tlicy may be modified by the ■will or reason. Habits easily become associated with other habits, and witli certain periods of time and states of the body. A\'hcn once acquired, they often remain constant throughout life. Several other points of resemblance between instincts and habits could be pointed out. As in repeating a well-known song, so in instincts, one action follows another by a sort of rhythm ; if a ])erson be in- terrupted in a song, or in repeating any thing by rote, he is generally forced to go back to recover the habitual train of thought : so P. Huber found it Avas with a caterpillar, which makes a very complicated hammock ; for if he took a cater- pillar which had completed its hammock up to, say the sixth stage of construction, and put it into a hammock completed up only to the third stage, the caterpillar simply reperformed the fourth, fifth, and sixth stages of construction. If, however, a cater])illar were taken out of a hammock made up, for in- stance, to the third stage, and were put into one finished up to the sixth stage, so that much of its work was already done for it, far from feeling the benefit of this, it was much embarrassed, and, in order to complete its hammock, seemed forced to start from the third stage, where it had left off, and thus tried to complete the already-finished work. If we suppose any habitual action to become inherited — and I think it can be shown that tliis does sometimes happen — then the resemblance between Avhat originally was a habit and an instinct becomes so close as not to be distinguished. If Mozart, instead of playing the jiiano-forte at three years old with wonderfully little jiracticc, had played a tune with no l)ractice at all, he might truly be said to have done so instinc- tivclv. But it would be a serious error to suppose that the greater number of instincts have been acquired by habit in one generation, and then transmitted by inheritance to succeeding generations. It can be clearly shown that the most wonder- ful instincts with which we arc acquainted, namely, those of the hive-bee and of many ants, could not possibly have been acqtiired by habit. It will be universally admitted that instincts are as imjior- tant as corporeal structure for the welf:ire of each species, un- der its present conditions of life. lender changed conditions of life, it is at least possible that slight modifications of in- stinct might be profitable to a species ; and if it can be shown Chap. VII. INSTINCT. 203 that instincts do vary ever so little, then I can see no dilTiculty in natural selection preserving^ and continually accumulating variations of instinct to any extent that was profitable. It is thus, as I believe, that all the most complex and wonderful in- stiijcts have originated. As modifications of corporeal struct- ure arise from, and arc increased b}'", use or habit, and are diminished or lost by disuse, so I do not doubt it has been with instincts. But I believe that the effects of habit arc of quite subordinate importance to the effects of the natural selec- tion of what may be called spontaneous variations of instincts; tliat is, of variations produced by tlie same unknown causes which produce slight deviations of bodily structure. No comj^lex instinct can possibly be produced througli nat- ural selection, except by the slow and gradual accumulation of numerous, slight, yet profitable, variations. Hence, as in the case of corporeal structures, we ought to find in nature, not the actual transitional gradations by which each complex in- stinct has been acquired — for these could be found only in the lineal ancestors of each species — but we ought to find in the collateral lines of descent some evidence of such gradations ; or we ought at least to be able to shoAV that gradations of some kind are possible ; and this we certainly can do. I have been surprised to find, making allowance for the instincts of animals having been but little observed except in Europe and North America, and for no instinct being- kno^vn among extinct species, how very generall^'^ gradations, leading to the most complex instincts, can be discovered. Changes of instinct may sometimes be facilitated by the same species having different instincts at different periods of life, or at different seasons of the year, or when placed imder difi'erent circumstances, etc. ; in which case cither the one or the other instinct might be pre- served by natural selection. And such instances of diversity of instinct in the same species can be shown to occur in nature. Again, as in the case of corporeal structure, and conform- ably to my theorj-, the instinct of each species is good for it- self, but has never, as far as we can judge, been produced for the exclusive good of others. One of the strongest instances of an animal apparently performing an action for the sole good of another, witli which I am acquainted, is that of ajihides vol- untarily yielding, as was first observed l)y Hu})er, their sweet excretion to ants ; that they do so voluntarily, the following facts show : I removed all the ants from a group of about a dozen aphides on a dock-plant, and prevented their attendance 204 INSTINCT. CiiAr. VII. during several hours. After this interval, I felt sure that the aphides would Avant to excrete. I watched thcni for some time through a lens, but not one excreted ; I then tickled and stroked them with a hair in the same manner, as well as I could, as tlie ants do with their antennre; but not one excreted. Afterward I allowed an ant to visit them, and it immediately seemed, by its eager way of running about, to be well aware what a rich Hock it had discovered ; it then began to play witli its anteun;i3 on the abdomen first of one aphis and then of an- other; and each, as soon as it felt the antenna?, immediately lifted up its abdomen and excreted a limpid drop of sweet juice, which was eagerly devoured by the ant. Even the quite young aphides behaved in this manner, showing that the action Avas instinctive, and not the result of experience. It is certain, from the observations of Huber, that the aphides show no dislike to the ants : if the latter be not present they are at last compelled to eject their excretion. But as the excretion is extremely viscid, it is no doubt a convenience to the aphides to have it removed ; tlierefore, probably they do not excrete solely for the good of the ants. Although there is no evidence that any animal performs an action for the exclusive good of another species, yet each tries to take advantage of the in- stincts of others, as each takes advantage of the weaker bodily structure of other species. So, again, certain instincts cannot be considered as absolutely perfect ; but as details on this and other such points are not indispensable, they may be here passed over. As some degree of variation in instincts under a state of nature, and the inheritance of such variations, arc indispensable for the action of natural selection, as many instances as pos- sible ought to be here given ; but want of space prevents me. I can only assert that instincts certainly do vary — for instance, the migratory instinct — both in extent and direction, and in its total loss. So it is with the nests of birds, which vary partly in dependence on the situations chosen, and on the nature and temperature of the country inhabited, but often from causes Avliolly unknown to us : Audubon has given several remarkable cases of differences in the nests of the same species in the Northern and Southern United States. A\''hy, it has been asked, if instinct be variable, has it not granted to the bee " the ability to use some other material Avhen wax was deficient ? " But what other material could bees use? They Avill work with and use, as I have seen, wax hardened with vermilion and Chap. VII. CHANGES OF HABIT OR INSTINCT. 205 softened" with lard. Andrew Knight observed that his bees, instead of laboriously collecting propolis, used a cement of wax and turpentine, with which he had covered decorticated trees. It has lately been shown that bees, instead of search- ing flowers for their pollen, will gladly use a very different substance, namely, oatmeal. Fear of any particular enemy is certainly an instinctive quality, as may be seen in nestling birds, though it is strengthened by experience, and by the sight of fear of the same enemy in other animals. The fear of man is slowly acquired, as I have elsewhere shown, by the various animals which inhabit desert islands ; and we see an instance of this even in England, in the greater wildness of all our large birds in comparison with our small birds; for llie large birds have been most persecuted by man. '\\'e may safely attribute the greater Avildness of our large birds to this cause ; for in iminhabited islands large birds are not more fear- 'ful than small ; and the magjDie, so wary in England, is tame in Norwa}', as is the hooded crow in Egypt. That the mental qualities of animals of the same kind, bom in a state of nature, vary much, could be shown by many facts. Several cases could also be given of occasional and strange habits in wild animals, which, if advantageous to the species, might give rise, through natural selection, to new instincts. But 1 am well aware that these general state- ments, without the facts in detail, Avill produce but a feeble effect on the reader's mind. I can only repeat my assurance, that I do not speak without good evidence. Inherited Chanr/cs of Ilah'it or Instinct in Domesticated Animals. The possibilit}', or even probability, of inherited varial ions of instinct in a state of nature Avill be strengthened In' briefly considering a few cases under domestication. Wc shall thus b(; enabled to see the part Avhich habit and the selection of so-called accidental or spontaneous variations have plaj-ed in modifying the mentiil ([ualilies of our domestic animals. It is notorious how much domestic animals vary in their mental qualities. With cats, for instance, one naturally takes to catch- ing rats, and another mice, and these tendencies are known to be inherited. One cat, according to Mr. St. John, alwavs brought home game-birds, another hares or rabbits, and another Inmtcd on marshy ground and almost nightly caught wood- cocks or snipes. A number of curious and authentic instancca 20G CHANGES OF HABIT OR INSTINCT Chap. VII. could be g'iven of various shades ol' disjKjsitiou and of taste, • and likcAvisc of the oddest tricks, associated with certain frames of mind or periods of time, being inherited. But let us look to the familiar case of the breeds of the dof^ : it cannot be doubted that younj^ pointers (I have myself seen a striking instance) "will sometimes point and even back other dogs the very first time that they are taken out ; retrie\-ing is certainly in some degree inherited by retrievers ; and a ten- dency to run round, instead of at, a flock of sheep, by shep- herd-dogs. I cannot see that these actions, performed with- out experience by the young, and in nearly the same manner by each indi\'idual, performed with eager delight by each breed, and "without the end being known — for the young pointer can no more know that he points to aid his master, than the white butterfly knows why she lays her eggs on the leaf of the cabbage — I cannot see that these actions differ essentially from true instincts. If we were to behold one kind of wolf, when young and without any training, as soon as it scented its prey, stand motionless like a statue, and then slowly crawl forward with a peculiar gait ; and another kind of wolf rushing round, instead of at, a herd of deer, and driving them to a distant point, we should assur- edly call these actions instinctive. Domestic instincts, as they may be called, are certainly far less fixed than natural in- stincts; but they have been acted on by far less rigorous selec- lion, and have been transmitted for an incomparably shorter period, imder less fixed conditions of life. HoAV strongly these domestic instincts, habits, and dis- positions, are inherited, and how curiously they become mingled, is well shown when diflferent breeds of dogs are crossed. Thus it is known that a cross with a bull-dog has afiFected for many generations the courage and obstinacy of greyhounds ; and a cross Avith a greyhound has given to a whole famih^ of shepherd-dogs a tendency to hunt hares. These domestic instincts, when thus tested by crossing, resem- ble natural instincts, which in a like manner become curiously blended together, and for a long period exhibit traces of the instincts of either parent : for example, Le Koy describes a dog, whose great-grandfather was a wolf, and this dog showed a trace of its wild parentage only in one way, by not com- ing in a straight line to his master, when called. Domestic instmcts are sometimes spoken of as actions which have bec^.me inherited solelv fi"om long - continued CiiAP. VII. IN DOMESTICATED ANIMALS. 207 and compulsory habit, but this is not true. No one would ever have thought of teaching, or probably could have taught, the tumbler-pigeon to tumble — an action which, as I have witnessed, is performed by young birds, that have never seen a pigeon tumble. We may believe that some one pigeon showed a light tendency to this strange habit, and tlie long-continued selection of the best individ- uals in successive generations made tumblers what they now are ; and near Glasgow there are house-tumblers, as I hear from Mr. Brent, which cannot fly eighteen inches high without going head over heels. It may be doubted whether any one would have thought of training a dog to point, had not some one dog naturally shown a tendency in this line ; and this is known occasionally to happen, as I once saw, in a pure terrier : the act of pointing is probably, as many have thought, only the exaggerated pause of an animal preparing to sjiring on its prey. When the first tendency to point was once displayed, methodical selection and the inherited effects of compulsory training in each successive generation would soon complete the work ; and unconscious selection is still in progress, as each man tries to procure, without intending to improve the breed, dogs which will stand and hunt best. On the other hand, habit alone in some cases has sufficed ; hardly any animal is more diflicult, in most cases, to tame than the young of the Avild rabbit ; scarcely any animal is tamer than tlic young of the tame rabbit; bull can hardly suppose* that domestic rabbits have often been selected for tameness alone; so that we must attribute at least the greater part of the inherited change from extreme wildness to extreme tameness, to habit and long-continued close confinement. Natural instincts are lost under domestication : a remark- able instance of this is seen in those breeds of fowls which very rarely or never become " brooould have intersected or broken into each other, if the spheres had been completed ; but this is never permitted, the bees building pel-fectly Hat walls of wax between the spheres which thus tend to intersect. Hence each cell con- sists of an outer spherical portion, and of two, three, or more perfectly flat surfaces, according as the cell adjoins two, three, or more other cells. When one cell rests on three other cells, which, from the spheres being nearly of the same size, is very frequently and necessarily the case, the three flat surfaces arc united into a p}Tamid ; and this pyramid, as Huber has re- marked, is manifestlv a gross imitation of the three-sid(Ml jiyram- 'dal baseji of th(> cell of the liivc'liee. As in the fells of the 10 218 SPECIAL INSTINCTS. Chap. VII. hive-bee, so licre, the three plane surfaces in anyone cell neces- sarily enter into the construction of three adjoining cells. It is obvious that the Meli])ona saves wax, and, what is more im- portant, lal)or, by this manner of buildinjr; for the Hat walls between the adjoining cells are not double, but are of the Siune thickness as the outer spherical portions, and yet each flat por- tion forms a part of two cells. Rc^flecting on this case, it occurred to me that if the Meli- pona had made its spheres at some given distance from each other, and had made them of erjnal sizes, and had arranged them s^-nnnetrically in a double layer, the resulting structure would probably have been as perfect as the comb of the hive- bee. Accordingly, I wrote to Prof. Miller, of Cambridge, and this geometer has kindly read over the following statement, drawn up from his information, and tells me that it is strictly correct : If a number of equal spheres be described with their centres placed in two parallel layers, with the centre of each sphere at the distance of radius X 1^ 2, or radius X 1.414:'21 (or at some lesser distance), from the centres of the six surrounding spheres in the same layer; and at the same distance from the centres of the adjoining spheres in the other and parallel layer ; then, if planes of intersection between the several spheres in both layers be formed, there will result a double layer of hexagonal prisms united together by pyramidal bases formed of three rhombs ; and the rhombs and the sides of the hexagonal prisms will have every angle identically the same with the best meas- urements Avhich have been made of the cells of the hive-bee. But I hear from Prof. Wyman, who has made numerous care- ful measurements, that the accuracy of the workmanship of the bee has been greatly exaggerated ; so much so, that, as he adds, whatever the typical form of the cell may be, it is rarely, if ever, realized. Hence, we may safely conclude that, if Ave could slightly modify the instincts already possessed by the Melipona, and in themselves not very wonderful, this bee would make a structure as wonderfully perfect as that of the hive-bee. "We must sup- pose the Melipona to have the power of forming her cells truly spherical, and of equal sizes ; and this would not be very sur- prising, seeing that she already does so to a certain extent, and seeing what perfectly cvlindrical burrows in wood many insects can make, apparently by turning round on a fixed jioint. AVc must suppose the Melipona (o arrange her cells in level layers, Chap. VII. CELL-MAKING INSTINCT. 219 as she already does her cylindrical cells ; and wc uiiist further suppose, and this is the greatest difliculty, that she can some- how judge accurately at what distance to stand from her fellow- laborers when several are making their spheres; but she is already so far enabled to judge of distance, that she alwa3-s describes her spheres so as to intersect largely ; and then she unites tlie points of intersection by perfectly flat surfaces. We have further to suppose, but this is no difficulty, that after hexagonal ])risnis have been formed by the intersection of ad- joining spheres in the same layer, she can jirolong the hexagon to any length requisite to hold the stock of hone}*; in the same way as the rude humble-bee adds cylinders of wax to the circu- lar mouths of her old cocoons, ]5y such modifications of in- stincts, in themselves not very wonderful — hardly more wonder- ful than those which guide a bird to make its nest — I believe tluit the hive-bee has acquired, through natural selection, her inimitable architectural powers. But this theory can be tested by experiment. Following the example of Mr. Tegetmeier, I separated two combs, and put between them a long, thick, rectangular strip of wax ; the bees instantly began to excavate minute circular pits in it ; and as they deepened these little pits, they made them wider and wider until they were converted into shallow basins, appearing to the eye jierfectly true or parts of a sphere, and of about the diameter of a cell. It was most interesting to me to observe that, •wherever several bees had begun to excavate these basins near together, they had begun their work at such a distance from each other, that by tlie time the basins had acquired the above- stated width (i. e., about the width of an ordinary cell), and were in d<^pth about one-sixth of the diameter of the sphere of M'hiih they formed a part, the rims of the Ixisins intersected or broke into each other. As soon as this occurred, the bees ceased to excavate, and began to build up flat walls of wax on the lines of intersection between the basins, so that each hex- agonal prism was built upon the scalloped edge of a smooth l)asin, instead of on the straight edges of a three-sided pyramid as in the case of ordinary cells. I then put into the hive, instead of a thick, rectangular \)\cce of wax, a thin and narrow, knife-edged ridge, colored with vermilion. The bees instantlv began on both sides to ex- cavate little basins near to each other, in the same way as be- fore; but the ridge of wax was so thin, that the bottoms of the basins, if they had been excavated to the same depth as in tlio 220 SPECIAL INSTINCTS. Cuap. VII. former experiment, would liavc broken into eacli otlier from the opposite sides. The bees, liowcver, did not sufl'er this to happen, and they stopped their excavations in due time ; so that the basins, as soon as they had been a little deepened, came to have bottoms with flat sides ; and these flat sides, formed by little thin plates of the vermilion wax left un- p^nawed, were situated, as far as the eye could judge, exactly along the planes of imaginary intersection between the basins on tiie opposite sides of the ridge of wax. In some parts, only small portions, in other parts, large portions of a rhombic jilate had been left between the opposed basins ; but the work, from the mmatural state of things, had not been neatly performed. The bees must have worked at very nearly the same rate in circularly gnawing away and deepening the basins on both sides of the ridge of vennilion wax, in order to have thus suc- ceeded in leaving flat plates between the basins, by stopping work at the jilanes of intersection. Considering how flexible thin wax is, I do not see that there is any dilficulty in the bees, while at work on the two sides of a strip of wax, perceiving when they have gnawed the wax away to the proper thinness, and then stopping their work. In ordinarv combs it has appeared to me that the bees do not always succeed in working at exactly the same rate from the opposite sides ; for I have noticed half-completed rhombs at the base of a just-commenced cell, which were slightly concave on one side, where I suppose that the bees had excavated too quickly, and convex on the opposed side where the bees had ■worked less quickly. In one well-marked instance, I jiut the coml) back into the hive, and allowed the bees to go on work- ing for a sliort time, and again examined the cell, and I found that the rhombic plate had been completed, and had become pcrfectli/ JJat : i^ was absolutely impossil)le, from the extreme thinness of the little plate, that they could have effected this by gnawing away the convex side ; and I suspect that the bees in such cases stand on opposite sides and push and bend the ductile and warm -wax (which, as I have tried, is easily done) into its ])roper intermediate plane, and thus flatten it. From the experiment of the ridge of vermilion wax we can Bee that, if the bees were to build for themselves a thin wall of wax, they could make their cells of the proper shape, by standing at the proper distance from each other, by excavat- ing at the same rate, and by endeavoring to make equal spheri- cal hollows, but never allowing the s])hcres to break into each CuAP. VII. CELL-MAKING INSTINCT. 221 other. Now bees, as may be clearly seen hy examining' the od^e of a ^rowinp^ comb, do make a roug'li, circumferential ^vall or rim all round the com)) ; and they g'liaw this away from the op])()sitc sides, always workinj]^ circularly as they deepen each cell. They do not make the whole three-sided pyramidal base of any one cell at the same time, but only tliat one rhom- bic plate Avhich stands on the extreme growing marg-in, or the two plates, as the case may be ; and th(>y never complete the upper edges of the rhoml)ic plates, until the hexagonal walls are comnifmced. Some of these statements dilTer from those made by the justly-celebrated elder Uuber, but I am convinced of their accuracy ; and, if I had space, I could show that they are conformable with my theory. Huber's statement, that the very first cell is excavated ouJ of a little parallel-sided wall of wax, is not, as far as I liavo seen, strictly correct ; the first commencement ha^'ing always bben a little hood of wax ; but I will not here enter on these details. We see how important a part excavation plays in the construction of the cells ; but it would be a gn^at error to sup- pose that the bees cannot l)uild up a rough Avail of M'ax in the proper position — that is, along the plane of intersection be- tween two adjoining si)heres. I have several specimens show- ing clearly that they can do this. Even in the rude circum- ferential rim or wall of wax round a growing comb, flexures may sometimes be observed, corresponding in position to the planes of the rhombic basal plates of future cells. But the rough wall of wax has in every case to be finished ofl", l)y being largely gnawed away on both sides. The manner in which the bees build is curious ; they always make the first rough wall from ten to twenty times thicker than the excessively thin finished wall of the cell, which will ultimately be left. \\^o shall understand how they work, by supposing masons first to pile up a broad ridge of cement, and then to Ix^gin cutting it away erjually on l)()th sides near the ground, till a smooth, very thin wall is left in the middle ; the masons always piling up the cut-away cement, and adding fresh cement on the sum- mit of tilt; ridge. "We shall thus have a thin wall steadily growing upward but always crowned by a gigantic coping. From all tlu; cells, both those just commenced and those com- pleted, Ijcing tluis crowned by a strong coping of wax, the bees can cluster and crawl over the comb without injuring the delicate hexagonal walls. These walls, as Prof. Miller has kindly ascertained for me, vary greatly in thickness ; being, 222 SPECIAL INSTINCTS. Cuap. VII. on an average of twelve measurements made near tlie border of tlic comb, Tp^ of an inch in thickness ; whereas the basal rhomboidal plates are thicker nearly in the proportion of three to two, having a mean tliickness, from twenty-one measure mcnts, of -rAs of an inch, liy the above singular maimer of building, strength is continually given to the coml), with the utmost ultimate economy of wax. It seems at first to add to the difficulty of understanding how the cells are made, that a multitude of bees all work to- gether ; one bee after Avorking a sliort time at one cell going to another, so that, as Hubcr has stated, a score of individuals work even at the commencement of the first cell. I was able practicall}'" to show this fact, by covering the edges of tlie hex- agonal walls of a single cell, or the extreme margin of the cir- cumferential rim of a growing comb, with an extremely thin layer of melted vermilion wax ; and I invariably found that the color was most delicately difiused by tlie bees — as delicately as a painter could have done with his brush — by atoms of the colored wax having been taken from the spot on which it had been placed, and Avorked into the growing edges of the cells all round. The Avork of construction seems to be a sort of balance struck between many bees, all instinctively standing at the same relative distance from each other, all trying to SAveep equal spheres, and tlien building up, or leaving un- gnaAved, the planes of intersection betAveen these spheres. It was really curious to note in cases of difficulty, as Avhen two pieces of comb met at an angle, hoAV often the bees Avould pull down and rebuild in different ways the same cell, sometimes recurring to a shaj^e which they had at first rejected. When bees haA'e a place on Avhich they can starid in their proper positions for Avorking — for instance, on a slip of Avood, placed directly under the middle of a comb groAving doAvnAvard so that the comb has to be built OA'er one face of the slip — in this case the bees can lay the foundations of one Avail of a ncAV hexagon, in its strictly proper place, projecting beyond the other com])leted cells. It suffices that the bees should be en- al)led to stand at their proper relative distances from each other and from the A\-alls of the last completed cells, and then, by striking imaginary spheres, they can build up a Avail inter- mediate betAveen tAvo adjoining s])heres ; but, as far as I have seen, they ncAxr gnaw aA\-ay and finish off the angles of a cell till a large part both of that cell and of the adjoining cells has been built. This capacity in bees of laying doAvn under ccr- CiiAP. VII. rELL-MAKINO INSTLXCT. 223 tain circumsluuccs a rough wall in its proper place l>elwccn two just-commenced cells, is important, as it bears on a fact, which seems at first quite subversive of the foregoing theory ; namely, that the cells on the extreme margin of Avasp-combs are some- times strictly hexagonal ; but I have not space here to enter on this subject Nor does there seem to me any great difii- culty in a single insect (as in the case of a queen-wasp) making hexagonal cells, if she were to work alternately on the inside and outside of two or three cells commenced at the same time, always standing at the proper relative distance from the parts of the cells just Ix^guji, sweeping spheres or cylinders, and building up intermediate planes. As natural selection acts only by the accumulation of slight modificiitions of structure or instinct, each profitable to the individual under its conditions of life, it may reasonably be asked, how a long and graduated succession of motlified archi- tectural instincts, all tending toward the prcsent perfect plan of construction, could have profited the progenitoi-s of the hive- bee ? I think the answer is not diflicult: cells constructed like those of the bee or the wasp gain in strength, and save much in labor and space, and in the materials of which they are constructed. With respect to the formation of wax, it is known that bees are often hard pressed to get sufficient nectar, and I am informed by Mr. Tegetmeier that it has been experi- mentally proved that from twelve to fifteen poimds of dry sugar arc consumed by a liive of bees for the secretion of each pound of wax; so that a prodigious quantity of fluid nectar nmst be collected and consumed by the bees in a hive for the secretion of the wax necessary for the construction of their combs. Moreover, many bees have to remain idle for many days during the jirocess of secretion. A large store of honey is indispensable to support a largt; stock of bees during the winter ; and the security of the hive is known mainly to depend on a large number of bees being supported. Hence the saving of wax by largely saving honey and the time consumed in col- lecting the honey must be an important element of success to any family of bees. Of course, the success of the species ma\- be dependent on the number of its enemies, or parasites, or on (juite distinct caiuses, and so be altogether independent of the quantity of honey which the bees could collect. But let us suppose that this latter circumstance determined, as it proba- bly often has determined, whether a bee allied to our liumble- bces could exist in large nuinbere in any country ; and let us 224 SPECIAL INSTINCTS. Chap. VII, further suppose lliat tlie community lived tlirou;^h the winter, and consequently required a store of honey; there can in this case be no doubt that it would be an advantage to our imagi- nary humble-bee, if a slight modification in her instincts led her to make her waxen cells near together, so as to intci*sect ;•- little ; for a wall in common even to two adjoining cells would save some little labor and wax. Hence it would continually be more and more advantageous to our humble-bees, if they were to make their cells more and more regular, nearer to- gether, and aggregated into a mass, like the cells of the Jleli- ])ona ; for in this case a large part of the bounding surface of each cell would serve to bound the adjoining cells, and much labor and wax Avould bo saved. Again, from the same cause, it Avould be advantageous to the Melipona, if she were to make her cells closer together, and more regular in every way than at present ; for then, as we have seen, the sjiherical sm- faces would wholly disappear and be replaced by plane sur- faces ; and the Melipona Avould make a comb as perfect as that of the hive-bee. Beyond this stage of perfection in architect- ure, natural selection could not lead ; for the comb of the hive- bee, as far as we can see, is absolutely perfect in economizing labor and wax. Thus, as I believe, the most v.'onderful of all known in- stincts, that of the hive-bee, can be explained by natural selec- tion having taken advantsige of numerous, successive, sUght modifications of simpler instincts; natural selection having, by slow degrees, more and more perfectly led the bees to sweej) etjual spheres at a given distance from each other in a double layer, and to build up and excavate the wax along the planes of intersection ; the bees, of course, no more knowing that they swept their spheres at one particular distance from each other, than they know what are the several angles of the hexagonal prisms and of tlie basal rhombic plates ; the motive jiower of tlie process of natural selection having been the con- struction of cells of due strength and of the proper size and shape for tlie larv.o?, this being etlected Avith the greatest pos- sible economy of labor and Avax ; that individual swarm which thus made tlie best cells with least lalior, and least waste of honey in tlie secretion of wax, having succeeded best, and liav- ing transmitted by inheritance their newly-acquired economical instincts to new swarms, which in their turn will have had tho best chance of succeeding in the struggle for existence. CiiAi-. VII. OBJECTIONS TO NATDKAL SELECTION. 225 Objectiojis to the Tlicory of Natural Selection as cq^plied to Instincts y' Axuter and Sterile Insects. It has been objected to the foregoing view of the origin of instincts that " the variations of structure and of instinct must have been simultaneous and accurately adjusted to each other, as a modification in the one without an immediate corre- sponding change in the other Avould have been fatal." The force of this olijection rests entirely on the assumption that the changes in both instinct and structure are abrupt. To take as an illustration the case of the larger titmouse (Parus major) al- luded to in the last chapter : this l)ird often holds the seeds of the yew between its feet on a branch, and hannners away till it gets into the kernel. Now what special diiliculty would there be in natural selection preserving all the slight individ- ual variations in the shape of the beak, Avhich were better and better adapted to break ojien the seeds, until a beak was formed, as well constructed for this purpose as that of the nut- hatch, at the same time that habit, or compulsion, or sponta- neous variations of taste, led the bird to become more and more of a seed-eater ? In this case the beak is supposed to be slowly modified by natural selection, subsequently to, but in accordance with, slowly-changing habits or taste ; but let the feet of the titmouse vary and grow larger from correlation with tlie beak, or from any other imknown cause, and it is not improbable that such larger feet would lead the bird to climb more and more until it acquired the remarkable climbing instinct and jwwer of the nuthatch. In this case a gradual change of struct- ure is supposed to lead to changed instinctive habits. To take one more case : few instincts are more remarkable than that which leads the swift of the Eastern Islands to make its nest wholly of inspissated saliva. iSoine birds build their nests of nnid, believed to be moistened with saliva ; and one of the swifts of North America makes its nest (as I have seen) of sticks agglutinated with saliva, and even with flakes of this substance. Is it, then, very improbable that the natural selec- tion of individual swifts, wliich secreted more and more saliva, should at last produce a species with instincts leading it to neglect other matc^rials^ and to make its nest exclusively of inspissated saliva? And so in other cases. It must, however, be admitted tliat in many instances we cannot conjecture whetlier it was instinct or structure which first varied. No doubt many instincts of very dillicult explanation could 226 OBJECTIONS TO THE THEOICY CriAP. VII. be 0])poscJ to tlic theory of natunil selection — cases, in -which Ave cannot sec how an instinct could ])ossibly liave originated; cases, in ■which no intermediate gradations arc known to exist ; cases of instinct of such trifling importance, that they could hardly have been acted on by natural selection ; cases of in- stincts almost identically the same in animals so remote in the scale of Nature, that we cannot account for their similarity by inheritance from a common progenitor, and consequently must believe that they were independently acciuircd through natural selection. I will not here enter on these several cases, but will confine myself to one special difficulty, which at first ap- jieared to me insuperable, and actuall}' fatal to the whole theory. I allude to the neuters or sterile females in insect- communities : for these neuters often differ widely in instinct and in structure from both the males and fertile females, and yet, from being sterile, they cannot propagate their kind. The subject well deserves to be discussed at great length, but I Avill here take only a single case, that of working or sterile ants. Hoav the workers have been rendered sterile is a difriculty : but not much greater than that of any other strik- ing modification of structure ; for it can be shown that some insects and other articulate animals in a state of nature occa- sionally become sterile ; and if such insects had been social, and it had been profitable to the community that a numl^er should have been annually born capable of work, but incapable of procreation, I can see no especial difficulty in this having been efT(M'ted through natural selection. But I must pass over this preliminary difficulty. The great difiiculty lies in the working ants differing widely from both the males and the fertile females in structure, as in the shape of the thorax, and in being destitute of wings and sometimes of eyes, and in in- stinct. As far as instinct alone is concerned, the Avonderful difference in this respect between the Avorkers and the perfect females, would have been better exemplified by the hive-bee. If a working ant or other neuter insect had been an ordinary animal, 1 should have mihesitatingly assumed that all its char- acters had been slowly acquired through natural selection ; namely, by individuals having been born with slight profitable modifications, which were inherited by -the offspring; and that these again varied and again were selected, and so onward. But with the Avorking ant we have an insect differing greatly from its parents, yet. absolutely sterile ; so that it could never have transmitted successively-acquired modifications of struct- Chap. VII. OF NATURAL SELECTION. 227 lire or instinct to its proc^ony. It may well be asked how is it possible to reconcile this case with the theory of natural selection ? First, let it be remembered that wc have innumeral)le in- stances, both in our domestic productions and in those in a state of nature, of all sorts of differences of inherited structure which are correlated with certain ages, and with either sex. We have differences correlated not only Avith one sex, but with that short period when the reproductive system is active, as in the nuptial plumage of many birds, and in the hooked jaws of the male salmon. We have even slight differences in the horns of diQ'erent breeds of cattle in relation to an artifi- cially imperfect state of the male sex ; for oxen of certain breeds have longer horns than tlie oxen of other breeds, rela- tively to the h'ugth of the horns in both the bulls and cows of these same breeds. Hence I can see no great difficulty in any character becoming correlated with the sterile condition of certain members of insect communities : the difficulty lies in understanding how such correlated modifications of structure could have been slowly accumulated by natural selection. Tliis dilficulty, though ajipearing insuperable, is lessened, ' or, as I believe, disajjpears, when it is remembered that selec- tion may l)e applied to the family, as well as to the individual, and may thus gain the desired end. Thus, breeders of cattle wish the flesh and fat to be well marbled together ; the animal has been slaughtered, but the breeder has gone witli confidence to the same stock and has succeeded. Such faith may be placed in the power of selection, that probably a breed of cattle, always yielding oxen Avith extraordinarily long horns, could be slowly formed by carefully Avatching Avhich individual bulls and cows, Avhen matched, produced oxen Avith the longest horns ; and yet no one ox Avould ever have propagated its kind. Here is a better and real illustration : according to M. Vcrlot, some varieties of the double annual stock of various colors, from having been long carefully selected to the right degree, always produce by seed a large projinrtion of plants l)earing double andf|nite sterile floAvers ; so that, if the A'ariety had not likewise yielded others, it Avould at once have become extinct; but it always yields some single and fertile plants, Avhich differ from ordinary single A'arieties only in their power of producing the two forms. Thus the fertile plants producing single flowers may be compared Avith the males and females of an ant-comnumity, and the sterile double-flowered plants, Aviiich 228 OBJECTIONS TO THE THEOKY Chap. VII. arc rcg'ularly produced in l:ir^e numbers, with tlu; iiKiny st4?rilo neuters of the same coininunity. Thus I l>elieve it has been "vvith social insects : a slij^ht modification of structure, or of in- stinct, correlated with tlic sterile condition of certain members of the conmiunity, has been advantageous to the community: consequently the fertile males and females of tlie same cona- munity flourished, and transmitted to their fertile ofl'sprinfr^ a tendency to produce sterile members having the same modifi- cation. And I believe that this process has been repeated, luitil that prodigious amount of difference between the fertile and sterile females of the same species has been produced, which wo see m so many social insects. But we have not as yet touched on the climax of the difE- culty : namely, the fact that the neuters of several ants differ, not only from the fertile females and males, but from each other, sometimes to an almost incredible degree, and are thus divided into two or even three castes. Tiie castes, moreover, do not generally graduate into each other, but are perfectly well defined ; being as distinct from each other as are any two species of the same genus, or rather as any two genera of the same family. Thus in Eciton, there are working and soldier neuters, wnth jaws and instincts extraordinarily difi'ercnt : in Cryptocerus, the workers of one caste alone carry a wonderful sort of shield on their heads, the use of which is quite imknown : in the Mexican JMyrmecocystus, the workers of one caste never leave the nest ; they are fed by the workers of another caste, and they have an enormously-developed abdomen which se- cretes a sort of honey, supplying the place of that excreted by the aphides, or the domestic cattle, as they may be called, which our European ants guard and imprison. It will indeed be thought that I have an overweening con- fidence in the principle of natural selection, when I do not ad- mit that such wonderful and Avell-established facts at once an- jiiliilate the theory. In the simpler case of neuter insects all of one caste, which, as I believe, have been rendered different from the fertile males and females through natural selection, we may conclude from the analogy of ordinary variations, that the successive, slight, profitable modifications did not first arise in all the neuters in the same nest, but in some few alone ; and that by the survival of the communities with females which ])r()(lure most neuters having the advantageous modification, all the neuters ultimately come to be thus characterized. Accord- ing to this view, Ave ought occasionally to find in the same nest CuAr. YII. OF NATURAL SELECTION. 229 neuter insects, presenting gradations of structure ; and this we do find, even frcquenth'-, considering liow few neuter insects out of Europe have been carefully examined, ^[r. F. JSinith lias shown that the neuters of several British ants differ sur- ])risingly from each other in size, and sometimes in color ; and that the extreme forms can be linked together by individuals taken out of the sanu; nest ; I have myself compared j^erfect gradations of this kind. It sometimes happens that the larger or the smaller sized workers are the most numerous ; or that both large and small are numerous, while those of an interme- diate size are scanty in numbers. Formica flava has larger and smaller workers, with some few of intermediate size ; and, in this species, as Mr, F. Smith has observed, the larger workere have simj)lc eyes (ocelli), which though small can be plainly distinguished, whereas the smaller workers have their ocelli rudimentary. IlaWng carefully dissected several specimens ot these workers, I can affirm that the eyes are far more rudimen- tary in the smaller workers than can be accounted for merely by their proportionally lesser size ; and I fully believe, though I dare not assert so positively, that the workers of intermediate size have their ocelli in an exactly intermediate condition. So that here we have two bodies of sterile workers in the same nest, differing not only in size, but in their organs of vision, yet conn(^cted by some few members in an intermediate condition. I may digress by adding, that if the smaller Avorkers had been the most useful to the community, and those males and females had been continually selected, which produced more and more of the smaller workers, until all the workers were in this condi- tion, we should then have had a species of ant with neuters nearly in the same condition with those of Mynnica. For the workers of Mynnica have not even rudiments of ocelli, though the male and female ants of this genus have Avcll-develoi)ed ocelli. I may give one other case : so confidently did I expect to find gradations in important points of structure between the differ- ent castes of neuters in the same species, that I glaiUy availed myself of Mr. F. Smith's offer of numerous specimens from the same? nest of tlie driver-ant (Aiiomma) of West Africa. The reader will peihajis best a]i])reciate the amount of difference in these workers, by my giving not the actual measurements, but a strictly aecurate illustration : the difference was the same as if we were to sec a set of workmen building a house, of whom many were five feet four inches high, and many sixteen feet 230 OBJECTIONS TO THE TIIEOPwY Chap. VII. high ; but wc must suppose that the larger workmen had heads four instead of three times as l:)ig as those of the smaller men, and jaws nearly five times as big. The jaws, moreover, of the working ants of the several sizes differed wonderfully in shape, and in the form and number of the teeth. But the important fact f(^r us is, that, though the workers can be grouped into castes of different sizes, yet they graduate insensibly into each other, as does the widely-different structure of their jaws. I speak confidently on this latter point, as Sir J. Lubbock made draw- ings for me, with the camera lucida, of the jaws which I dis- sected from the workers of the several sizes. Mr. Bates, in his most interesting "Naturalist on the Amazons," has described analogous cases. With these facts before me, I believe that natural selection, by acting on the fertile ants or parents, could form a species which should regularly produce neuters, either all of large size with one form of jaw, or all of small size M-ith jaws having a Avidely-different structure; or lastl}^, and this is the clima.x of dilliculty, one set of workers of one size and structure, and at the same time another set of workers of a different size and structure ; a graduated series having first been formed, as in the case of the driver-ant, and then the extreme forms having been produced in greater and greater numbers, through the survival of the parents which generated them, until none with an intermediate structure were produced. An analogous explanation has been given by Mr. Wallace, of the equally com[)lcx case, of certain Malayan butterflies regularly appearing under two or even three distinct female forms ; and by Fritz Miiller, of certain Brazilian crustaceans likewise appearing under two widely-distinct male forms. But this subject need not here be discussed. I have now explained how, as I believe, the wonderful fact of two distinctly-defined castes of sterile workers existing in the same nest, both widely different from each other and from their parents, has originated. We can see how useful their production niay have been to a social comnnmity of ants, on the same, principle that the division of labor is useful to civil- ized man. Ants, however, work by inherited instincts and by inherited organs or tools, while man works by acquired knowl- edge and manufactured instruments. But I must confess, that, Avith all my faith in natural selection, I should never have an- ticipated tiiat this principle could have been efficient in so high a degree, had not the case of these neuter insects convinced me Chap. VII. OF NATURAL SELECTION. 231 of the fact. I have, therefore, discussed this case, at some little but wholly insuniciout length, in order to show tlio power of nat- ural selection, and likewise because this is by far the most serious special dillieulty which my theory has encountered. The case, also, is very intercstinir, as it proves that with animals, as with plants, any amount of modification may be cUc'cted ])y the ac- cumulation o( numerous, slight, spontaneous variations, which are in any way jirolitable, without exercise or hal)it havini^ been brouf^ht into play. For peculiar hal^its confined to the workers or sterile females, however long they mif^ht be fol- lowed, could not possibly affect the males and fertUe female, which alone leave descendants. I am surprised that no one has hitherto advanced this demonstrative case of neuter insects, against the well-known doctrine of inherited habit, as advanced by Lamarck. iSummari/. I have endeavored in this chapter briefly to show that the mental qualities of our domestic animals vary, and that the vari- ations are inlierited. Still more briefly I have attempted to show that instincts A'ary slightly in a state of nature. No one will dispute that instincts are of the highest importance to each animal. Therefore there is no real diiliculty, under changing conditions of life, in natural selection accumulating to any ex- tent slight modifications of instinct which are in any way use- ful. In some cases habit or use, and disuse, have proliably come into play. I do not pretend that the facts given in this chapter strengthen in any great degree my theory ; but none of the cases of diiliculty, to the best of my judgment, annihilate it. On the other hand, the fact that instincts are not always ab- solutely perfect, and are liable to'mistakes ; that no instinct can be shown to have been produced for the good of other animals, tliough animals take advantage of the instincts of others; that tlie canon in natural history of " Natura non facit saltuin,"isap]ili(able to instincts as well as to corporeal struct- ure, and is plainly explicable on the foregoing views, but is otherwise inexplicable — all tend to corroborate the theory of natural selection. This theory is, also, strengthened by some few other facts in regard to instincts ; as by that common case of closely-allied, but distinct, species, when inhabiting distant parts of the world and living under considerably diffcM-ent conditions of lifCj yet often retaining nearly the same instincts. For instance, 232 SUMMARy. Chap. VII. we can understand, on the principle of inheritance, how it is that the thrush of tropical South America hues its nest -with mud, in the same peculiar manner as docs our British thrush : how it is that the liornbills of Africa and India have the same extraordinary instinct of plastering' up and imprisoning the females in a hole in a tree, with only a small hole left in the plaster throug-h Avhich the males feed them and the young Avhen hatched : how it is that the male wrens (Troglodytes) of North America build "cock-nests," to roost in, like the males of our kitty-wrens — a habit wholly unlike that of any other known bird. Finally, it may not be a logical deduction, but to my imagination it is far more satisfactory to look at such instincts as the young cuckoo ejecting its foster-brothers — ants making slaves — the lnrva3 of ichneumonidas feeding within the live bodies of caterj:)illars — not as specially endowed or created instincts, but as small consequences of one general laAV, lead- ing to the advancement of all organic beings — namely, multi- ply, vary, let the strongest live and the weakest die. Ci:\v. Vlil. lIYBiaDISM. 23J CHAPTER VIII. IIYBKIDISM. Distinction Ijctwpon the Stcrilifir of First CropfCF and of Hybrids— Slorilil.v various in JX'iXTCf, nut uiiivtM>jil. all'i'ctcd l)y cIojji! Interbrccdincr, removed by Domesti- cation—Lawf LTovi'miiiL,' llie Sterility of Hybrids— Sterility not a special Endow- ment, bat incidental on oilier DilTerences, not accnmiilated by Natural Selection — Causes of the Sterility f)f First Crosses and of Hybrids— i'urallilisrn between llie Ell'ectrt of Clianjred Conditions of Life and of Crossin;,'— Diinnrnldsm and , Triuiorpliism — Fertility of Varieties when crossed and of their Monjirel Ollsprinij not universal— Hybrids and Mongrels compared independently of tlicir Fertility —Summary. The view generally entertained by naturalists is that species, when intercrossed, have been specially endowed witli sterility, in order to prevent their confusion. This view cer- tainly seems at first hifi;hly pro])able, for s])ecies within the same country could hardly have been kept distinct had they been capable of freely crossing-. The subject is in many ways important for us, more especially as the sterility of species when first crossed, and that of tlieir hybrid offspring, cannot have been acquired by the continued preservation of successive, ]irolit:d)lc degrees of sterility. It is, as I hope to show, inci- dental on differences in tlie reproductive system of the jiarciit- spceics, and is not cither a specially acquired or endoAVcd quality. In treating this subject, two classes of facts, to a large ex- tent fundamentally different, have generally been confounded together ; namely, the sterility of two species when first crossed, antl the sterility of the hybrids produced from them. Pure species have of course their organs of reproduction in a perfect condition, yet when intercrossed they produce either few or no offs])ring. Hybrids, on the other hand, have their reproductive organs functionally impotent, as may ])e dearly seen in the; statt; of the malt; element in both ])hints and ani- mals ; though the formative; organs themselves are perfect in structure, as far as the microscope reveals. In the first case the two sexual elements which go to form the embryo are per- 234 DEGKEES OF STERILITY. Chap. YIII. feet ; in the second case they are either not at all developed, or are imperfectly developed. This distinction is important, ■when the cause of the sterility, -which is common to the two cases, has to be considered. The distinction probably has been slurred over, owiniii; to the sterility in botli cases being' looked on as. a special endowment, Ijoyond the province of our reason- ing powers. The fertility of vaiietics, that is of the forms known or be- lieved to have descended from common parents, when inter- crossed, and likewise the fertility of their mongrel offspring, is, with reference to my theory, of equal importance with the sterility of species ; for it seems to make a broad and clear dis- tinction between varieties and species. Defjrees of Sterility. — First, for the sterility of species when crossed and of their hybrid offspring. It is impossible to study the several memoirs and works of those two conscientious and admirable observers, Kolrcuter and Gartner, who almost de- voted their lives to this subject, without being deeply im- pressed with the high generality of some degree of steriUty. Kolreuter makes the rule universal ; but then he cuts the knot, for in ten cases in which he found two forms, considered by most authors as distinct species, quite fertile together, he un- hesitatingly ranks them as varieties. Gartner, also, makes the rule equally universal ; and he disputes the entire fertility of Kolrcuter's ten cases. But in these and in many other cases, Gartner is obliged carefully to count the seeds, in order to show that there is any degree of sterility. He always compares the maximum number of seeds produced by two species when first crossed, and the maximum produced by their hybrid off- spring, with the average number produced by both pure par- ent-species in a state of nature, lint a serious cause of error seems to me to be here introduced : a plant, to be hybridized, nuist be castrated, and Avhat is often more imjiortant, must be secluded in order to prevent pollen being brought to it by in- sects from other plants. Nearly all the plants experimented on by Gartner were potted, and were kept in a chamber in his house. That these processes are often injurious to the fertility of a plant cannot be doubted ; for Giirtner gives in his table about a score of cases of plants Avhich he castrated, and arti- ficially fertilized with their own pollen, and (excluding all cases such as the Leguminosa', in which there is an acknowledged difliculty in the manipidation) half of these twenty plants had their fertility in some degree impaired. Moreover, as Giirlncr Chap. VIII. DEGREES OF STERILITY. 235 ropcalcdly cnissed some forms, such as the common red and blue pimpernels (Anagallis arvensis and ccerulea), wliicli the best botanists rank as varieties, and found them absolutely sterile, -sve may doubt whether many species are really so ster- ile, when intercrossed, as he believed. It is certain, on the one hand, that the sterility of various species "when crossed is so dilferent in deg'ree and graduates away so insensibly, and, on the other hand, that the fertility of pure sjiecies is so easily aflected by various circumstances, that for all practical purposes it is most difficult to say where perfect fertility ends and sterility begins. I think no better evidence of this can be required than that the two most ex- perienced observers who have ever lived, namely, Kolrcuter and Gartner, should luivc arrived at diametrically opposite con- clusions in regard to the very same species. It is also most instructive to compare — but I have not space here to enter on details — the evidence advanced by our best botanists on the question whether certain doubtful forms should be ranked as species or varieties, with the evidence from fertility adduced by different hybridizers, or by the same author, from ex])eri- ments made during different years. It can thus be shown that neither sterility nor fertility alfords any clear distinction ];c- tween species and varieties; but tliat the evidence from this source graduates away, and is doubtful in the same degree as is the evidence derived from other constitutional and structural differences. In regard to the sterility of hybrids in successive generations; though Giirtner was enabled to rear some hybrids, carefully guarding them from a cross with either pure parent, for six or seven, and in one case for ten generations, yet he asserts posi- tively that their fertility never increased, but generally de- creased greatly and suddenly. With respect to this decrease, it may first be noticed that, when any deviation in structure or constitution is common to both parents, this is often transmitted in an augmented degree to the oiTspring; and both sexual ele- ments in hyljritl ])lants are already allected in some degree. 15ut I believe in nearly all these cases, that the fertility has been diminished by an independent cause, namely, by too close interbreeding. I have collected so large a body of facts, show- ing on the one liand that an occasional cross with a distinct individual or variety increases the vigor and fertility of the offspring, and on the other hand that very clc>8e interbreeding lessons their vigor and fcrtihty, that I must admit the correct- 23G DEGREES OF STERILITY. Cu.vp. VIII. ness of this almost universal belief among breeders. Hybrids are seldom raised b}-- experimentalists in great numl)ers ; and as the parent-species, or other allied hybrids, generally grow in the same garden, the visits of insects must be carefully pre- vented during the ilowering season: hence hyljrids will gener- ally have to be fertilized during each generation by their own individual pollen ; and this would probably be injurious to their fertility, already lessened by their hybrid origin. I am strength- ened in this conviction by a remarkable statement repeatedly made by Giirtner, namely, that if even the less fertile hybrids be artilicially fertilized with hybrid pollen of the same kind, tlieir fertility, notwithstanding the frequent ill elFects from manipulation, sometimes decidedly increases, and goes on in- creasing. Now, in the process of ai'tilicial fertilization, pollen is as often taken by chance (as I know from my own experi- vncc) from the anthers of another tlower, as from the anthers of the tlower itself which is to be fertilized ; so that a cross be- tween two flowers, though probably often on the same plant, would be thus etfected. Moreover, whenever complicated ex- jieriments are in jirogress, so careful an observer as Gartner would have castrated his hybrids, and this wovdd have insured in each generation a cross with pollen from a distinct tlower, either from the same plant or from another plant of the same hj'brid nature. And thus the strange fact of an increase of fertility in the successive generations of art tficicdhf -fertilized hylirids, in contrast Avith those spontaneously self-fertilized, may, as I believe, be accounted for by too close interbreeding having been avoided. Now let us turn to the results arrived at l)y a third most experienced hybridizer, namely, the Hon. and Rev. W. Herbert. He is as emphatic in his conclusion that some hybrids are per- fectly fertile — as fertile as the pure parent-species — as are Kiil- rcuter and Gartner that some degree of sterility between dis- tinct species is a imiversal law of Nature. He experimented on some of the very same species as did Gartner, The diflVr- ence in their results may, I think, be in part accounted for by Herliert's great horticultural skill, and by his having hot-houses at his command. Of his many important statements I will here give only a single one as an examjile, namcdy, that " every ovule in a pod of Crinum capense fertilized by C. revolutuni ])roduced a plant, which I never saw to occur in a case of its natural fecundation." So that here wc have perfect, or even more than commonly perfect, fertility in a first cross between two distinct species. Chap. VIII. DEGREES OF STEKILITY. 237 This case of tlie Crinum leads me to refer to a singular fact, namely, that individual plants of certain species of Lobelia, Vcrbascuni, and Passillora, can easily be fertilized by pollen from a distinct species, but not by pollen from the same; plant, though this pollen can be proved to l)e perfectly sound by fer- tilizing other plants or species. In the genus llippeastrum, in Corydalis as shown by Prof. Hildebrand, in various orchids as shown by Mr. Scott and Fritz Mliller, all the individuals are in this })eculiar condition. So that with some species, certain abnormal individuals, and in other species all the individuals, can actually be hybridized much more readily than they can be fertilized by jiollen from the same individual plant ! To give one instance, a bulb of llippeastrum aulicum produced four flowers ; three were ferlilizod by Herbert with their own pollen, and the fourth was subsequently fertilized by the pollen of a compound hybrid descended from three distinct species: the result was, that " the ovaries of the first three flowers soon ceased to grow, and after a few days perished entirely, whereas the pod impregnated by the pollen of the hybrid made vigorous growth and rapid progress to maturity, and bore good seed, which vegetated freely." Mr. Herbert tried similar experi- ments during many y(iars, and always with the same result. With those plants in which certain individuals alone fail to be fertilized by their own pollen, though thej^ appear cjuite healthy and although both ovules and pollen are perfectly good with reference to other species, yet they must be in some way in an unnatural condition. Tliese cases serve to show on what slight and mysterious causes the lesser or greater fertility of a species sometimes depends. The practical experiments of horticulturists, though not made with scientific ])recision, deserve some notice. It is no- torious in how complicated a manner the species of Pelargo- nium, Fuchsia, Calceolaria, Petunia, Rhododendron, etc., have been crossed, yet many of these hybrids seed freely. For in- stance, Herbert asserts that a hybrid from Calceolaria integri- folia and jilanlaginea, species most widely dissimilar in general habit, "reproduced itself as perfectly as if it had been a natu- ral species from the mountains of Chili." I have taken some pains to ascertain the degree of fertility of some of the complex crosses of Khod(jdendrons, and I am assured that many of thcrn are perfectly fertile. Mr. C. Noble, for instance, informs mc that he raises stocks for grafting from a hvbrid between Rhod. Poiifieiiin and Catawbiense, and that this hybrid "seeds as 238 DEGREES OF STEKILITY. Chat. VIII. freely as it is possible to imac^ine." Had liybrids, when fairly treated, always p^ono on decreasing in fertility in each succes- sive generation, as Gartner believed to be the case, the fact would have been notorious to nursery-men. Horticulturists raise large beds of the same hybrids, and such alone are fairly treated, for by insect-agency the several individuals of the same hybrid variety are allowed to freely cross with each other, and the injurious influence of close interbreeding is thus ]irc- vented. Any one may readily convince himself of the eih- ciency of insect-agency by examining the lloAvers of the more sterile kinds of hybrid Rhododendrons, which produce no pol- len, for he will find on their stigmas plenty of pollen brought from other flowers. In regard to animals, much fewer experiments have been carefully tried than with jilants. If our systematic arrange- ments can be trusted, that is, if the genera of animals are as distinct from each other as are the genera of plants, then we may infer that animals more widely separated in the scale of Nature can be more easily crossed than in the case of plants ; l)ut the hybrids themselves are, I think, more sterile. 1 doubt whether any case of a perfectly-fertile hybrid animal can be considered as thoroughly well authenticated. It should, how- ever, be borne in mind that, owing to few animals breeding freely under confinement, few experiments have been fairly tried : for instance, the canary-bird has been crossed Avith nine other finches, but, as not one of these nine species breeds freely in confinement, we have no right to expect that the first crosses between them and the canary, or that their Inbrids, should be perfectly fertile. Again, with resjiect to the fertility in successive generations of the more fertile hybrid animals, 1 hardly know of an instance in which two fiimilies of the same hyl>rid have been raised at the same time from difierent par- ents, so as to avoid the ill-effects of close interbreeding. On the contrary, brothers and sisters have usually been crossed in each STiccessive generation, in opposition to the constantly- repeated admonition of every breeder. And in this case, it is not at all surprising that the inherent sterility in the hybrids should have gone on increasing. If we were to act thus, and pair brothers and sisters in the case of any pure animal, which from any cause had the least tendency to sterility, the breed would assuredly be lost in a few generations. Although I do not know of any thoroughly well-autlienli- catcd cases of perfectly-fertile liybrid animals, I have reason to Chap. Vlll. DEGREES OF STERILITY. 239 believe that tlio liyl)ri(ls from Cervulus vaginalis and Rccvesii, and from Pliasianus colchicus with P. toniuatus, arc perfectly fertile. It has lately been asserted in France, that two such distinct species as the hare and the raljbit, Avhen they can be |Tot to breed together, produce offspring, which are highly fer- tile when crossed with one of the parent-species. The hybrids from the common and Chinese geese (A. cygnoides), species which are so dillerent that they are generally ranked in dis- tinct genera, have often bred in this country with either pure parent, and, in one single instance, they have bred inter se. This was eifected by Mr. Eyton, who raised two hybrids from the same parents, but from difVerent hatches ; and from these two birds he raised no less than eight hybrids (grandchildren of the pure geese) from one nest. In India, however, these cross-bred geese must be far more fertile ; for I am assured by two emi- nently-capable judges, namely, Mr. Blyth and Captain Hutton, that whole flocks of these crossed geese are kept in various parts of the country ; and as they are kept for profit, where neither pure parent-species exists, they must certainly be high- ly or perfectly fertile. Tlie various races of each kind of domesticated animals are quite fertile when crossed together ; yet in many cases they are descended from two or more wild species. From this fact wc must conclude either that the aboriginal parent-species produced at first perfectly fertile hybrids, or that the hybrids subsequently reared under domestication became quite fertile. This latter alternative, which was first propounded by Pallas, seems tlie most probable, and can, indeed, hardly be doubted. It is, for instance, almost certain that our dogs are descended from several wild stocks ; yet, witli perhaps the exception of certain indigenous domestic dogs of South America, all are quite fertile together; and anahigy makes me greatly doubt whether the several aljoriginal species would at first have freely bred together, and have produced quite fertile hybrids. So, again, I have lately acquired decisive evidence tliat the crossed offspring from the Indian humped and common cattle are inter se perfectly fcrtih^; and from the observations by Ililtimeyer on their important osteijlogical differences, as well as from tliose by Mr. IJlytli on their differences in habits, voice, constitution, etc., these two forms must be regarded as good and distinct species as any in the world. According to this view of the origin of many domestic animals, we must either give up tlie belief of the almost universal sterility of distinct 240 LAWS GOVERNING THE STERILITY Chap. VIII. species of animals when crossed ; or we must look at sterility, not as an indelible characteristic, but as one capable of being removed by domestication. Finally, considering^ all the ascertained facts on the inter- crossing of plants and animals, it may be concluded that some degree of sterility, both in first crosses and in hybrids, is an extremely general result ; but that it cannot, under our present state of knowledge, be considered as absolutely univ'crsal. Laics f/overnlng the Sterility of First Grosses and of Hybrids. We will now consider a little more in detail the circum- stances and rules governing the sterility of first crosses and of hybrids. Our chief object will be to see whether or not the rules indicate that species have specially been endowed with this quality, in order to prevent their crossing and blending to- gether in utter confusion. The following rules and conclusions are chiefly drawn \\\) from Gartner's admirable work on the hybridization of plants. I have taken much pains to ascertain how far the rules apply to animals, and, considering how scanty our knowledge is in regard to hybrid animals, I have been sur- prised to find how generally the same rules apply to both king- doms. It has been already remarked, that the degree of fertilitj'', both of first crosses and of hybrids, graduates from zero to per- fect fertility. It is surprising in how many curious ways this gradation can be shown ; but only the barest outline of the facts can here be given. When pollen from a ])lant of one family is placed on the stigma of a plant of a distinct family, it exerts no more influence than so much inorganic dust. From this absolute zero of fertility, the pollen of dilferent species of the same genus applied to the stigma of some one of the species, 3-ields a jierfect gradation in the number of seeds pro- duced, up to nearly comj)lete or even quite comi)lete fertility ; and, as we have seen, in certain abnormal cases, even to an excess of fertility, beyond tiiat which the plant's own pollen j^roduces. So in hybrids themselves, there are some which never have produced, and probably never would produce, even with the pollen of the pure parents, a single fertile seed : but in some of these cases a first trace of fertility mvcy be detected, by the jiollen of one of the pure parent-sjiecies causing the flower of the hybrid to wither earlier than it otherwise would have done ; and the early withering of the flower is well known Chap. VIII. OF FIRST CROSSES AND OF HYBRIDS. 241 to be a sign of incipient fertilization. From this extreme degree of sterility wc have self-fertilized hybrids jiroduting a greater and greater number of seeds up to perfect fertility, Hyljrids from two species which are very difiicult to cross, and which rarely produce any offspring, are generally very sterile ; but the parallelism between the dilliculty of making a first cross, and the sterility of the hybrids thus jiroduced — two classes of facts which are generall}- confounded together — is by no means strict. There are many cases, in Avhich two pure species, :is in the genus Yerbascum, can be united with un- usual facility, and produce numerous hybrid-offspring, yet these hybrids are remarkabh' sterile. On the other hand, there are sp(M-ies which can be crossed very rarelv, or with extreme diflli- culty, but the hybrids, ■when at last produced, are very fertile. Even within the limits of the same genus, for instance in Dian- thus, these two opj^osite cases occur. • The fertility, both of first crosses and of hybrids, is more easily affected by unfavorable conditions, than is the fertility of pure species. But the degree of fertility is likewise innately variable ; for it is not always the same when the same two species arc crossed under the same circumstances, but depends in part upon the constitution of the individuals which happen to have been chosen for the experiment. So it is with hybrids, for their degree of fertility is often found to differ greatly in the several individuals raised from seed out of the same ca})sule and exposed to the same conditions. By the term systematic affinity is meant, the resemblance between species in structure and in constitution, more espe- cially in the structure of parts which are of high physiological importance and which differ little in the allied sjiecies. Now the fertility of fust crosses between species, and of the hybrids protluced from them, is largely governed by their systematic affmity. This is clearly shown by hybrids never having lieen raised l)etween species ranked by systematists in distinct fami- lies ; and, on the other hand, by very closely-allied species gener- ally uniting Mith facility. But the correspondence between systematic affmit}- and the facility of crossing is by no means strict, A nudtitude of cases could be given of very closely- allied species which Avill not imite, or only with extreme diffi- culty ; and, on the other hand, of very distinct species which unite with the utmost facility. In the same family there may be a genus, as Dianthus, in which very many species can most readilv be crossed ; and another genus, as ISilcne, in which the II 242 LAWS GOVERNING THE STERILITY Chap. VIIL most persevering efforts have failed to produce between ex- tremely-close species a single hybrid. Even within the limits of the same genus, wc meet with this same difference ; for in- stance, the many species of Nicotiana have been more largely crossed than the species of almost any other genus ; but Giirt- ner found that N. acuminata, which is not a particularly dis- tinct species, obstinately failed to fertilize, or to be fertilized by, no less than eight other species of Nicotiana. Many analo- gous facts could be given. No one has been able to point out what kind, or what amount, of difference in any recognizalile character is suflicient to prevent two species crossing. It can be shown that plants most Avidely different in habit and general appearance, and having strongly-marked differences in every part of the flower, even in the pollen, in the fruit, and in the cotyledons, can be crossed. Annual and perennial plants, deciduous and ever- green trees, plants inhabiting difl'erent stations and fitted for extremely different climates, can often be crossed with ease. By a reciprocal cross between two species, I mean the case, for instance, of a stallion-horse being first crossed with a fe- male-ass, and then a male-ass with a mare : these two species may then be said to have been reciprocally crossed. There is often the widest possible difference in the facility of making reciprocal crosses. Such cases are highly important, for they prove tliat the capacity in any two species to cross is often completely independent of their systematic affinity, or of any difference in their whole organization, except in their rej^ro- ductive systems. The diversity of result in reciprocal crosses between the same two species was long ago observed by Kiil- reuter. To give an instance : Mirabilis jalapa can easily be fertilized by the pollen of M. longiflora, and the hybrids thus 25roduced are suiliciontly fertile ; but Kolreuter tried more than two hundred times, during eight following j-ears, to fertilize reciprocally M, longiflora v»itli the pollen of ^I. jalapa, and utterly failed. Several other equally striking cases could be given. Thuret has observed the same fact with certain sea- weeds or Fuci. Gartner, moreover, found that this difference of facility in making reciprocal crosses is extremely common in a lesser degree. He has observed it even l)etween closely-re- lated forms (as Matthiola annua and glabra) which many bota- nists rank only as varieties. It is also a remarkable fact, that hybrids raised from reciprocal crosses, though of course com- pounded of the very same two species, fhe one species having Chap. VIII. OF FIRST CIJOSSES AND OF HYBRIDS. 243 first been used as the ftithcr and tlien as the mot her, though thev rarely ditFer in external characters, yet generally difier in fertility in a small, and oeeasionally in a high degree. Several other singular rules could be given iVom Gartner ; for instance, some species have a remarkable power of crossing with other species ; other species of tlie same genus have a re- markablt; power of impressing their likeness on their hybrid ollspring ; but these two powers do not at all necessarily go together. There are certain hybrids which, instead of having, as is usual, an intermediate character between their two par- (mts, always closely resemble one of them ; and such hybrids, tliougli externally so like one of their pure parent-species, are witli rare exceptions extremely sterile. So again among hy- brids which arc usuall}' intermediate in structure between their jiarents, exceptional and abnormal individuals sometimes are l)orn, Avhich closely resemble one of their pure parents; and these hybrids are almost always utterly sterile, even Avhen the other hybrids raised from seed from tlie same capsule have a considerable degree of fertility. These facts .show how com- pletely fertility in the hybrid is independent of its external lesemblance to either pure parent. Considering the several rules now given, Avhich govern the fertility of first crosses and of hybrids, we see that when forms, wliich must be considered as good and distinct species, are united, tlieir fertility graduates from zero to perfect fertility. hybrids produced from this cross ; that the fertility of hy- brids is not related to the degree in which the}' resemljle in external appearance either parent ; and, lastly, that the facili- ty of making a first cross between any two species is not al- ways governed by their systematic aiHnity or degree of re- semblance to each other. This latter statement is clearly proved by the dillerencc in the result of reciprocal crosses be- tween the same two species, for, according as the one species or the other is used as the father or the mother, there is gen- erally some difieriMice, and occasionally the widest possible dif- ference, in the facility of effecting a union. Tlie hybrids, more- over, produ(<'d from reciprocal crosses often dilTer in fertility. Now do these complex and singular rules indicate that .spe- cies have b(>en endowed with sterility simply to prevent their 244 LAWS GOVERNING THE STERILITY Chap. VIIL becoming confounded in nature ? I think not. For why should the sterility be so extremely difTerent in degree, Avhen various species are crossed, all of Avhicli we must suppose it would l)e equally important to keej") from blending together? Why should the degree of sterility lie innately variable in the individuals of the same species ? Why should some species cross with facility, and yet produce very sterile hybrids ; and odier species cross with extreme dilliculty, and yet produce fairly fertile hybrids ? Why should there often be so great a diflerence in the result of a reciprocal cross between the same two species ? Why, it may even be asked, has the production of hybrids been permitted ? To grant to species the special power of producing hybrids, and then to stop their further l)ro]iagation by dilferent degrees of sterility, not strictly related to the facility of the first union between their parents, seems a strange arrangement. The foregoing rules and facts, on the other hand, appear to me clearly to indicate that the sterility both of first crosses and of hybrids is simplv incidental or dependent on unknown differences in their reproductive systems; the differences being of so peculiar and limited a nature, that, in reciprocal crosses between the same two species, the male sexual element of the one will often freely act on the female sexual element of the other, but not in a reversed direction. It will be advisable to explain a little more fully by an example what I mean by ster- ility being incidental on other differences, and not a specially- endowed quality. As the capacity of one plant to be grafted or budded on another is so unimportant for its Avelfare in a state of nature, I prcsmne that no one will suppc^se that this capacity is a specialli/ endowed quality, but will admit that it is incidental on differences in the laws of growth of the two plants. We can sometimes see the reason why one tree will not take on another, from differences in their rate of growth, in the hardness of their wood, in the period of the flow or na- ture of their sap, etc. ; but in a multitj^de of cases we can as- sign no reason whatever. Great diAersity in the size of two plants, one being Avoody and the other herbaceous, one being evergreen and the other deciduous, and adaptation to widely- different climates, do not alwaA'S prevent the two grafting to- gether. As in hybridization, so with grafting, the capacity is limited by systematic alhniU', for no one has been able to graft trees together belonging to ((uite distinct families: and, on the other hand, closely-allied species, and varieties of the same CiiAi-. Vin. OF FIRST CROSSPIS AND OF HYBRIDS. o|5 species, can usually, but not iuvariabl}', be grafted with case, liut this capacity, as in hybridization, is by no means absolute- ly jToverned by systematic allinity. Althoup^'h many distinct genera within the same family have been grafted together, in otiier cases species of the same genus will not take on each other. The j)ear can be grafted far more readily on the rpiince, which is ranked as a distinct genus, than on the apple, wliich is a member of the same genus. Even dilferent varieties of the pear take with different degrees of facility on the quince ; so do dilTerent varieties of the apricot and peach on certain varieties of the plum. As Gartner found that there was sometimes an innate dif- ference in dilferent individuals of the same two species in crossing ; so Sageret behoves this to be the case with different individuals of the same two species in being grafted together. As in recipi-ocal crosses, the facility of effecting a union is often very far from equal, so it sometimes is in grafting; the couimon gooseberry, for instance, cannot be grafted on tlie cur- rant, Avhereas the currant will take, though with difficulty on the gooseberry. AVe have seen that the sterility of hybrids, which have their reproductive organs in an imperfect condition, is a differ- ent case from the dilHculty of uniting two pure species, which have their reproductive organs perfect ; yet these two distinct cases run to a certain extent parallel. Something analogous occurs in grafting ; for Thouin found that three species of llo- bini;i, which seeded freely on their own roots, and which could bo grafted with no great difficulty on another species, when thus grafted were rendered barren. On the otiier hand, cer- tain species of Sorbus, when grafted on other species, yielded twice as much fruit as Avhen on their own roots. AVe are re- minded by this latter fact of the extraordinary cases of Ilip- jieastrum, Passillora, etc., which seed much more freely wIkmi fertilized with the ])ollen of a distinct species, than wIk.mi fer- tilized with }K)llen from tlie same plant. Wc thus see, that, althougli there is a clear and great dif- ference between the mere adhesion of grafted stocks, and the union of the male and female elements in the act of reproduc- tion, yet that there is a rude degree of parallelism in the re- sults of grafting and of crossing distinct species. And as wc must look at the curious and complex laws governing the facility with which tre(\s can be grafted on each other as inci- dental on unknown differences in their vegetative SA'stcms, so 246 CAUSES OF TIJE STEUILITY Cu\r. VIII. I believe that the still more complex laws governing!: the facili- ty of first crosses are incidental on unknown dillerences in their reproductive systems. These dilTcrences, in both cases, follow to a certain extent, as might have been expected, sys- tematic allinity, by Avhieh every kind of resemblance and dis- similarity between organic beings is attempted to be ex- jiressed. The facts by no means seem to me to indicate that the greater or lesser dilFiculty of either grafting or crossing va- rious species has been a special endowment ; although in the Ciise of crossing, the difficulty is as important for the endur- ance and stabiUty of specific forms, as in the case of grafting it is unimportant for their Avelfare. Origin and Causes of the Ster'tUty of First Crosses and of JLjbrids. At one time it appeared to me probable, as it has to others, that the sterility of iirst crosses and of hybrids might have been slowly acquired through the natural selection of slightly- lessened degrees of fertility, which s]iontaneously appeared, like any other variation, in certain indivitluals of one variety "when crossed with another variety. For it would clearly be advantageous to two varieties or incipient species, if they could be kept from blending, on the same principle that, when man is selecting at the same time two varieties, it is necessary that he should keep them separate. In the Iirst place, it may be remarked that distinct regions are ofteii inhabited by groups of species and by single species, which, when brought together and crossed, are found to be more or less sterile ; now it could clearly have been of no advantage to such sejiarated species to have been rendered mutually sterile, and consequently this could not have been effected through natiu-al selection ; but it may perhaps be argued that, if a species were rendered sterile with some one compatriot, sterility with other species would follow as a necessary contingency. In the second place it is almost as much opposed to the theory of natural selection as to that of special creation, that in reciprocal crosses the male element of one form should be rendered utterly impotent on a second form, Avhile at the same time the male element of this second form is enabled freely to fertilize the Iirst form ; for this peculiar state of the reproductive system could not possibly be advantageous to either species. In considering the probability of natiual selection having CuAr. VIII. OF FIRST CROSSES AND OF IIYBKIDS. 247 come into action, in rendering species mutually sterile, one frrcat clifTiculty will he found to lie in the existence of many graduated steps from slin^litly-lesscned fertility to absolute sterility. It may be admitted, on the principle above ex- plained, that it would \n-oi\t an incipient species if it were rendered in some slight degree sterile when crossed with its j)ar(>nt-form or with some other variety ; for thus fewer bas- tartUzed and deteriorated odspiing would be produced to com- mingle their blood with the new species in jirocess of forma- tion. But he who will take the trouble to rellcct on the steps by which this first degree of steriUty could be increased through natural selection to that high degree which is common with so many species, and which is iniiversol with species which have been diflerentiated to a generic or family rank, will find the subject extraordinarily complex. After mature rellection it seems to me that this could not have been effected through natural selection ; for it could have been of no direct advantiigc to an individual animal to breed badly with another indi\-idual of a dilVcrent variety, and thus to leave few offspring ; conse- quently such individuals could not have been preserved or se- lected. Or take the case of two species which, in their present state when crossed, produce few and sterile oflspring ; now, what is there which coidd favor the survival of those individuals which ha})pe!ied to be endowed in slightly-higher degree Avith nnitual infertility, and which thus approached by one small step toward absolute sterility ? Yet an advance of this kind, if the theory of natural selection be brought to bear, must have in- cessantly occurred with many species, for a multitude are mutu- ally quite barren. With sterile neuter insects we have reason to believe that modifications in their structure and fertility have been slowly accumulated by natural selection, from an advan- tage having been thus indirectly given to the conununity to which they belonged over other communities of the same spe- cies ; but an individual animal not belonging to a social com- numity, if rendered slightly sterile when crossed with some other variety, would not thus itself gain any advantage or in- directly give any advantage to the other individnals of the same variety, thus leading to their preservation. From these considerations I infer, as far as animals are concerncil, that the various degrees of lessened fertility which occur with species when crossed cannot have been slowly accumulated by means of natural s(>l(>ction. With plants, it is possible that the case may be somewhat 248 CAUSES OF THE STERILITY Chap. VIII. (lifTcront. With many kinds, insects constantly ciirry pollen from neigliboriniT plants to the stif^mas of each flower ; and "with some species this is effected by the •wind. Now, if the pollen of a variet}', when deposited on the stij]^ia of the same va- riety, should become by spontaneous variation in ever so slig'ht a degree prepotent over the pollen of other varieties, this would certainly be an advantage to the variety ; for its own pollen would thus obliterate the efl\3cts of the pollen of other varieties, and prevent deterioration of character. And the more pre- ])(^tcnt the variety's own pollen could be rendered through nat- ural selection, the greater the advantage would be. We know from the researches of Giirtncr that, with s^xjcies which are mutually sterile, the pollen of each is always prepotent on its own stigma over that of the other species ; but we do not know whether this prepotency is a consequence of the mutual sterility, or the sterility a consequence of the prepotency. If the latter view be correct, as the prepotency became stronger through natural selection, from being advantageous to a spe- cies in process of formation, so the sterility consequent on pre- potency would at the same time be augmented; and the final result would be various degrees of sterility, such as occurs with existing species. This view might be extended to animals, if the female before each birth received several males, so that the sexual element of the prepotent male of her own variety ob- literated the effects of the access of previous males belonging to other varieties, but we have no reason to believe, at least with terrestrial animals, that this is the case ; as most males and females pair for each birth, and some few for life. On the whole we may conclude, that with animals the ster- ility of crossed species has not been slowly augmented, through natural selection ; and as this sterility follows the same general laws in the vegetable as in the animal kingdom, it is improba- ble, though apparently possible, that with plants crossed spe- cies should have been rendered sterile by a different process. From this consideration, and remembering that species Avhich have never coexisted in the same countiy, and Avhich therefore could not have received any advantage from having been ren- dei'ed mutually infertile, yet are generally sterile when cix)ssed ; and bearing in mind that in reciprocal crosses between the same two species there is sometimes the widest difference in their sterility, we must give up the belief that natural selection has come into play. We are thus driven to our former prop- osition, namely, that the sterility of first crosses, and indi- Chap. VIII. UF FIRST CROSSES AND OF HYBRIDS. 249 rcctly of hybrids, is simply incidental on unknow n ditFcrcnccs in tli(^ reproductive sx'slems of the parent-species. We may now try and look a little closer at tlic probable nature of these dilVerenies, Avhicli induce sterility in first crosses and in hybrids. I*iire species and hybrids difl'er, as already re- marked, in the state of their reproductive ore that when organic beings are placed under new and unnatural conditions, and when hybrids are produced by the unnatural crossing of two species, the reproductive system, independently of the general state of health, is affected by sterility in a very similar manner. In the one case, tlie condi- tions of life have been disturbed, though ofteit in so slight a degree as to be inappreciable by vis ; in the other case, or that of hybrids, the external conditions have remained the same, but the organization has been disturbed by two different struct- ures and constitutions having been blended into one. For it is scarcely possible that two organizations should be com- pounded into one, without some disturbance occurring in the de- velopment, or periodical action, or mutual relations of the differ- ent parts and organs one to another or to the conditions of life. When hyl)rids are able to breed inter se, they transmit to their offsj)ring from generation to generation the same compounded organization, and hence we need not be surprised that their sterility, thougli in some degree variable, does not diminish; it is even a]it to increase, this being generally the result, as l)eforc explained, of too close interbreeding. The above view of the sterility of hybrids being caused by two different constitutions being conlounded into one, has lately been strongly maintained l)y Max Wichura ; but it must be owned that the sterility (as will be immediately ex])lained) which affects the offspring of dimorphic and trimorpliic plants, when individuals l)elonging to the same form are united, makes this view rather doulitful. It should, however, be borne in mind that the sterilitv of these plants has been acfpiired for a special purpose, and may differ in origin from that of hybrids. It must be owned that we cannot understand, on the above 252 STERILITY OF HYBRIDS. Chap. VIII. or any other view, scvenil facts with respect to the sterility of hybrids ; for instance, the unequal fertility of hybrids produced from reciprocal crosses; or the increased sterility in those hy- brids Avhich occasionally and exceptionally reseniljle closely cither pure parent. Nor do I pretend that the foroi^oing re- marks go to the root of the matter ; no explanation is oflered why an organism, when placed under unnatural conditions, is rendered sterile. All that I have attempted to show is, that in two cases, in some respects allied, sterility is the common result — in the one case from the conditions of life having been disturljed, in the other case from the organization or constitu- tion having been disturbed by two organizations being com- pounded into one. A similar parallelism apparently extends to an allied yet very different class of facts. It is an old and almost universal belief, founded on a considerable body of evidence, that slight changes in the conditions of life are beneficial to all living things. We sec this acted on by fiirmcrs and gardeners in their frequent exchanges of seed, tubers, etc., from one soil or climate to another, and back again. During the convalescence of animals, great benefit is derived from almost any cliange in the habits of life. Again, both with plants and animals, there is abundant evidence that a cross between indiWduals of the same species, which difltr to a certain extent, gives vigor and fertility to the oflspring; and that close interbreeding con- tinued during several generations between the nearest rela- tions, especially if these be kept under the same conditions of life, almost always induces weakness and sterility. Hence it seems that, on the one hand, slight clianges in the conditions of life benefit all organic beings, and, on the other hand, that slight crosses, that is, crosses between the males and females of the same species, which have varied and become slightly different, give vigor and fertility to the offspring. But we have seen that greater changes, or changes of a jiarticular nature, often render organic beings in some degree sterile ; and that greater crosses, tliat is, crosses between males and females which have become widely or specifically different, produce hybrids which are generally sterile in some degree. I cannot persuade myself that this parallelism is an accident or an illusion. Both series of facts seem to be connected to- gether by some common but unknown bond, which is essen- tially related to the principle of life ; this principle apparently being that life, as Mr. Herbert Spencer has remarked, depends Chap. VIII. DIMORPHISM AND TRIMOKPIllSM. 253 on, or consists in, the incessant action and reaction of various forces, which, as throui^liout Nature, are always tending toward an e(iuilihriuiii ; and when this tendency is slip^htly disturbed by any change, tlie vital forces apparently gain in power. Meciprocal Dimorphism and Trimorpliism. This subject may he here briefly discussed, and will l)e found to throw some light on hybridism. Several plants belong- ing to distinct orders present two forms, which exist in al)out equal numbers, and whicli differ in no respect except in tlieir re])roductive organs ; one form having a long pistil with short stamens, tlie other a short ])istil with long stamens; botli with dilVerently-sized pollen-grains. With trimorphic plants there are three forms likewise difl'ering in the lengths of their pistils and stamens, in the size and color of the pollen-grains, and in some otlier respects ; and as in each of the three forms there are two sets of stamens, there arc altogether six sets of stamens, and three kinds of pistils. These organs are so proportioned in length to each other, that, in any two of the forms, half the stamens in each stand on a level with the stigma of the third fonn. Now I have shown, and the result has been confirmed Ijy other observers, that, in order to obtain full fertility Avith these ])lants, it is necessary that the stigma of the one form should be fertilized by pollen taken from the stamens of corre- sponding height in the other form. So that with dimorphic species two unions, wliich may be called legitimate, arc fully fertile ; and two, which may be called illegitimate, are more or less infertile. AVith tnmorj>hic species six imions are legiti- mate or fully fertile, and twelve are illegitimate or more or less infertile. The infertility which may be observed in various dimorphic and trimorphic ])lants, when they are illegitimately fertilized, that is, by ]io]Um) taken from stamens not coi'responding in height villi the pistil, dilTers much in degree, up to absolute and utter sttM-ility ; just in the same manner as occurs in cross- ing distinct species. As the degree of sterility in the latter case depends in an eminent degree on the conditions of life being more or less favorable, so I have found it witli iUegitimate imions. It is well known that if pollen of a distinct species be placed on the stigma of a flower, and its own ]iollen be afterward, even after a considerable interval of time, placed on the same stigma, its action is so strongly prepotent that it g.54 RECIPKOCAL DIMORPHISM Cuap. VIII. generally amiiliilatcs the effect of the foreign pollen; so it is with the pollen of the several forms of the same species, for legitimate pollen is strongly prepotent over illegitimate pollen, when botii are placed on the same stigma. I ascertained this by fertilizing several flowers, first illegitimately and twenty- four hours afterward legitimately, with pollen taken from a peculiarly-colored variety, and all the seedlings were similarly colored; this shows that the legitimate pollen, though applied twenty-four hours subsequently, had wholly destroyed or pre- vented the action of the previously-applied illegitimate pollen. Again, as in making reciprocal crosses between the same two species, there is occasionally a great difference in the result, so the same thing occurs Avith trimorphic plants ; for instance, the mid-styled form of Lythrum salicaria Avas illegitimately fertilized with the greatest case by pollen from the longer sta- mens of the short-styled form, and yielded many seeds ; but the latter form did not yield a single seed when fertilized by the longer stamens of the mid-styled form. In all these respects and in others which might have been adduced, the forms of the same imdoubted species when ille- gitimately united behave in exactly the same manner as do two distinct S[)ecics wheii crossed. This led me carefully to ob- serve during four years many seedlings, raised from several illegitimate unions. The chief result is that these illegitimate ])lants, as they may be called, are not fully fertile. It is possi- ble to raise from dimorphic species both long-styled and short- styled illegitimate plants, and from trimorphic plants all three illegitimate forms ; these can then be properly united in a le- gitimate maimer. When this is done, there is no apparent reason why they should not yield as many seeds as did their parents when legitimately fertilized. But such is not the case ; they are all infertile, but in various degrees; some being so utterly and incurably sterile that they did not yield during four seasons a single seed or even seed-capsule. The sterility of these illegitimate plants, when united with each other in a legitimate manner, may be strictly compared with that of hy- brids when crossed inter sc. When on the other* hand a hybrid is crossed with either pure parent-species, the sterility is usu- ally much lessened : and so it is when an illegitimate plant is fertilized by a legitimate plant. In the same manner as the sterility of hybrids does not always run parallel with the difii- culty of making the first cross between the two j^arent-species, so the sterility of certain illegitimate plants was usually great, f'liAr. VIII. AND TRIMORPIIISM. 205 while tlic sterility of the union from which they were derived was hy no means f^n\it. ^Vith liybrids raised from the same seed-capsule the deforce of sterility is innately variable, so it is in a marked manner with illefritimate plants. Lastly, many hybrids arc profuse and persistent flowcrers, while other and more sterile hybrids produce few flowers, and arc weak, miser- able dwarfs ; exactly similar cases occur with the illegitimate oflspring of various dimordhic and trimorphic plants. Altogether there is the closest identity in character and be- havior between illegitimate plants and hybrids. It is hardly an exaggeration to maintain that the former are hybrids, but jjroduced within the limits of the same species by the imjiroper imion of certain forms, while ordinary hybrids are produced from an improper union between so-called distinct species. We have also already seen that there is the closest similarity in all respects between first illegitimate unions and first crosses l)e+wcen distinct species. This will perhaps be made more fully ajiparcnt by an illustration : we may suppose that a bot- anist found two well-marked varieties (and such occur) of the loiig-stjled form of the trimorphic Lythrum salicaria, and that he determined to try by crossing whether they Mere specifi- cally distinct. He would find that they yielded only about one- fifth of the proper number of seed, and that they behaved in all the other above-specified respects as if they had been two dis- tinct species. But to make the case sure, he would raise plants from his supposed hybridized seed, and he would find that the seedlings were miserably dwarfed and utterly sterile, and that they behaved in all other respects like ordinary hybrids. He might then maintain that he had actually proved, in accordance with the common view, that his two varieties were as good and as distinct species as any in the world ; but he would be completely mistaken. The facts now given on dimorphic and trimorphic plants an» important, because they show us, first, that the ])hysi()logi- cal test of lessened fertility, both in first crosses and in hyl)ri(ls, is no safe critericm of specific distinction ; secondly, because we may conclude that there is some unknown bond which connects (li(> infertility of illegitimate unions with that of their illegiti- mate oflspring, and we an* led to extend the same view to lirst crosses and hybrids ; thirdly, because we find, and this seems to me of especial importance, that two or three forms of the same species may exist and may differ in no respect, except in their reproductive organs, and yet be sterile when united in 256 FERTILITY OF VARIETIES Chap. VIII. certain -ways. With dimorpliic plants, tlie unions between the two distinct forms arc alone quite fertile, and jmxluce quite fertile ofl'spring-, while unions between individuals belonging to the same form are more or less sterile ; so that the result is exactly the reverse of what occurs with distinct sj)ecies. With dimorphic plants the resultant sterility is quite indejiendent of any difference in general structure or constitution, for it arises from the union of individuals belonging not only to the same species, but to the same form. It must, therefore, depend on the nature of the sexual elements, which are so adapted to each other, that the male and female elements occurring in the same form do not suit each other, Avhile those occurring in the two distinct forms are mutually suited to each other. From these considerations, it seems proljable that the sterility of distinct species when crossed, and of their hybrid progeny, dejiends ex- clusively on the nature of their sexual elements, and not on any general difference in structure or constitution. We are, indeed, led to this same conclusion by considering reciprocal crosses, in which the male of one species cannot be imited, or can be imited with great dilhculty, with the female of a sec- ond species, while the converse cross can be effected with per- fect facility ; for this difference in the facility of making recip- rocal crosses and in the fertility of their offspring must be at- tributed cither to the male or to the female element in the first species having been differentiated, with reference to the sexual elements of the second species in a higher degree than in the converse case. That excellent observer, Gartner, likewise came to this same conclusion, namely, that species when crossed are sterile owing to differences confined to their repro- ductive systems. Fertility of Varieties tchen crossed, and of their Mongrel Offqyring. It may be urged, as an overwhelming argument, that there must be some essential distinction between species and varie- ties, inasmuch as the latter, however nuich tliey may differ from each other in external ajipearance, cross with perfect facil- it>-, and yield perfectly fertile offspring. Witli some excep- tions, presently to be given, I fully admit that this is the rule. But the subject is surrounded by difliculties, for, looking to va- rieties, produced under nature, if two forms hitherto reputed to be varieties be found in any degree sterile together, they are Chap. VIII. WHEN CROSSED. 257 .it onre ranked l)v ina.sl naturalists as species. For instance, the l)lue and red ijiiiipi'rnel, wliich are considered by most bot- anists as varieties, are said l)y Giirtncr not to be quite fertile when crossed, and he consequently ranks them as undoubted species. If we thus art»-uo in a circle, the fertility of all varie- ties produced under nature will assuredly have to be granted. If we turn to varieties, produced, or supposed to have been j)roduced, under domestication, we arc still involved in doubt. For when it is stated, for instance, that the German Spitz dog crosses more easily with the fox than do other dogs, or that (v-rtain South American indigenous domestic dogs do not rcadilv unite with European dogs, the explanation which will o^'cur to every one, and jirobably the true one, is, that these dogs are descended from aboriginally distinct species. Never- theless the perfect fertility of so many domestic varieties, dif- fering widely from each other in appearance, for instance those of -the pigeon, or of the cabbage, is a remarkable fact; more especially when we reflect how many species there arc, which, though resembling each other most closely, are utterly sterile wiien intercrossed. Several considerations, how'ever, render the fertility of domestic varieties less remarkable. In the first ])hice, it may be observed that the amount of external dill'er- cnce between two species is no suni guide to their degree of mutual sterility, so that similar differences in the case of vari- eties would be no sure guide. It is almost certain that with species the cause lies exclusively in differences in their sexual constitution. Now the conditions to which domesticated ani- mals and cultivated plants have been subjected, have had so little tendency tow^ard modifying the reproductive system in a manner leading to mutual sterility, that we have good grounds for admitting the directly opposite doctrine of Pallas, namely, that such conditions generally eliminate this tendency; so that the domesticated descendants of species, which in their natural state would have becMi in some degree sterile when crossed, become ])(;rfectly fertile together. With plants, so far is cul- tivation from giving a tendency toward sterility between dis- tinct species, that in several well-authenticated cases already alluded to, certain ])lants have been affected in an opposite mann(^r, for they^ have become self-impotent, while still retain- ing the ea|)a(itv of fertilizing and being fertilized by, other species. If the Pallasian doctrine of the elimination of sterility through long-continued domestication be admitted, and it can hardly be rejected, it becomes in the highest degree improl> 258 FERTILITV OF VARIETIES Chap. VIII. tiblc tliat similar circumstances should both induce and elimi- nate the same tendency ; though in certain cases, with species liaving a peculiar constitution, sterility might occasionally be thus induced. Tims, as I believe, we can understand why with domesticated animals varieties have not been produced which arc mutually sterile ; and why with plants only a few such cases, immediately to l)e given, have been observed. The real difficulty in our present subject is not, as it ap- pears to me, why domestic varieties have not become mutually infertile when crossed, but why this has so generally occun-ed Avith natural varieties as soon as they have been modified in a sufficient and permanent degree to take rank as species. "We are far from precisely knowing the cause ; nor is this surpris- ing, seeing hovv' profoundly ignorant we are in regard to the normal action of the reproductive system. But we can see that species, owing to their struggle for existence with numerous competitors, must have been exposed to more uniform condi- tions during long periods of time, than have been domestic varieties ; and this may well make a wide difference in the re- sult. For we know how commonly wild animals and plants, when taken from their natural conditions and subjected to cap- tivity, are rendered sterile ; and the reproductive functions of organic beings, which have always lived and been slowly mod- ilied under natural conditions, would probably in like manner be eminently sensitive to the influence of an unnatural cross. Domesticated productions, on the other hand, which, as shown by the mere fact of their domestication, were not originally highly sensitive to changes in their conditions of life, and winch can now generally resist with undiminished fertility repeated changes of conditions, might be expected to produce varieties, Avhich would be little liable to have their reproduc- tive powers injuriously affected by the act of crossing with other varieties which had originated in a like manner. I have as yet spoken as if the varieties of the same species were invariably fertile when intercrossed. But it is impossible to resist the evidence of the existence of a certain amount of sterility in the few following cases, which I will briefly abstract. Tiie evidence is at least as good as that from which we believe in the sterility of a multitude of s]iecies. The evidence is, also, derived from hostile Avitncsses, who in all other cases consider fertility and sterility as safe critej-ions of specific distinction. Giirtner kept during several years a dwarf kind of maize with yellow seeds, and a tall variety with red seeds growing ncai Chap. VIII. WHEN CROSSED. 059 each oilier in his r.arclcn ; and although these j)lants liavc separated sexes, they never naturally crossed. lie then fertil- ized thirteen flowers of the one with the pollen of the other; but only a sing-le head jiroduced any seed, and this one head produced onl}' live j^rains. Manipulation in this case could not have l)een injurious, as the plants have separated sexes. No one, I believe, has suspected that these varieties of maize arc distinct species; and it is important to notice that the hybrid plants thus raised were themselves j^erfecthj fertile ; so that even Gartner did not venture to consider the two varieties as specilically distinct. Girou do Buzareingues crossed three varieties of gourd, which, like the maize, has separated sexes, and he asserts that their mutual fertilization is by so much the less easy as their differences are greater. How far these experiments may be trusted, I know not ; but the forms experimented on are rapked by Sageret, who mainly founds his classification by the test of infertility, as varieties; and Naudin has come to the same conclusion. The following case is far more remarkable, and seems at fh-st quite incredible ; but it is the result of an astonishing number of experiments made during many years on nine spe- cies of Verbascmn, by so good an observer and so hostile a witness as Gartner; namely, that the yellow and white varie- ties when crossed i)roduce less seed than the similarly-colored varic;ties of the same species. Moreover, he asserts that, when yellow and white varieties of one species are crossed with yel- \o\v and white varieties of a distinct species, more seed is pro- duced by the crosses between the similarly-colored flowers than between those which are diflerently colored. ]\lr. Scott, also, has experimented on the species and varieties of Verbas- cum ; and, although unable to ctrnfinn Gartner's results on the crossing of the distinct species, he finds that the dissimilarly- colored varieties of the same species yield fewer seeds, in the proportion of 8G to 100, than the similarly-colored varieties, ^'et these varieties difl'er in no respect, except in the color of their flowers; and one variety can sometimes be raised from the seed of another. Kolrcuter, whose accuracy has been confirmed by every subscijucnt observer, has proved th(» remarkal)le fact that one particular variety of the connnon tobacco was more fertile than the other variiMies, when cro.ssed with a widely-distinct species. He experimented on live fonns, which are commonly reputed o(;,j HYBRIDS AND MONGRELS COMPARED. Chap. VIII. to be varieties, and which he tested by the severest trial, namel}', by reciprocal crosses, and he found their mongrel ofl- sprinf^ perfectly fertile. But one of these five varieties, when used either as the father or mother, and crossed with the Nico- tiana glutinosa, always yielded hybrids not so sterile as those which were produced from the four other varieties when crossed with Nicotiana glutinosa. Hence the reproductive system of this one variety must have been in some manner and in some degree modified. From these facts it cannot be maintained that varieties when crossed are invariably (luitc fertile : from the great diffi- culty of ascei'taining the infertility of varieties in a state of nature, for a supposed variety, if proved to be infertile in any degree, would almost universally be ranked as a species ; from man attending only to external characters in liis domestic va- rieties, and from such varieties not having been exposed for a very long period to uniform conditions of life. From these sev- eral considerations we may conclude that fertility does not con- stitute a fundamental distinction between varieties and species when crossed. The general sterility of crossed species may safely be looked at not as a special acquirement or endowment, but as incidental on changes of an vmknown nature in their sexual elements. Hybrids and 3fonr/rels compared^ independently of their Fertility. Independently of the quest i(m of fertility, the offspring of species when crossed, and of varieties when crossed, may be compared in several other respects. Giirtuer, whose strong wish it was to draw a distinct line between species and varie- ties, could find very few, and, as it seems to me, quite unim- portant differences between the so-called hybrid offspring of species, and the so-called mongrel offspring of varieties. And, on the other hand, they agree most closely in many im2:)ortant respects. I shall here discuss this subject with extreme brevity. The most important chstinction is, that in the first generation mon- grels are more variable than hybrids ; but Gartner admits that hybrids from species which have long been cultivated are often variable in the first generation; and 1 have myself seen striking instances of this fact. Gartner further admits that liybrida between very closely-allied species are more variable than Cii4.r. VIII. IIVIJRIDS AND MONGRELS COMPARED. ogi those from very distinct species ; and this shows that the dif- ference in the degree of variability graduates away. When mongrels and the more fertile liybrids are propagated for sev- eral generations, an extreme amount of variability in the off- spring in both cases is notorious ; but some few instances of botli hybrids and mongrels long retaining a imifonn character could be given. The variability, however, in the successive generations of mongrels is, perhaps, p^eater than in hybrids. This greater variability in mongrels than in hybrids docs not seem at all surprising. For the parents of mongrels are varieties, and mostly domestic varieties (very few ex])eriments having been tried on natural varieties), and this imjjlies that there lias been recent variability, Avhich would often continue and be added to that arising from the act of crossing. The slight variability of hybrids in the first generation, in contrast with the succeeding generations, is a curious fixct, and deserves attention. For it bears on the view which I have taken of one of the causes of ordinary variability ; namely, that the repro- ductive systeni from being eminently sensitive to changed con- ditions of life, fails under these circumstances to perform its proper function of producing offspring identical in all respects with the parent-form. Now, hybrids in the first generation are descended from sjiecies (excluding those long cultivated) which have not had their reproductive systems in any way allected, and they are not variable ; but hybrids themselves have their reproductive systems seriously affected, and their descendants are highly variable. Biit to return to our comparison of mongrels and hybrids : Gfirtner states that mongrels are more liable than hybrids to revert to cither parent-form ; but this, if it be true, is certainly only a difference in degree. Mort>over, Gartner expressly states that hylirids from long-cultivated plants are more subject to reversion than hybrids from species in their natural state; and this probably explains the singular difference in the results arrived at by dilferent obser\'ers : thus, Max Wichura doubts whether hybrids ever revert to their parent-forms, and he ex- perimented on uncultivated species of willows ; while Naudin, on the other hand, insists in the strongest terms on the almost universal tendencv to reversion in hybrids, and he experiment- ed chiedy on cultivated plants. Gartner further stiites that when any two species, although most closely allied to each other, are crossed with a third species, the hybrids arc widely different from each other; Avhereas, if two vtM-y distinct varie- 262 HYBRIDS AND MONGRELS COMPARED. Chap. VIII. ties of one species are crossed with another species, the hybrids do not diiler much. But this conclusion, as far as I can make out, is founded on a sinp^le experiment; and seems directly opposed to tlie results of several experiments made by Kol- reuter. Such alone arc the imimportant dilTercnces which Gartner is able to point out between hybrid and moiig'rel plants. On the other hand, the dco-recs and kinds of resemblance in mon- g^rels and in hybrids to their respective parents, more especially in hybrids produced from nearly related species, follow accord- ing to Giirtncr the same laws. When two species are crossed, one has sometimes a prepotent power of impressing its likeness on the hybrid ; and so I believe it to be with varieties of plants. With animals one variety certainly often has this prepotent power over another variety. Hybrid plants produced from a reciprocal cross, generally resemble each other closely ; and so it is with mongrel plants from a reciprocal cross. Both hybrids and mongrels can be reduced to either pure parent-form, by repeated crosses in successive generations Avith cither parent. These several remarks are apparently applicable to animals ; but the subject is here much complicated, partly owing to the existence of secondary sexual characters ; but more especially owing to prepotency in transmitting likeness running more strongly in one sex than in the other, both when one species is crossed with another, and when one variety is crossed with another variety. For instance, I think those authors are right, Avho maintain that the ass has a prepotent power over the horse, so that botli the mule and the hinny more resemble the ass than tlie horse ; but that the prepotency runs more strongly in the male-ass than in the female, so that the mule, which is the off- spring of the male-ass and mare, is more like an ass, than is the hinny, Avhich is the offspring of the female-ass and stallion. Much stress has been laid by some authors on the supposed fact, that it is only Avith mongrels that the offspring are not intermediate in character, but closely resemble one of their parents ; but this does sometimes occur Avitli hybrids, yet I grant much less frequently Avith them than Avitli mongrels. Looking to the cases Avliich I have collected of cross-bred ani- mals closely resembling one parent, the resemblances seem cliiefly confined to characters almost monstrous in their nature, and Avhich have suddcidy appeared — such as albinism, melanism, deficiency of tail or horns, or additional fingers and toes; and do not relate to characters Avhich have been sloAvly ac(|uired Chap. VIU. SUMMARY. 263 through selection. Consequently, sudden reversions to the perfect character of either parent would be much more likely to occur with mono-rels, Avhicli are descended from varieties often sudd(Mily produced and semi-monstrous in character, than with hvbrids, which are descended from species slowly and naturally produced. On the whole, I entirely agree witli Dr. Prosper Lucas, who, after arranging an enormous body of facts with respect to animals, comes to the conclusion that the laws of resemblance of the child to its parents are the same, whether the two parents dilTer little or nmch from each other, namely in the imion of individuals of the same variety, or of different varieties, or of distinct species. Independently of the question of fertility and sterility, in all other respects there seems to be a general and close simi- larity in the offspring of crossed species, and of crossed vari- eties. If we look at species as having been specially created, an'd at varieties as having been produced by secondary laws, this similarity would be an astonishing fact. But it harmo- nizes perfectly with the view that there is no essential dis- tinction l^etwcen species and varieties. Summary of Chapter. First crosses between forms sufficiently distinct to be ranked as species, and their hybrids, are very generally but not univer- sally sterile. The sterility is of all degrees, and is often so slight that the most careful experimentalists have arrived at diametrically ojiposite conclusions in ranking forms by this test. The sterility is innately variable in individuals of the same species, and is eminently susceptible to the action of favorable and imfavorable conditions. The degree of sterility does not strictly follow systematic affinity, but is governed by several curious and complex laws. It is generally different, and sometimes widely different, in reciprocal crosses between tlie same two species. It is not always equal in degree in a first cross and in the hybrids produced from this cross. In the same manner as in grafting trees, the capacity of one species or variety to take on another, is incidental on differ- ences, generally of an unknown nature, in their vegetative sys- tems, so in crossing, the greater or less facility of one species to unite with another is incidental on imknown differences in their reproductive systems. There is no more reaiion to think that species have been specially endowed with various degrees 2G4 6UMMAEY. Chap. VIH. of sterility to prevent their crossing and blending in nature, than to think that trees have been specially endowed with vari- ous and somewhat analogous degrees of ditliculty in being grafted together in order to prevent their inarching in our forests. Tlie sterility of first crosses and of their hylirid progeny lias not, as far as we can judge, been acquired through natural se- lection. In the case of first crosses it seems to depend on several circumstances ; in some instances in chief part on the early death of the embryo. In the case of liybrids, it perhajis depends on tlieir whole organization having been disturbed by lieing compounded from two distinct forms ; tlie sterility being closely allied to that Avhich so frequently aii'ects pure species, when exposed to unnatural conditions of life. This view is supported by a parallelism of another kind : namely, that, first, the crossing of forms only slightly diiferentiated favors the vigor and fertility of their offspring, while close interbreeding is injurious ; and secondly, that slight changes in the conditions of life apparently add to the vigor and fertility of all organic beings, while greater changes are often injurious. But the facts given on the sterility of the illegitimate unions of dimor- phic and trimorphic plants and of their illegitimate progeny, render it probable that some unknown bond in all cases con- nects the degree of fertility of first unions with that of their offspring. The consideration of these facts on dimorphism, as well as the results of reciprocal crosses, clearly leads to the conclusion that the primary cause of tlie sterility is confined to differences in the sexual elements. But why, in the case of species, the sexual elements should so generally liave become more or less modified, leading to their mutual infertility, we do not know% It is not surprising that the diniculty in crossing any two species, and the sterility of their hybrid-offspring, should in most cases correspond, even if due to distinct causes ; for both depend on the amount of difference between the species which are crossed. Nor is it surprising that the facility of effecting a first cross, and the fertility of tlie hybrids thus produced, and the capacity of being grafted together — though this latter capacity evidently depends on widely-different circumstances — should all run, to a certain extent, parallel with the system- atic afiinity of the forms subjected to experiment ; for system- atic airiiiity includes resemblances of all kinds. First crosses between forms known to be varieties, or snffi- CuAP. VIII. SUMMARY. 205 cicntly alike to be considered as varieties, and their mongrel ofl'spring, are very generally, but not, as is so often stated, invariably fertile. Nor is this almost universal and perfect fertility surprising, Avhen we remember liow liable we are to argue in a circle with respect to varieties in a state of nature ; and when we remember that the greater number of varieties have been jiroduced under domestication by the selection of mere external difl'crences, and that they have not been long exposed to uniform conditions of life. It should also be espe- cially kept in mind, that long-continued domestication tends to eliminate sterility, and is therefore little likely to induce this same cjuality. Independently of the question of fertility, in all other respects there is the closest general resemblance between hybrids and mongrels, in their variability, in their power of absorbing each other by repeated crosses, and in their inheri- tance of characters from both parent-forms. Finally, then, although we are profoundly ignorant of the precise cause of the sterility of first crosses and of h^'brids, the facts given in this chapter do not seem to me opposed to the belief that vari- eties and species are not fundamentally different. 12 2GG IMPEKFECTIO^' OF THE Chap. IX. CHAPTER IX. ON TIIK EMPKRFECTIOX OF THE GEOLOGICAIi EECOED. On the Absence of Intermediate Varieties at the Present Day— On the Nature of Ex- tinct Intermediate Variotii's : on their Number— On the Lapse of Time, as in- ferred from the Ilate of Denudation and of Deposition — On the Lapse of Time as estimated V)y Years — On the Poorness of our Paleontologieal Collections — on the Denudation of Granitic Areas— On the Intermittence of Geolo'jrical Formations — On the Absence of Intermediate Varieties in any one Formation— On the sudden Appearance of Groups of Species— On their sudden Appearance in the lowest known Fossiliferous Strata — Antiquity of the Uubitablc Earth. In the sixth chapter I enumerated tlie chief objections which iTiio;lit be justly urged against the views maintained in this A-olume. Most of them have now been discussed. One, namely, the distinctness of specific forms, and their not being blended together by innumerable transitional links, is a very obvious dilliculty. I assigned reasons why such links do not commonly occur at the present day, under the circumstances apparently most favorable for their presence, namely, on an extensive and continuous area with graduated physical condi- tions. I endeavored to show that the life of each species depends in a more important manner on the presence of other alrcady-deiined organic forms, than on climate ; and, therefore, that the really governing conditions of life do not graduate away quite insensibly like heat or moisture. I endeavored, also, to show that intermediate varieties, from existing in lesser numbers than the forms which they connect, will generally be beaten out and exterminated during the course of further modification and improvement. The main cause, however, of innumerable intermediate links not now occurring everywhere throughout Nature depends on the very process of natural selection, through which new varieties continually take the places of and exterminate their parent-forms. But just in pro- portion as this process of extermination has acted on an enor- mous scale, so must tlie number of intermediate varieties, which have formerly existed, be truly enormotis. ^V^ly, then, CuAP. IX. GEOLOGICAL RECORD. 267 is not every pfColor;ical formation and every stratum full of such iiitermcdiate links V GeolojTy assuredly does not reveal any such tinclv-^radiiated organic; chain; and this, perhaps, is the most obvious and serious objection which can be urf^ed ar^ainst the theor3^ The explanation lies, as I believe, in the extreme imperfection of the geological record. In the first place, it should always be borne in mind what sort of intermediate forms must, on the tiieory, have formerly existed. I have found it diilicult, when looking at any two species, to avoid picturing to myself forms directbj intermedi- ate between them. But this is a wholly false view ; we should alwa3's look for forms intermediate between each species and a common but unknown progenitor ; and the progenitor will generally have ditlered in some respects from all its modified descendants. To give a simple illustration: the fantail and pouter pigeons have both descended from the rock-pigeon; if we possessed all the intermediate varieties which have ever existed, we should have an extremely close series between both and the rock-pigeon ; but we should have no varieties directly intermediate between the fantail and jiouter ; none, for instance, combining a tail somewhat expanded with a crop somewhat enlarged, the characteristic features of these two breeds. These two breeds, moreover, have become so much modified, that, if we had no historical or indirect evidence re- garding their origin, it would not have been possible to have determined, from a mere comparison of their structure with tliat of the rock-pigeon, C. livia, whether they had descended irom this species or from some other allied species, such as C. oenas. So with natural species, if we look to forms very distinct, for instance to the horse and tapir, we have no reason to sup>- pose that links ever existed directly intermediate between them, but between each and an unknown connnon parent. The common parent will have had in its whole; organization much general resemblance to the tapir and to the horse ; but in some ])oints of structure may have difTcMcd considerably from both, even perhaps more than they differ from each other. Hence, in all such cases, we should be unable to recognize the parent- form of any two or more s])ecies, even if we closely compared the structure of the parent with that of its modified descend- ants, unless at the same time we had a nearly jierfect chain of the intermediate links. It is just possible by the theory that one of two living 208 THE LAPSE OF TIME. Chap. IX, forms might have ilescended from the other; lor instance, a horse from a tapir ; and in this case direct intermediate links will have existed between them. But such a case "would im- ply that one form had remained for a very long period unal- tered, while its descendants had undergone a vast amount of change ; and the principle of competition between organism and organism, between child and parent, will render this a very rare event ; for in all cases the new and improved forms of life tend to supplant the old and unimproved forms. By the theory of natural selection all living species have been connected with the parent-species of each genus, by dif- ferences not greater than we see between the varieties of the same species at the present day ; and these parent-species, now generally extinct, have in their turn been similarly con- nected with more ancient species ; and so on backward, always converging to the common ancestor of each great class. So that the number of intermediate and transitional links, be- tween all living and extinct species, must have been incon- ceivably great. But assuredly, if this theory be true, such have lived upon the earth. On the Lapse of Time, as inferred from the Mate of Deposi- tion and Extent of Denudation. Ind(^pendently of our not hnding fossil remains of such in- finitely numerous connecting links, it may be objected that time cannot have sufficed for so great an amount of organic change, all changes having been etlected very sIomIv. It is hardh' possible for me even to recall to the reader, who is not a practical geologist, the facts leading the mind feebly to com- prehend the lapse of time. He who can read Sir Cliarles Ly ell's grand Avork on the Principles of Geology, which the future historian will recognize as having produced a revolu- tion in natural science, 3'et does not admit how vast have been the past periods of time, may at once close this volume. Not that it suffices to study the Princij^les of Geology, or to read special treatises by d liferent observers on separate formations, and to mark how each author attempts to give an inadequate idea of the duration of each formation or even of each stratum. We can best gain some idea of jiast time by knowing the agen- cies at work, and learning how nuich of the surface of the land has l)cen denuded, and how nnich sediment has been deposited. As I.yell has well remarked, the extent and thickness of our CiiAP. IX. THE LAl'SK OF T1MI<:. 2G9 sciliincntiiry formations arc the result and the measure of the rlrnudation which the eartli's crust lias elsewhere underrionethshire of 12,000 feet; yet in these cas(>s there is nothing on the sm-face of the land to show such prodigious movements ; tlie jiile of rocks on cither side of the crack having been smoothly SAvej)t away. Chap. IX. THE LAPSE OF TIME. 271 Oil the other Imnd, in all parts of the world the piles of Kcdimentary strata arc of wonderful thickness. In the Cordil- lera I estimated one mass of cono:lomcrate at ten thousand feet ; and althou_Q;h conf2;lomerates have probably been accu- nuilated at a quicker rate than liner sediments, yet from being formed of worn and rounded pebbles, each of which bears the stamp of time, they are f:^ood to show how slowly tlic mass must have been heaped tos, would thus be removed from the mean level of the whoh* area in the course of six million vears. This seems an astonishing result, and some considerations lead to the sus- picion that it may be much too large, but even if halved or (juartered it is still very surprising. Few of us, however, know what a million really means: Mr. Croll gives the following 272 Tilt: LAPSE OF TIME. Cuap. IX. illustration : take a narrow strip of paper, 83 feet 4 inclies in length, and stretch it along the Avail of a large hall ; then mark off at one end the tenth of an inch. This tenth of an inch will represent one hundred years, and the entire strip a million years, But let it be borne in mind, in relation to the subject of this work, what a hundred years implies, represented as it is by a measure utterly insignificant in a hall of the above dimen- sions. Several eminent breeders, during a single lifetime, have so largely modified some of the higher animals, Avhich propa- gate their kind much more slowly than most of the lower ani- mals, that they have formed Avliat Avell deserve to be called new sub-breeds. Few men have attended AA-ith due care to any one strain for more than half a century, so that a hundred years represent the Avork of two breeders in succession. It is not to be supposed that species in a state of nature ever change so quickly as domestic animals under the guidance of methodical selection. The comparison Avould be in every AA'ay fairer Avitli the results Avhich follow from unconscious selection, that is, the preservation of the most useful or beautiful animals, Avith no intention of modifying the breed ; but by this process of unconscious selection, various breeds haA"e been sensibly changed in the course of two or three centuries. Species, however, probably change much more slowly, and within the same country only a few change at the same time. This slowness follows from all the inhabitants of the same country being already so avcU adapted to each other that ncAV places in the polity of Nature do not occiir until after long in- tervals, Avhen changes of some kind in the physical conditions or through immigration haA^e occurred; and indiA^idual differ- ences or variations of the right nature, by Avliich some of the inhabitants might be better fitted to their new places under the altered circumstances, might not at once occur. According to the standard of years Ave have no means of determining how long a period it takes to modify a species. Mr. Croll, judging from the amount of heat-energy in the sun and from the date Avhich he assigns to the last glacial epoch, estimates that only sixty million years lia\'e elapsed since the deposi- tion of the first Cambrian formation. This appears a A'ery short period for so many and such great mutations in the forms of life as have certainly since occurred. It is admitted that many of the elements in the calculation are more or less doubt- ful, and Sir "W. Thomson giA'Cs a Avide margin to the possible age of the habitable Avorld. But, as Ave have seen, Ave cannot Chap. IX. PALEONTOLOGICAL COLLECTIONS. 273 comprehend what tlio firrures 00,000,000 really imply ; and during this, or perhaps a lonfrcr roll of years, the land and the waters have everywhere teemed with livinj^ creatures, all ex- posed to the struggle for life and undergoing change. On the Poorness of oitr Paleontological Collections. Now let us turn to our richest geological museums, and what a paltry display we behold ! That our collections arc very imperfect is admitted by every one. The remark of that admira1:)lc paleontologist, Edward Forbes, should not be for- gotten, namely, that numbers of our fossil species are known and named from single and often broken specimens, or from a few specimens collected on some one spot. Only a small portion of the surface of the earth has been geologically explored, and no part with sufficient care, as the important discoveries made eVery year in Europe prove. No organism wholly soft can be preserved. Shells and bones Avill decay and disappear when left on the bottom of the sea, where sediment is not accumu- lating. I believe we often take an erroneous view, when we tacitly admit to ourselves that sediment is being deposited over nearly the whole bed of the sea, at a rate sufficiently quick to embed and preserve fossil remains. Throughout an enormously large proportion of the ocean, the bright blue tint of the water bespeaks its purity. The many cases on record of a formation conformably covered, after an immense interval of time, by another and later formation, Avithout tlic underly- ing bed having suffered in the interval any wear and tear, seem explicable only on the view of the l)ottom of the sea not rarely lying for ages in an unaltered condition. The remains Avhich do become embedded, if in sand or gravel, will when the beds are upraised generally be dissolved by the percolation of rain- water chargetl with carbonic acid. Some of the many kinds of animals which live on the beach between high and low water mark seem to be rarely jireserved. For instance, the several species of the Chthamalin;!:? (a sub-family of sessile cirripcdes) coat the rocks all over the world in infinite numbers: they are all slridly littoral, with the exception of a single Medit(;rranean species, which inhabits deep water, and this has been f(jund fossil in Sicily, whereas not one other species has hitherto been found in any tertiary formation : yet it is not known that the genus Chthamalus existeil during the T^halk period. Lastly, many great deposits requiring a vast 274 THE .POORNESS OF OUR Chap. IX, length of time for llicir Jiccuinuliilion, are entirely destitute of organic remains, witliout our being able to assign any reason : one of tlic most striking instances is that of the Flysch forma- tion, -wliich consists of shale and sandstone, several thousand, occasionally even six thousand feet, in thickness, and extending for at least 300 miles from Vienna to Switzerland ; and, al- though this great mass has been most carefully searched, no fossils, except a fcAv vegetable remains, have been found. "With respect to the terrestrial productions which lived during the Secondary and Paleozoic periods, it is superfluous to state that our evidence from fossil-remahis is fragmentary in an extreme degree. For instance, not a land-shell until quite recently was known belonging to cither of these vast periods, with the exception of one species discovered by Sir C. Lyell and Dr. DaAvsou in the carboniferous strata of North Amei'ica, of which shell above a hundred specimens have now been col- lected. In regard to mammiferous remains, a single glance at the historical table published inLyell's Manual will bring home the Iruth, how acciilental and rare is tlicir preservation, far better than pages of detail. Nor is their rarity surprising, when we remember how large a proportion of the bones of tertiary mam- mals have been discovered either in caves or in lacustrine deposits; and that not a cave or true lacustrine bed is known belonging to the age of oiu- secondary or paleozoic formations. But the imperfection in the geological record largely results from another and more important cause than any of the forego- ing ; namely, from the several formations being separated from each other l^y wide intervals of time. This doctrine has been most emphatically admitted by many geologists and paleon- tologists, who, like E. Forbes, entirely disbelieve in the change of species. When we see tiie formations tabulated in written works, or when we follow them in Nature, it is diihcult to avoid believing that they arc closely consecutive. But we know, for instance, from Sir R. Murchison's great work on Rus- sia, what wide gaps there are in that country between the superimposed formations ; Sf) it is in North America, and in many otlier parts of the world. The most skilful geologist, if his attention had been confined exclusively to tliese large ter- ritories, Avould never have suspected that, during the periods which were blank and barren in his own country, great piles of sediment, charged Avith new and peculiar forms of life, had elsewhere been accumulated. And if, in each separate territory, hardly any idea can be formed of the length of time which has- CiiAP. IX. PALEONTOLOGICAL COLLECTIONS. 075 elapsed lictwoL'u Iho consecutive formations, we may infer that this could nowhere be ascertained. The frequent and great changes in the miiieralogieal composition of consecutive forma- tions, generall}^ implying great changes in the geography of the surronnding lands, whence the sediment has been derived, accords with tlie belief of vast intervals of time having elapsed between each formation. But we can, I thinlc, see wliy the geological formations of each region are almost invariably intermittent; that is, have not followed each other in close sequence. Scarcely any fact struck me more when examining many hundred miles of the South American coasts, which have been uj)raised several hun- dred feet Avithin the recent period, than the absence of any recent deposits sufliciently extensive to last for even a short geological period. Along the whole west coast, Avhich is in- liabited by a peculiar marine fauna, tertiary beds are so poorly cTeveloped, that no 'record of several successive and peculiar marine faunas will probably be preserved to a distant age. A little refl(^ction will explain Avhy, along the rising coast of the western side of South America, no extensive formations with recent or tertiary remains can anywhere be found, though the supply of sediment must for ages have been great, from the enormous degradation of the coast-rocks and from muddy streams entering the sea. The exjilanation, no doubt, is, that the littoral and sub-littoral deposits are continually worn away as soon as they nro, lirought up by the slow and gradual rising of the land witliin the grinding action of the coast-waves. We niay, I think, conclude that sediment must be accumu- lated in extremely thick, solid, or extensive masses, in order to withstand the incessant action of tlie waves, wlien lirst up- raised and during successive oscillations of level, as well as the subsequent subaerial degradation. Such thick and exten- sive accumulations of sediment may be formed in two waj-s; either in profound depths of the sea, in which case the bottom will not be inhabited by so many and such varied f(jrms of life as the more shallow seas; and the* mass Avhen upraised will give an imperfect record of the organisms which existed tliroughout the world during the period of its accunuilation; or, sediment may be deposited to any thickness and extent over a shallow bottom, if it continue slowly to subside. In this latter case, as hmg as the rate of subsidence and the suj> ply of sediment nearly balance each other, the sea will remain shallow and favorable for many and varied fonns, and thus a 276 THE POORNESS OF OUR Chap. IX. rich fossiliferous formation, thick cnougli, \\hvn upraised, to resist a large amount of denudation, may be formed. I am convinced that nearly all our ancient formations, which are throughout the greater part of their thickness rlcJi in fos- sils, have thus been formed during subsidence. Since publish- ing my views on this subject in 1845, 1 have watched the prog- ress of Geology, and have been surprised to note how author after author, in treating of this or that great formation, has come to the conclusion that it was accumulated during. sub- sidence. I may add that the only ancient tertiary formation on the west coast of South America, which has been bulky enough to resist such degradation as it has as yet suffered, but which will hardly last to a distant geological age, was deposit- ed during a downward oscillation of level, and thus gained considerable thickness. All geological facts toll us plainly that each area has under- gone numerous slow oscillations of level, atid apparently these oscillations have affected Avide spaces. Consequently forma- tions rich in fossils and sufficiently thick and extensive to re- sist subsequent degradation, may have been formed over wide spaces during periods of subsidence, but only where the sup- ply of sechment was sufficient to keep the sea shallow and to embed and preserve the remains before they had time to deca3% On the other hand, as long as the bed of the sea remained sta- tionary, thich deposits could not hare been accumulated in the shallow parts, which are the most favorable to life. Still less could this have happened during the alternate periods of ele- vation ; or, to speak more accurately, the beds which were then accumulated Avill generally have been destroyed by being upraised and brought within the limits of the coast-action. These remarks appl}'" chiefly to littoral and sub-littoral de- jiosits. In the case of an extensive and shallow sea, such as that within a large part of the Malay Archipelago, Avhere the depth varies from 30 or 40 to 60 fathoms, widely-extended for- mation might be formed during a jicriod of elevation, and yet not suffer excessively fr©m denudation during its slow up- heaval ; but the thickness of the formation could not be great, for owing to the elevatory movement it would be less than the depth in whidi it was formed ; nor would the deposits be much consolidated, nor be cajiped by ovcrlj-ing formations, so that it would run a good chance of being worn away by atmos- ])heric dc^gradation and by the action of the sea during subse- quent oscillations of level. It has, however, been suggested CiiAr. IX. TALEONTOLOGICAL COLLECTIONS. 277 by Mr. Hopkins, lliat if one part of the area, after rising and before being denuded, subsided, the deposit formed during the rising movement, though not thick, might afterward become protected by fresh accumuhitions, and thus be jireserved for a long period. Mr. Hopkins also expresses his belief that sedimentary beds of considerable horizontal extent have rarely been completely destroyed. But all geologists, excepting the; few who believe that our present metamorphic schists and plutonic rocks once formed the primordial nucleus of the globe, Avill admit that tlicse latter rocks have been denuded on an enormous scale. For it is scarcely possible that such rocks could have been solidilied and crystallized while imcovered ; but if the meta- morphic action occurred at profound depths of the ocean, the former protecting mantle of rock may not have been very thick. Admitting, then, that gneiss, mica-schist, granite, diorite, etc., w-ere once necessarily covered up, how can we account for the naked and extensive areas of such rocks in many jiarts of the world, except on the belief that they have subsequently been completely denuded of all overl^ang strata ? lliat such ex- tensive areas do exist cannot be doubted : the granitic region of Parime is described by Humboldt as being at least nineteen times as large as Switzerland. South of the Amazon, Boue colors an area comjiosed of rocks of this nature as equal to tliat of Spain, France, Italy, part of German}-, and the British Isl- ands, all conjoined. This region has not been carefully ex- plored, but, from tlie concurrent testimony of travellers, the granitic area is very large : thus. Von Eschwege gives a de- tailed section of these rocks, stretching from Rio dc Janeiro for 260 geograjihical miles inland in a straight line ; and I trav- elled for 150 miles in another direction, and saw nothing but granitic rocks. Numerous specimens, collected along the whole c(jast from near Rio Janeiro to tlie mouth of the Plata, a dis- tance of 1,100 geographical miles, were examined l)y me, and they all belonged to this class. Inland, along the whole north- ern bank of the Plata, I saw, besides modern tertiary beds, only one small patch of slightly-metamorphosed rock, ^\■hich alone could have formed a part of the original capping of the granitic scries. Tuining to a well-known region, namely, to the United States and Canada, as shown in Prof. II. I). Rogers's beauti- ful map, I have estimated the areas l)y (Hitting out and weigh- ing the paper, and I find that liie metamorphic (including "the Henii-metamorphic") and granitic rocks exceed, in the i)n)p()rtion 273 ABSENCE OF INTERMEDIATE VARIETIES Chap. IX. of 19 to 1:2.5, the Avholc of the newer Paleozoic formations. In many rcg'ions the metamorphic and granitic rocks would be seen to be much more widely extended, if all the sedimentary beds Averc removed which rest unconformably on them, and whicli could not have formed part of the original mantle under which they were crystallized. Hence it is probable that in some parts of the world whole formations have been completely de- nuded, with not a wreck left behind. One remark is here worth a passing notice. During peri- ods of elevation the area of the land and of the adjoining shoal parts of the sea will be increased, and new stations will often be formed — all circumstances favorable, as previously explained, for the formation of new varieties and species ; but during such periods there will generally be a blank in the geological record. On the other hand, during subsidence, the inhabited area and number of inhabitants Avill decrease (excepting on the shores of a continent when fii-st broken up into an archipelago), and consequently during subsidence, though there will bo much ex- tinction, few new varieties or species will be formed ; and it is during these very periods of subsidence that the deposits which are richest in fossUs have been accumulated. On the Absence of JVumcrous Intermediate Varieties in any one Single Formatio7i, From these several considerations, it cannot be doubted that the geological record, viewed as a whole, is extremely im- perfect ; but if we conline our attention to any one formation, it becomes much more diilicult to understand why we do not therein find closely-graduated varieties between the allied species which lived at its commencement and at its close. Several cases arc on record of the same species presenting va- rieties in the upper and lower parts of the same formation : thus, Trautschold gives a number of instances with Ammon- ites ; and Hilgendorf has described a most curious case of ten graduated forms of Planorbis multiformis in the successive beds of a fresh-water formation in Switzerland. Although each formation has indisputably required a vast number of years for its deposition, several reasons can be given why each should not connnonly include a graduated series of links be- tween the species which lived at its commencement and close; but I cannot assign due proportional weight to the following considerations. CiiAr. IX. IN ANY SINGLE lORMATION. 219 Although each formation may mark a very long lapse of years, each probably is short compared with the period requi- site to change one sjieoies into another. I am aware that two paleontologists, whose opinions are worthy of much deference, namely, Broim and AVoodward, have concluded that the aver- age duration of each fcjrmation is twice or thrice as long as the average duration of specific forms. But insuperable difficul- ties, as it seems to me, prevent us from coming to an}"^ just conclusion on this head. W^hen we see a species first appear- ing in the middle of any formation, it would be rash in the ex- treme to infer that it had not elsewhere previously existed. So, again, when we find a species disappearing before the last layers have been deposited, it would be equally rash to sup- pose that it then became extinct. We forget how small the area of Europe is compared with the rest of the world ; nor have the several stages of the same formation throughout Eu- rope been correlated Avith perfect accuracy. With marine animals of all kinds, we may safelv infer a large amount of migration during climatal and other changes ; and when we see a species first appearing in any formation, the probability is, that it only then first iimnigrated into that area. It is well known, for instance, that several species ajipeared somewhat earlier in the paleozoic beds of North America than in those of Europe; time having apparently been required for their Tnigration from the American to the European seas. In examining the latest deposits in various quai'ters of the world, it has everywhere been noted that some few still existing spe- cies are common in the deposit, but have become extinct in the immediately surrounding sea ; or, conversely, that some arc now abundant in the neighboring sea, but are rare or absent in tliis particular deposit. It is an excellent lesson to reflect on the ascertained amount of migration of the inhabitants of Eu- rope during the glacial epoch, which forms only a part of one whole geolf)gical period ; and likewise to reflect on the changes of level, on the extreme change of climate, and on the great lapse of time — all included within this same glacial period. Yet it may be doubted whether, in any quarter of the world, sedi- mentary deposits, indiidlitf/ fossil rcmahis, have gone on ac- cumulating within the same area during the whole of this period. It is not, for instance, probable that S(Mliment was de- posited during the whole of the glacial jieriod near the mouth of the Mississippi, within that limit of (ie])th at which marine animals can best flourish ; for we know that great geographical 280 ABSENCE OF INTERMEDIATE VARIETIES Chap. IX. changes oocurrcd in other parts of America duriiifr this space of time. "\\'h(ui such beds as were deposited in shallow water near the mouth of the Mississippi during some part of the gla- cial period shall have been upraised, organic remains will prob- ably lirst appear and disappear at different levels, owing to the migrations of species and to geographical changes. And in the distant future, a geologist, examining these beds, would be tempted to conclude that the average duration of life of the embedded fossils had been less than that of the glacial period, instead of having been really far greater, that is, extending from before the glacial ejioch to the present day. In order to get a perfect gradation between two forms in the upper and lower parts of the same formation, the deposit will have to go on continuously accumulating during a very long period, so that there may be time sufficient for the slow procT^ss of modification ; hence the deposit will have to be a very thick one ; and the species undergoing change will have to live in the same district throughout this whole time. But we have seen that a thick formation, fossiliferous throughout its entire thickness, can accumulate only during a period of subsidence ; and to keep the depth approximately the same, which is necessary that the same marine species may hve on the same space, the supply of sediment must nearly counter- balance the amount of subsidence. But this same movement of subsidence will tend to submerge the area whence the sedi- ment is derived, and thus diminish the supply while the down- ward movement continues. In fact, this nearly exact balancing between the supply of sediment and the amount of subsidence is probably a rare contingency ; for it has been observed, by more than one paleontologist, that very thick deposits are usually barren of organic remains, except near their upper or lower limits. It would seem that each separate formation, like the whole pile of formations in any country, has generally been intermit- tent in its accumulation. When we see, as is so often the case, a formation composed of beds of dilVercnt mineralogical com- position, we may reasonably suspect that the jirocess of deposi- tion has been much intcrnij)t(Ml, as a change in the currents of the sea, and a supply of sediment of a different nature will generally have been due to geographical changes requiring much time. Nor will the closest ins]iection of a formation give any idea of the time which its deposition has consumed. Many instances could be given of beds only a few feet in thickness, riiAr. IX. IN ANY SINGLE FORMATION. ogj roiM(\scntIiig formations, elsewhere thousands of feet in thick- ness, and which must have required an enormous period for (heir accumuhition ; yet no one ignorant of this fact woukl have snspecled the vast lapse of time represented by the thinner for- mation. Many cases could be f]fiven of the lower beds of a formation havin*^ been upraised, denuded, submcrj^ed, and then i('-covercd by the upper beds of the same formation — facts, shcnving what wide, yet easily-overlooked, intervals have oc- curred in its accumulation. In other cases we have the plainest evidence in great fossilized trees, still standing upright as they grew, of many long intervals of time and changes of level (luring the process of deposition, which would never even have been suspected, had not the trees chanced to have been pre- served : tluis Sir C Lyell and Dr. Dawson found carboniferous beds 1,400 feet tliick in Nova Scotia, with ancient root-bearing strata, one above the other, at no less than sixty-eight diiferent levels. Hence, when the same species occur at the bottom, middle, and top of a formation, the probability is that they have not lived on the same spot during the whole period of depo- sition, but have disappeared and reapj)eared, perhaps many times, during the same geological period. So that, if such spe- cies were to undergo a consid(n-al)le amount of modification during any one geological period, a section would not include all the fin(^ intermediate gradations which must on our theory have existed between them, but aljrupt, though perhaps slight, changes of form. It is all-important to remember that natin-alists have no golden rule by which to distinguish species and varieties ; they grant some little variability to each species, but when they meet with a somewhat greater amount of dilference between any two forms, they rank both as species, uidess tliey are en- abled to coimect them together by the closest intermediate gradations. And this from the reasons just assigned we can seldom hope to effect in any one geological section. Supposing B and C to b(; two sjiecies, and a third. A, to l)e found in an older and underlying bed; even if A were strietly intermediate l)(!tween B and C, it would simply be ranked as a tliird and distinct sjiecies, unl(\ss at the sam«; time it could be most closely coimected with either one or l)oth forms 1)>' intermediate varieties. Nor should it be forgotten, as before explaitied, that A might ])(; the actual ]irogenitor of B and C, and yet might not necessarily be strictly intermediate between them in all re- opccts. So that we might obtain the parcnt>specics and its 282 ABSENCE OF INTERMEDIATE VAEIETIP:S Chap. IX. several modified descendants from tlu^ lower and upper beds of the same formation, and unless "\ve obtained numerous transi- tional gradations, we should not reco^ize their blood-relation- ship, and should consequently be compelled to rank them as distinct species. It is notorious on what excessively slight differences many paleontologists liave founded their species; and they do this the more readily if the specimens come from different sub- stages of the same formation. Some experienced conchologists are now sinking many of the very fine species of D'Orbigny and others into the rank of varieties ; and on this view we do find the kind of evidence of change which on tlie theory we ought to find. Look, again, at the later tertiary deposits, which include many shells believed by the majority of natural- ists to be identical with existing species ; but some excellent naturalists, as Agassiz and Pictet, maintain that all these ter- tiary species are specifically distinct, though the distinction is admitted to be very slight ; so that here, unless we believe that these eminent naturalists have been misled by their imaginations, and that these late tertiary species really present no difference whatever from their living representatives, or unless we believe that the great majority of naturalists are wrong, and that the tertiar}' sjiecies are all truly distinct from the recent, we have evidence of the frequent occurrence of sliglit modifications of the kind required. If we look to rather wider intervals of time, namely, to distinct but consecutive stages of the same great formation, we find that the embedded fossils, though almost universally ranked as specifically different, yet are far more closely related to each other than are the species found in more widely-separated formations ; so that here again we have un- doubted evidence of change in the direction required by the theory ; but to this latter subject I shall have to return in the following chapter. With animals and plants that propagate rapidly and do not wander much, there is reason to suspect, as we have for- merly seen, that their varieties are generally at first local ; and that such local varieties do not spread widely and supplant their parent-forms imtil they have been modified and perfected in some considerable degree. According to this A'icw, the chance of discovering in a fonnation in any one country all the early stages of transition between any two such forms is small, for the successive changes are supposed to liave been local, or confined to some one spot. Most marine animals have a wide Chap. IX. IN ANY SINGLE FORMATION. 033 range ; and wo have soon tliat with plants it is those wliicli have the widest rani>-o, that oftonest present varieties; so that, witli shells and other marine animals, it is pro])able that those which liavc had the widest range, far exceeding the limits of the known geological formations of Europe, have oftcnest given rise, ilrst to local varieties, and ultimately to new species; and this again would greatly lessen the chance of our being able to trace the stages of transition in any one geological for- mation. It is a more important consideration, leading to the same result, as lately insisted on by Dr. Falconer, namely, that the period during which each species underwent modification, though long as measured by years, was, from the reasons lately assigned, probably short in comparison with that during which it remained without undergoing any ch.ange. It should not be forgotten that, at the present day, with perfect specimens for examination, two forms can seldom be connected by intermediate varieties, and thus proved to be the same species, initil many specimens are collected from many ]ilaces ; and, in tlie case of fossil species, this can rarely be effected by paleontologists. "We shall, perhaps, best perceive the improbability of our being enabled to connect species by numerous fine, intermediate fossil links, by asking ourselves ^^•hether, for instance, geologists at some future period will be able to prove that our different breeds of cattle, sheep, horses, and dogs, are descended from a single stock or from several alwriginal stocks; or, again, Avhether certain sea-shells inliabit- ing the shores of North America, which are ranked by some conchologists as distinct species from their European represent- atives, and by other conchologists as only varieties, are really varieties, or are, as it is called, specifically distinct. This could be effected only by the future geologist discovering in a fossil state numerous intermediate gradations ; and such success is imj>r(>bable in tlu; iiighest degree. It has been assorted over and over again, l)y Avritors who believe in the immutability of species, that geology yields no linking forms. This assertion is entirely erroneous. As Sir J. Lubbock has nMuarkod, "Every species is a link between other allied forms." We clearly see this if we take a genus having a score of recent and extinct species and destroy four- lifths of thorn ; for in tliis case no one doubts that the ri'inain- dor will stand much more distinct from each other. If the ex- treme forn)s in the genus happen to have boon thus destroyed, 284 ABSENCE OF INTERMEDIATE VARIETIES Chap. IX. the genus itself in most cases will stand more distinct from ntlicr allied genera. The camel and the pig, or the horse and the tapir, are now obviously very distinct forms ; but if we add the several fossil quadrupeds which have already been discov- ered to the families including the camel and pig, these forma become joined by links not extremely wide apart. The chain of linking forms does not, hovrever, in these cases, or in any case, run straight from the one living form to the other, but takes a circuitous sweep through the forms which lived during long-past ages. What geological research has not revealed, is the former existence of infinitely numerous gradations, as fine as existing varieties, connecting nearly all our existing species w'itli extinct species. But this ought not to be ex- pected ; yet this has been repeatedly advanced as a most serious objection against my views. It may be worth while to sum up the foregoing remarks on the causes of the imperfection of the geological record under an imaginary illustration. The Malay Archipelago is about the size of liluvopc from the North Cape to the Mediterranean, and fi'om Britain to Russia ; and therefore equals all the geo- logical formations which have been examined with any accu- racy, excepting those of the United States of America. I fully agree with Mr. Godwin-x^usten, that the present condition of the Malay Archipelago, with its numerous large islands sep- arated by wide and shallow seas, probably represents the for- mer state of Europe, while most of our formations were accu- mulating. The Malay Archipelago is one of the richest re- gions of the "whole world in organic beings ; yet, if all the species were to be collected which have ever lived there, how imperfectly would they represent the natural history of the world ! But Ave have every reason to believe that the terrestrial productions of the archipelago would be preserved in an ex- tremely imperfect manner in the formations which we suppose to be there accumulating. Not many of the strictly littoral animals, or of those Avhich lived on naked submarine rocks, would be embedded ; and those embedded in gravel or sand would not endure to a distant epoch. Wherever sediment did not accumulate on the bed of the sea, or where it did not accu- mulate at a sufficient rate to protect organic bodies from decay, no remains could be preserved. Formations rich in fossils of many kinds, and of thick- ness sufficient to last to an age as distant in futurity as Chap. IX. IN ANY SINGLE FORMATION. 285 the secondary formations lie in the past, -wonkl f^cncrally be formed in the archipehigo only during poriods of subsi- dence. These periods of subsidence would be separated from each other by immense intervals of time, during which the area would be either stationary or rising ; while rising, the fossiliferous formations on the sleeper shores would be destroyed, almost as soon as accumulated, by the incessant coast-action, as we now see on the shores of South America ; even throughout the extensive and shallow seas witiiin the archipelago sedimentary beds could hardly be accumulated of great thickness during the periods of elevation, or become capped and protected by subsequent deposits, so as to have a good chance of enduring to a very distant future. During the periods of subsidence, there would probably be much ex- tinction of life ; during the periods of elevation, there would be much variation, but the geological record would then be least perfect. It may be doubted whether the duration of any one great period of subsidence over the whole or part of the archipelago, together with a contemporaneous accumulation of sediment, would exceed the average duration of the same specific forms; and these contingencies are indispensable for the preservation of all the transitional gradations between any two or more species. If such gradations were not all fully preserved, transitional varieties would merely appear as so many new and distinct species. It is also probable that each great period of subsidence would be interrupted by oscillations of level, and that slight climatal changes would intervene during such lengthy periods ; and in these cases the inhabitants of the archipehigo would migrate, and no closely-consecutive record of their modifications could be preserved in any one formation. Very many of the marine inhabitants of the archipelago now range: thousands of miles beyond its confines ; and analogy plainly leads to the belief that it would be chiefly these far-ranging species, though only some of them, which would oftenest produce new varieties ; and the varieties would at first generally be local or confined to one place ; but if jiossessed of any decided advantage, or when further modified and improveorhaj)s extremely-slight degree. 280 SUDDEN A^PEAPwA^X'E OF Chap. IX, and as they would Ijc found embedded in sliglilly-different sub-stages of the same formation, they would, according to the principles followed l)y many paleontologists, be ranked as new and distinct species. If, then, there be some degree of truth in these remarks, we have no right to expect to find, in our geological for- mations, an infinite number of those fine transitional forma which, on our theor}', have connected all the past and present species of the same gi-oup into one long and branching chain of life. We ought only to look for a few links, and such assuredly Ave do find — some more distantly, some more closely, related to each other ; and these links, let them be ever so close, if found in different stages of the same forma- tion, would, by many paleontologists, be ranked as distinct species. But I do not pretend that I should ever have suspect- ed how poor was the record in the best preserved geologi- cal sections, had not the absence of innumerable transitional links between the species which lived at the commencement and close of each formation, pressed so hardly on my theory. On the sudden Apjjearance of ichole Groups of allied /Species. The abrupt manner in which Avhole groups of species suddenly appear in certain formations, has been urged by several paleontologists — for instance, by Agassiz, Pictet, and Sedgwick — as a fatal objection to the belief in the trans- mutation of species. If numerous species, belonging to the same genera or families, have really started into life at once, the fact would be fatal to the theory of descent with slow modification through natural selection. For the devel- opment of a group of forms, all of which have descended from some one progenitor, nuist have been an extremely slow process; and the progenitors must have lived long ages before their mothfied descendants. But we continually over- rate the perfection of the geological record and falsely infer, because certain genera or families have not been foimd beneath a certain stage, that they did not exist before that stage. In all cases positive paleontological evidence may be implicitly trusted ; negative evidence is worthless, as experience has so often sho"wn. We continually forget how large the world is, compared with the area over which our geological formations have been carefully examined ; we CiiAP. IX. GROUPS OF ALLIED SPECIES. 287 forf^ot that g-ioups of species may elsewhere have long: existed, and liavc slowly multiplied, before they invaded the ancient archipela^^oes of Europe and the United States. We do not make due allowance for the enormous intervals of time which have elapsed between our consecutive forma- tions— loiiocr perhaps in many cases than the time required for the accumulation of each formation. These intervals will have g-iven time for the multiplication of species from some one or some few parent-forms ; and in the succeeding^ formation such groups of species will appear as if suddenly created. I may here recall a remark formerly made, namely, that it might require a long succession of ages to adapt an or- ganism to some new and peculiar line of life, for instance, lo liy through the air; and consequently that the transition- al forms would often remain confined to some one region ; but that, when this adaptation had once been effected, and a few species had thus acquired a great advantage over other organisms, a comparatively short time would 1)0 neces- sary to produce many divergent forms, ■which would spread rapidly and widely throughout the world. Prof. Pictet, in his excellent review of this work, in commenting on early transitional forms, and taking birds as an illustration, cannot see how the successive modifications of the ante- rior limbs of a supposed protot\'pe could possibly have been of any advantage. But look at the penguins of the Southern Ocean; have not these birds their front limbs in this precise intermediate state of "neither true arms nor true wings?" Yet these birds hold their place victoriously in the battle for life; for they exist in infinite numbers and of many kinds. I do not suppose that we here see the real transitional grades through which the wings of birds have passed ; but what special dilliculty is there in believing that it might profit the modified descendants of the ]ienguin, first to become enabled to flap along the surface of the sea like the logger- headed duck, and ultimately to rise from its surface and glide through the air? I will now give a fmv examples to illustrate the fore- going remarks, and to show how liable we are to error in supposing that whole groups of sj^ecies have suddenly been produced. Even in so short an interval as that between the iirst and second editions of Pictet's great work on Paleon- tologv, pu])lished in 18-i4-'4G and in ISSS-^o?, the conclusions 288 SUDDEN AITEAEANCE OF Cuap. IX. on the first appearance and disappearance of several groups of animals have l)ocn considerably modified ; and a third edition •would rociuire still further changes. I may recall the ■well-known fact that in geological treatises, published not many years ago, mammals were always spoken of as liaving abruptly come in at the commencement of the tertiary series. And now one of the richest known accumulations of fossil mammals, for its thickness, belongs to the middle of the secondary series ; and true mammals have been discov- ered in the new red sandstone at nearly the commencement of tliis great series. Cuvier used to urge that no monkey oc- curred in any tertiary stratum ; but now extinct species have been discovered in India, South America, and in Europe, as far back as the miocene stage. Had it not been for the rare acci- dent of the preservation of footsteps in the new red sandstone of the United States, who would have A^entured to suppose that, besides reptiles, no less than at least thirty kinds of birds, some of gigantic size, existed during that period ? Not a frag- ment of bone has been discovered in these beds. Notwith- standing that the number of joints shown in the fossil impres- sions corresponds with the number in the several toes of living birds' feet, some authors doubt whether the animals which left these impressions were really birds. Until quite recently these authors might have maintained, and some have maintained, that the whole class of birds came suddenly into existence during the eocene period; but now we know, on the authority of Prof. Owen, that a bird certainly lived during the deposition of the upper greensand ; and still more recently, that strange bird, the ArchcopterA'x, with a long, lizard-like tail, bearing a pair of feathers on each joint, and with its wings furnished with two free claAvs, has been discovered in the oolitic slates of So- Icnhofcn. Hardly any recent discovery shows more forcibly than this, how little we as yet know of the former inhabitants of the Avorld. I may give another instance, which, from having passed im- der my own eyes, has much struck me. In a memoir on Fossil Sessile Cirripedes, I have stated that, from the large number of existing and extinct tertiary species ; from the extraordinary abundance of the individuals of many sj:)ecies all over the world, from the Arctic regions to the equator, inhabiting A-arious zones of depths from the upper tidal limits to 50 fathoms ; from the perfect manner in which specimens are preserved in the oldest tertiary beds ; from the case with which even a frag- Chap. IX. GROUPS OF ALLIED SPECIES. 289 nicnt of a valve can be recorrnizetl ; from all these circum- stances, I infer that, had sessile cirripcdes existed durinn^ the secondary periods, they would certainly have Ijcen preserved and discovered ; and as not one species had then been discovered in oeds of this age, I concluded that this grcjit jrroup had been suddenly develo])ed at the commencement of the tertiary series. This was a sore trouble to me, adding as I thought one more instance of the abrupt appearance of a great group of species. But my work had hardly been published, when a skilful pale- ontologist, M. Bosquet, sent me a drawing of a perfect speci- men of an unmistakable sessile cirripede, Avhich he had him- self extracted from the chalk of Belgium. And, as if to make tlio case as striking as possil)le, this sessile ciiripede was a C'hthamalus, a very common, large, and ubiquitous genus, of which not one specimen has as yet been foimd even in any ter- tiary stratum. Hence we now positively know that sessile cirripedes existed during the secondary period ; and these cir- ripcdes might have been the progenitors of our many tertiary and existhig species. Still more recently Tyrgoma has been discovered by Mr, AA'oodward in the upper chalk. The case most frecjucntly insisted on by paleontologists of the apparently sudden appearance of a whole group of species, is that of the teieostean iishes, low down in the Chalk period. This group includes the large majority of existing species. Lately, Prof. Pictet has carried their existence one sub-stage further back ; and some paleontologists believe that certain nmch older fishes, of which the affinities are as yet imperfectly known, are really teieostean. Assuming, however, that the whole of them did appear, as Agassiz maintains, at the com- mencement of the chalk formation, the fact would certainly be highly remarkal)lc ; but I cannot see that it would be an in- superalile objection to these views, unless it could likewise be shown that tlie species of this group appeared suddenly and simultaneously throughout the world at tliis same period. It is almost superfluous to remark that hardly an}' fossil-lish are known from south of the equator; and by running tlu-ougli Pictet's Paleontology it will be seen that very few species are Ivtiown from several formations in Europe. Some few families of (ish now have a confined range ; the teieostean fish miglit formerly have had a simihirly confined range, and after having been largely developed in some one sea, might have spread widely. Nor have we any right to suppose that the seas of the world have always been so freely open from eouth to north 13 £90 GROUPS OF ALLIED SPECIES Cuap. IX. as they are at present. Even at tliis day, if the Malay Archi pehigo "svcre converted into land, the tropical parts of the In- dian Ocean would form a large and perfectly-enclosed basin, in Avliich any great group of marine animals might be multi- plied ; and here they would remain confined, until some of the species became adapted to a cooler climate, and were enabled to double the southern capes of Africa or Australia, and thus reach other and distant seas. From these considerations, from our ignorance of the geol- ogy of other countries beyond the confines of Europe and the United States, and fi'om the revolution in our paleontological knowledge eflected by the discoveries of the last dozen years, it seems to me to be about as rash to dogmatize on the succes- sion of organic forms throughout the world, as it would be for a naturalist to land for five minutes on a barren point in Aus- tralia, and then to discuss the number and range of its jjroduc- tions. On the sudden A2)2)earance of Groups of allied Species in the loicest Jcnoicn Fossil if eroiis Strata. There is another and allied difficulty, which is much more serious. I allude to the manner in Avhich many species in sev- eral of the main divisions of the animal kingdom suddenly appear in the lowest known fossiliferous rocks. Most of the arguments which have convinced me that all the existing species of the same group are descended from a single pro- genitor, apply with nearly equal force to the earliest known species. For instance, it cannot be doubted that all the Silu- rian trilobites are descended from some one crustacean, which must have lived long before the Silurian age, and which prob- ably difl'ered greatly from an>' known animal. Some of the most ancient Silurian animals, as the Nautilus, Lingula, etc., do not difter much from living sjiccies; and it cannot on our theory be sujiposed that these old species were the progenitors of all the species belonging to the same groups whicli have subseciucntly appeared, for they are not in any degree inter- mediate in character. Consequently, if the tlicory be true, it is indis]nitable that, before the lowest Silurian or Cambrian stratum was do- posited long periods elapsed, as long as, or probably far longer than, tlie whole interval from the Cambrian age to the present day; I'.nd that during these vast periods the world swarmed CuAP. IX. IN LOWEST FOSSILIFEROUS STRATA. ogj ■\vitli livinp; creatures. Here we encounter a formidable objec- tion ; for it sceins dou])tful whether the eartli in a lit state for the habitation of livin"^ creatures has lasted lono; cnouo-h. Sir W. Tiionipson concludes that the consolidation of the crust can hardly have occurred less than 20 or more than 400 million years ago, Ijut probably not less than 98 or more than 200 mill- ion years. Tliese very wide limits show how doubtful the data are ; and other elements may have to be introduced into tlie problem. Mr. C'roll estimates that about 00 million years have elapsed since the Cambrian period, but this, judging from the small amoiuit of organic change since the commencement of the Glacial epoch, seems a very short time for the many and great mutations of life, which have certainlv occurred since the Cambrian formation; and the previous 140 million years can hardly be considered as sufficient for the development of the varied forms of life which certainly existed toward the close of the Cambrian period. It is, however, probable, as Sir ^\^ Thompson insists, that the world at a verj- early period was subjected to more rapid and violent changes in its physical con- ditions than those now occurring; and such changes would have led to corresponding rapid changes in the organic being-s which inhabited the world at this remote period. To the question why we do not hnd rich fossiliferous de- posits belonging to these assumed earliest periods, I can give no satisfactory answer. Several eminent geologists, with Sir li. Murehison at their head, were until recently convinced that we beheld in the organic remains of the lowest Silurian stratum the first dawn of life. Other highly-competent judges, as Ja-qM and E. Forbes, have disputed this conclusion. We should not forget that only a small jiortion of the world is known with ac- curacy. Not long ago, M. Barrande added another and lower stage, abounding with new and peculiar species beneath the old Silurian system. Henmants of several forms have also been detected beneath BaiTande's so-called primordial zone in the Longmynd group, now divided into two stages, and con- stituting the Lower Cambrian system. The presence also of ])hosphatie nodules and bitunnnous matter in some of the low- est azoic rocks, jirobably indicates life at these periods. Now the great discovery of the Eozoon in the Laurent ian f(jrmation of Canada has been made, for, after reading Dr. Carpenter's description of tliis fossil, it is scarcely possible to doubt regard- ing its organic nature. There are tiiree great series of strata beneath tiie Siluriiin svslein in Canad:i, in the lowest of which 292 GROUPS OF ALLIED SPECIES Chap. IX. the Eozoon was found ; and Sir W. Logan states that their " united thickness may jiossibly far surpass that of all the suc- ceeding rocks, fiom llie base of the paleozoic series to the pres- ent time. We arc thus carried back to a period so remote, that the appearance of the so-called Primordial fauna (of Barraudc) may by some be considered as a comparatiyely modern eyent." The Eozoon belongs to the most lowly or- ganized of all classes of animals, but for its class is highly organ- ized ; it existed in countless numbers, and, as Dr, Dawson has re- marked, certainly preyed on other minute organic beings, which must have lived in great numbers. Thus the words, which I wrote in 1859, about the vast periods which had probably elapsed before the Cambrian system, are almost the same with those since used by Sir W. Logan. Nevertheless, the difliculty of assigning any good reason for the absence beneath the up- per Cambrian formations of vast piles of strata rich in fossils, is very great. It does not seem probable that the most an- cient beds have been quite worn away by denudation, or that their fossils have been wholly obliterated liy raetamorphic ac- tion, for if this had been the case we should have found only small remnants of the formations next succeeding them in age, and these -would always have existed in a partially-metamor- ])hoscd condition. But the descriptions which we possess of the Silurian deposits over immense territories in Russia and in North America, do not support the view, that the older a for- mation is, the more it has invariably suffered extreme denuda- tion and metamorphism. The case at present must remain inexplicable ; and may be truly urged as a valid argument against the views here enter- tained. To show that it may hereafter receive some explana- tion, I will give the following hypothesis : From the nature ot the organic remains which do not appear to have inhabited profound depths, in the several formations of Europe and of the United States; and from the amount of sediment, miles in thickness, of Avhich the formations arc composed, we may infer that from first to last large islands or tracts of land, whence the sediment was derived, occurred in the neighbor- hood of the now existing continents of Europe and North America. But we do not know what Avas the state of tilings in the intervals between the several successive fonnations ; whether Europe and the United States during these intervals existed as dry land, or as a submarine surface near land, on which sediment was not deposited, or as the bed of an open a lid unfathomable sea. Chap. IX. IN LOWEST FOSSILIFEROUS STRATA. 293 Lookinp^ lo the existing oceans, whicli are thrice as exten- sive as the hind, we see them studded with many ishmds ; but not one truly oceanic ishind (with the exception of New Zea- hind, if this can be called a truly oceanic island) is as ^-^et known to afford even a renmant of any jialeozoic or secondary forma- tion. Hence we may perhaps infer that, duriufr the paleozoic and secondary periods, neither continents nor continental islands existed where our oceans now extend ; for had they existed, ])aleozoicand secondary formations would in all j)roi)ability have been accumulated from sediment derived from their wear and tear ; and these would have been at least partiall}' upheaved by the oscillations of level, which must have intervened during these enormousl3'-long' periods. If, then, we may infer any thing from these facts, we may infer that, where our oceans now extend, oceans have extended from the remotest period of which we have any record ; and, on the other hand, that where cofitinents now exist, large tracts of hind have existc.'d, subjected no doubt to great oscillations of level, since the earliest Silurian j)criod. The colored map appended to my volume on Coral lleefs led me to conclude that the great oceans are still mainly areas of subsidence, the great archipelagoes still areas of oscil- lations of level, and the continents areas of elevation. IJut we have no reason to assume that things have thus remained from the beginning of the world. Our continents seem to have been formed by a jireponderance, during many oscillations of level, of the force of elevation ; but may not the areas of preponder- ant movement have changed in the lapse of ages ? At a period long antecedent to the Silurian epoch, continents may have ex- isted where oceans are now spread out ; and clear and open oceans may have existed where our continents now stand. Nor should we be justilied in assuming that if, for instance, the bed of the Pacific Ocean were now converted into a continent, we shouUl there find sedimentary formations in a recognizable con- dition older than the Silurian strata, suj^posing such to have been formerly deposited; for it might well happen that strata which had subsided some miles nearer to the centre of the earth, and which had been pressed on by an enormous weight of superincumbent water, might have undergcme far more met- amorphic action than strata Avhich have always remained nearer to the surface. The immense areas in some parts of the world, for instance in South America, of naked metamorphic rocks, which must have been heated under great pressure, have always seemed to me to require some special explanation; and 294 liirEEFECTION OF GEOLOGICAL RECOED. Chap. L\. we ma}' pcrliaps believe that avc sec, in these large areas, the many formations long anterior to the Silurian epoch in a com- pletely metamorphosed and denuded condition. The several difficulties here discussed — namely, that, though •\vc find in our geological formations many links between the species which now exist and which formerly existed, we do not find infiiutely numerous fine transitional forms closely jouiing them all together; the sudden manner in which several whole groups of species first appear in our European formations ; the almost entire absence, as at present known, of formations rich in fossils beneath the Cambrian strata, are all, undoubted- ly, of the most serious nature. A\'e see this in the fact that tlie most eminent paleontologists — namely, CuA-ier, Agassiz, Barrande, Pictet, Falconer, E, Forbes, etc., and all our great- est geologists, as Lyell, IMurchison, Sedgwick, etc., have unan- imously, often vehemently, maintained the immutability of species. But Sir Charles Lyell now gives the support of his high authority to the opposite side; and most other geologists and paleontologists are much shaken in their former belief. Those who believe that the geological record is in any d(>gree perfect, will undoubtedly at once reject the theory. For my part, following out Lyell's metaphor, I look at the geological record as a history of the world imperfectly kept, and written in a changing dialect ; of this history we possess the last vol- ume alone, relating only to two or three countries. Of this volume, only here and there a short chapter has been pre- served; and of each page, only here and there a few lines. Each word of the slowly-changing language, more or less dif- ferent in the successive chapters, may represent the forms of life, which arc entombed in our consecutive formations, and which falsely appear to us to have been abruptl}' introduced. On this vicAV, the difficulties above discussed are greatly dimin- ished, or even disappear. CHAr. X. GEOLOGICAL SUCCESSION. 295 CHAPTER X. ON TUE GEOLOGICAL SUCCESSION OF ORGA^^C BEINGS. On the Slow nnd Snccci>9ivc Appearance of New Species— On their Diflfcrcnt Rates of t'haii;,'C— Species once lost do not reappear — Groui)8 of Species follow the game (Jc-ncnil Rules in their Appearance and Uisappearunce as do Sinfrlc Species — On Extinction— On Sininltaneoiis C'hnn;,'es in the Forms of Life throu^rhout the World— On the Affinities of Extinct Species to cacli other and to Li\ in;; Spe- cies—On the State of Develo[)nKiit of Ancient Forms— On the Succession of tho same Typea within the same Areas- Summary of preccdinjj and present Chapter. Let us now see whether the several facts and laws relatint^ to the geological succession of organic beings better accord with the common view of the immutability of species, or with that of their slow and gradual modification, through descent and natural selection. New species have appeared very slowly, one after another, both on the land and in the waters. Lyell has shown that it is hardly possible to resist the evidence on this head in the case of the several tertiary stages ; and every year tends to fill up the blanks between the stages, and to make the percentage system of lost and new forms more gradual. In some of the most recent beds, though undoubtedly of high antitjuity if measured by years, only one or two species arc extinct, and only one or two arc new, having appeared there for the first time, either locally, or, as far as we know, on the face of the cartli. The secondary formations are more broken ; but, as Bronn has remarked, neither the appearance nor. disappear- ance of the many extinct species embedded in each formation has been simultaneous. Species of different genera iind classes have not changed at the same rate, or in the same degree. In the older tertiary beds a few living shells may still be foimd in the midst of a multitude of extinct fonns. Falconer has given a striking in- stance of a similar fact, for an existing crocodile is associated with many lost mammals and reptiles in the sub-IIimalayan deposits. The Silurian Liiigula dilTcrs but little from the 296 TJIE GEOLOGICAL SUCCESSION Chap. X. livinfj: spocics of this p^'onus ; whereas most of the other Silu- rian Molhiscs and all the; Crustaceans have changed greatly. The productions of the land seem to change at a quicker rate than those of the sea, of whicli a striking instance has lately been observed in Switzerland, There is some reason to believe that organisms high in the scale, change more quickly than those that are low ; though there are exceptions to this rule. The amount of organic change, as Pictet has remarked, is not the same in each successive so-called formation. Yet if we compare any but the most closely-related formations, all the species will be found to have undergone some change. When a species has once disappeared from the face of the earth, we have no reason to believe that the same identical form ever reappears. The strongest apparent exception to this latter rule is that of the so-called " colonies " of JM. Barrande, which intrude for a period in the midst of an older formation, and then allow the preexisting fauna to reappear ; but Lyell's ex- jjlanation, namely, that it is a case of temporary migration from a distinct geographical province, seems to me satisfac- tory. These several facts accord well Avith our theory, which in- cludes no fixed law of development, causing all the inhabitants of an area to change abruptl}^, or simultaneously, or to an equal degree. Tlic process of modification must be slow, and will generally affect only a few species at the same time ; for the variability of each species is quite independent of that of all others. Whether such variations or individual differences as may arise will be accumulated through natural selection in a greater or less degree, thus causing a greater or less amount of permanent modification, will depend on many complex con- tingencies— on the variations being of a beneficial nature, on the freedom of intercrossing, on the slowly-changing physical conditions of the country, on the immigration of new colonists, and on the nature of the other inhabitants with which the varying species come into competition. Hence it is by no means surprising that one species should retain the' same iden- tical form much longer than others ; or, if changing, that it should chang(! in a less degree. We find similar relations be- tween the inhaljitants of distinct countries ; for instance, the land-sliclls and coleopterous insects of jNIadeira have come to diller considerably from their nearest allies oh the continent of l^^urope, whereas the marine shells and birtls have remained unaltered. We can perhaps understand the apparently quicker Chap. X. OF ORGANIC BEINGS. 297 rate of change in terrestrial and in more higlily-org'anized pro- ductions compared with marine and lower productions, by the more complex relations of tlie hi<^her beings to tlieir organic and inor^^nic conditions of life, as explained in a former chap- ter. When many of the inhabitants of any area have become modified and imjiroved, we can imderstand, on the principle of competition, and from tlie all-important relations of organism to organism in the struggle for life, that any form which does not become in some deg'ree modified and improved, will be liable to extermination. Ilencc we see why all the species in the same region do at last, if we look to long-enougli intervals of time, become niodilicd, for otherwise they would become extinct. In members of the same class the average amount of change, during long and equal periods of time, may, perhaps, be nearly the same ; but as the accumulation of long'-enduring' for- mations, rich in fossils, depends on great masses of sediment being deposited on subsiding areas, our formations have been almost necessarily accumulated at wide and irregularly inter- mittent intervals of time ; consequently the amount of organic change exhibited by the fossils embedded in consecutive for- mations is not equal. Each formation, on this view, does not mark a new and complete act of creation, but only an occa- sional scene, taken almost at hazard, in an ever slowly-chan- ging drama. We can clearly understand why a species when once lost should never reappear, even if the very same conditions of life, organic and inorganic, should recur. For thougli the offspring of one species might be ada])t{Hl (and no doubt this has occurred in innunieral)le instances) to 1111 the place of another species' in tlie economy of Nature, and thus supplant it; yet the two forms — the- old and the new — would not be identically th.e same ; for both would almost certainly inherit different char- acters from their distinct progenitors, and organisms already differing would vary in a different manner. For instance, it is pist possible, if all our fantail jiigeons were destroyed, that fanciers might make a new breed hardly distinguishable from tlie present breed ; but if the parent rock-pigeon were likewise destroyed, and imdcr Nature we have every reason to believe that j»arent-fonns are generallv supplanted and exterminated bv tlieir improved offspring, it is incredible that a fantail, iden- tical with th(! existing breed, could he raised from any other species of pigeon, or even from any other well-established race 298 GEOLOGICAL SUCCESSION, Chap. X. of the domestic pigeon, for the successive variations would al- most certainly be in some degree different, and the newly- formed variety would probably inherit from its progenitor some characteristic differences. Groups of species, that is, genera and families, follow the same general rules in their appearance and disappearance as do single s))ecies, changing more or less qviickly, and in a greater or lesser degree. A group, when it has once disap- peared, never reappears ; that is, its existence, as long as it lasts, is contiruious. I am aware that there are some apparent excei:)tions to this rule, but the exceptions are surprisingly few, so few that E. Forbes, Pictet, and Woodward (though all strongly opposed to such views as I maintain), admit its truth ; and the rule strictly accords with the theor}'. For all the species of the same group, however long it may have lasted, are the modified descendants of each other, and of some com- mon progenitor. In the genus Lingula, for instance, the species which have successively appeared at all ages nmst have been connected by an unbroken series of generations, from the loAvest Silurian stratum to the present day. We have seen in the last chapter that many species of a group sometimes falsely appear to have come in abruptly in a body ; and I liavc attempted to give an explanation of tliis fact, which if true would be fatal to my views. But such cases are certainly exceptional ; the general rule being a gradual in- crease in number, until the group reaches its maximum, and then, sooner or later, a gradual decrease. If the number of the species included within a genus, or the number of the gen- era Avithin a family, be represented by a vertical line of vary- ing thickness, ascending through the successive geological for- mations in which the species are found, the line will sometimes falsely appear to begin at its lower end, not in a sharp point, but al^ruptly ; it then gradually thickens upward, often keep- ing for a space of equal thickness, and xdtimately thins out in the upper beds, marking the decrease and final extinction of the species. This gradual increase in number of the species of a group is strictly conformable Avith the theory, for the species of the same genus, and the genera of the same family, can in- crease only slowly and progressively ; the process of modifica- tion and the production of a number of allied forms necessarily being a slow and gradual process — one species first giving rise to two or three varieties, these being slowly converted in- to specie?, Avhich in their turn produce by equally slow steps Chap. X. EXTINCTION. 099 other varieties and species, and so on, like tlie branching of a great tree from a single stem, till the group becomes large. On Extinction. We have as yet spoken only incidentally of the disappear- ance of species and of groups of species. On the theory of natural selection the extinction of old forms and the produc- tion of ne\v and improved forms are intimately connected to- gether. The old notion of all tlie inhabitants of the earth hav- ing been swept away by catastrophes at successive periods is very generally given up, even by tliose geologists, as Elie de Beaumont, ]\Iurcliison, Barrande, etc., whose general views ■would naturally lead them to this conclusion. On the contrary, we have every reason to believe, from the study of the tertiary formations, that species and groups of species gradually disap- pear, one after another, first from one sj^t, then from another, and finally from tlie world. In some few cases, however, as by the breaking of an isthmus and the conseciuent irrujition of a multitude of new inhabitants into an adjoining sea, or by the final subsidence of an island, the process of extinction may have been rapid. Both single species and whole groups of species last for very unequal periods ; some groups, as we have seen, have endured from the earliest known dawn of life to the pres- ent day ; some have disappeared before the close of the paleo- zoic period. No fixed law seems to determine the length of time during which any single species or any single genus en- dures. There is reason to believe that tlic extinction of a whole group of species is generally a slower process than their production : if their appearance and disappearance be repre- sented, as before, by a vertical line of varying tliickness, tlie line is found to taper more gradually at its ujiper end, wliich marks tlie progress of extermination, than at its lower end, which marks the first appearance and the early increase in number of the species. In some cases, however, the extermina- tion of whole groups, as of ammonites toward the close of the secondary periofl, has been wonderfully sudden. The extinction of species has been involved in the most gratuitous mystery. Some authors have even supposed that, as the individual has a definite length of life, so liav<; species a definite duration. No one can have marvelled more than I have done at the extinction of species. "When I found in La Plata the tooth of a horse embedded witli the remains of Mas- 300 EXTINCTION, Chap. X. iodon, Mcgaihcrium, Toxodon, and other extinct monsters, ^vllich all coexisted with still liviiii? sliells at a very late geolo- gical period, I Avas filled Avilh astonishment; for, seeing that the horse, since its introduction by the Spaniards into South America, has run wild over the Avholc country, and has in- creased in numbers at an unparalleled rate, I asked myself what could so recently have exterminated the former horse un- der conditions of life apparently so favorable. But my aston- ishment was groundless. Prof. Owea soon perceived that the tooth, though so like that of the existing horse, belonged to an extinct species. Had this horse been still living, but in some degree rare, no naturalist Avould have felt the least sur- prise at its rarity ; for rarity is the attribute of a vast number of species of all classes, in all countries. If we ask ourselves why this or that species is rare, we answer that something is unfavorable in its conditions of life ; but what that something is, we can hardly ever tell. On the supposition of the fossil- horse still existing as a rare species, we might have felt certain, from the analogy of all other mammals, even of the slow-breed- ing ele])hant, and from the history of the naturalization of the domestic horse in South Amei'ica, that, xuider more favorable conditions, it would, in a very few years, have stocked the whole continent. 13ut wc could not have told what the un- favorable conditions Avere which checked its increase, Avhether some one or several contingencies, and at what period of the horse'^s life, and in what degree, they severally acted. If the conditions had gone on, however slowly, becoming less and less favorable, we assuredly should not have perceived the fact, yet the fossil-horse w^ould certainly have become rarer and rarer, and finally extinct — its place being seized on by some more successful competitor. It is most diilicult alwa3's to renaembcr that the increase of every creature is constantly being checked by imperceived hostile agencies ; and that these same unperceived agencies are amply sufficient to cause rarity, and finally extinction. So little is this subject understood, that I have heard surprise re- peatedly expressed at such great monsters as the Mastodon and the more ancient Dinosaurians having become extinct; as if mere bodily strength ga\e victory in the battle of life. Mere size, on the contrary, would in some cases determine, as has been remarked by Owen, quicker extermination from the greater amount of requisite food. Before man inhabited India or Africa, some cause must have checked the continued increase of the CiiAP. X. EXTINCTION. 301 existing; elephant. A lii;;lily-capablc judge, Dr. Falconer, be- lieves that it is chielly insects which, from incessantly harass- ing and weakening the elephant in India, check its increase ; and this was Bruce's conclusion with respect to the African elephant in Abj'ssinia. It is certain that insects and blood- sucking bats determine the existence of the larger naturalizec quadrupeds in several parts of South America. We sec in many cases in the more recent tertiary forma- tions, that rarity precedes extinction ; and we know that this has been the progress of events with those animals which have been exterminated, eillier locally or wholly, through man's agency. I may repeat what I published in 1815, uamel}', that to admit that sjiecies generally become rare before they be- come extinct — to feel no surprise at the rarity of a species, and yet to marvel greatly when the species ceases to exist, is much the same as to admit that sickness in the individual is t)ie forerunner of death — to feel no surprise at sickness, but, when the sick man dies, to wonder and to suspect that he died by some deed of violence. The theory of natural selection is grounded on the belief that each new variety, and ultimately each new species, is pro- duced and maintained by having some advantage over those with which it comes into competition ; and the consequent oxtinctioii of the less-favored forms almost inevitably follows. It is the same with our domestic productions ; when a new and slightly-improved variety has been raised, it at first sup- plants the less improved varieties in the same neighborhood; Avhen much improved, it is transported far and near, like our short-horn cattle, and takes the place of other breeds in other countries. Thus the a})pcarance of new forms, and the disap- pearance of old forms, both those naturally and those artifi- cially produced, are bound together. In nourishing groups, the number of new specific f(jrms which have been produced within a given time, has at some periods probably been greater than the number of the old specific forms which have been ex- terminated; but we know tliat species have not gone on indefi- nitely increasing, at least during the later geological epochs ; so that, looking to later times, we may believe that the pro- duction of new forms has caused the extinction of about the same number of old forms. llie competition will generally be most severe, as formerly explained and illustrated by examples, between the forms which are most like each other in all respects. Hence the 302 EXTINCTION. Chap. X. improved and modified dcsrcudants of a species will generally cause the externiiiiatiou of the parent-species; and if many new forms have been developed from any one species, the nearest allies of that species — i. e., the species of the same genus — will he the most liable to extermination. Thus, as I believe, a number of new speci(3s descended from one species, that is, a new genus, comes to sujiplant an old geiuis, belonging to the same family. But it must often have happened that a new species belonging to some one group has seized on the placfa occupied by a species belonging to a distinct group, and thus have caused its extermination. If many allied forms be de- veloped from the successful intruder, many will have to yield tlunr places ; and it Avill generally be the allied forms which will suffer from some inherited inferiority in common. But whetlicr it be species belonging to the same or to a distinct class, which have yielded their places to other modified and improved species, a few of the sufferers may often be preserved for a long time, from being fitted to some peculiar line of life, or from inhabiting some distant and isolated station, where they will have escaped severe competition. For instance, some species of Trigonia, a great genus of shells in the second- ary formations, survive in the Australian seas; and a few mem- bers of the great and almost extinct group of Ganoid fishes still inhabit our fresh waters. Therefore the utter extinction of a group is generalh', as Ave have seen, a slower process than its production. With res^DCct to the apparently svidden extermination of whole families or orders, as of Trilobites at the close of the paleozoic period and of Ammonites at the secondary period, we must remember what has been already said on the probable wide intervals of time between our consecutive formations ; and in these intervals there may have been much slow exter- mination. I\Ioreover, when, by sudden immigration or by un- usually rapid development, many species of a new group have taken possession of an area, many of the older species will have been exterminated in a correspondingly rapid manner; and the fonns Avhich thus yi(^^ld their places vrill commonly be allied, for they Avill partake of the same inferiority in common. Thus, as it seems to me, the manner in which single species and Avliole groups of species become extinct accords well with the theory of natural selection. We need not marvel at ex- tinction ; if we must marvel, let it be at our own presumption in imagining for a moment tliat wc understand the many com- CiiAr. X. FOKMS OF LIFE CHANGING. 3O3 plex cuntinoencies, on which ilic existence of e;ich species de- pends. If "SVC forrvers maintain that the existing productions of the United States are more closely related to those which lived in Europe during certain late tertiary stages, than to the present inhabit- ants of Europe ; and if this be so, it is e\ddent that fossilifer- ous beds now being deposited on the shores of North America would hereafter be liable to be classed with somewhat older European beds. Nevertheless, looking to a remotely future epoch, there can be little doubt that all the more modern ma- rine formations, namely, the upper pliocene, the pleistocene and strictly modern beds, of Europe, North and South Amer- ica, and Australia, from containing fossil remains in some de- gree allied, and from not including those forms which are found only in the older underlying deposits, would be correctly ranked as simultaneous in a geological sense. The fact of the forms of life changing simultaneously, in the above large sense, at distant parts of the Avorld, has greatly struck those admirable observers, MM. do Verneuil and d'Arcli- iac. After referring to the parallelism of the paleozoic forms of life in various pails of Europe, they 'add : " If. struck by tliis strange sequence, Ave turn our attention to North AmcricM, and there discover a series of analogous iihenomena, it wi!.' CiiAP. X. THROUGHOUT THE WORLD 305 appear certain that all these modifications of species, tlieir ex- tinction, and the introduction of new ones, cannot be owin^r to move chani^es in marine currents or other causes more or less local and temporarv, but de])cnd on general laws which govern the whole animal kingdom." M. Barrande has made forcible remarks to j)recisely tin; same eflect. It is, indeed, quite futile to look to changes of currents, climate, or other physical con- ditions, as the cause of these great mutations in the forms of lift; throughout the world, under the most different climates. Wc must, as Barrande has remarked, look to some special law. AVe shall see this more clearly when Ave treat of the present distribution of organic l)cings, and find how slight is the rela- tion between the ])hysical conditions of various countries, and the nature of their inhabitants. This great fact of the parallel succession of the forms of life througlu^ut the world, is explicable on the theory of natural selection. New species arc formed by having some advantage over older forms ; and the forms, whicli are already dominant, or have some advantage over the other forms in their own country, would be the most likely to give birth to the greatest number of new varieties or incipient sjiccies. We have distinct evidence on this head, in the plants which are dominant, that is, which are commonest and most widely diffused, producing the greatest numljcr of new varieties. It is also natural that the dominant, varying, and far-spreading species, which already 'lave invaded to a certain extent the territories of other species, should be those which would have the best chance of spreading still further, and of giving rise in new countries to other new varieties and species. The process of diffusion would often be very slow, depending on climatal and geographical changes, on strange accidents, and on the gradual acclimatization of new species to the various climates through which they might have to jxiss, but in the course of time the dominant forms would generally succeed in spreading, and would ultimately prevail. Th(' diffusion would, it is probable, be slower with the terres- trial inhabitants of distinct continents than with the marine inhabitants of llie continuous sea. We might therefore expect to find, as we do find, a less strict degree of parallelism in the succession of the productions of the land than with those of the sea. Thus, as it seems to me, the parallel, and,- taken in a large sense, simultaneous, succession of the same forms of life through- out the world, accords well witli tlic j>rinciple of new species 306 FORMS OF LIFE CHANGING Chap. X. havinfi^ been formed by dominant species spreading widely and varying; the new species thus produced being themselves dom- inant, owing to their having had some advantage over their already dominant parents, as well as over other species, ana again spreading, varying, and producing new forms. The old forms which are beaten and Avhicli yield their places to the new and victorious forms, will generally be allied in groups, from inheriting some inferiority in common ; and therefore, as new and imjjroved groups spread throughout the world, old groups disappear from the world ; and the succession of forms every- where tends to corres])ond both in their first appearance and final disappearance. There is one other remark connected with this subject worth making. I have given my reasons for believing that most of our great formations, rich in fossils, were deposited during periods of subsidence ; and that blank intervals of vast dura- tion, as far as fossils are concerned, occurred during the periods when the bed of the sea was cither stationary or rising, and likewise when sediment was not thrown down quiclcly enough to embed and preserve organic remains. During these long and blank intervals I suppose that the inhabitants of each re- gion underwent a considerable amount of modification and ex- tinction, and that there was much migration from other parts of the world. As we have reason to believe that large areas are affected by the same movement, it is probable that strictly contemporaneous formations have often been accumulated over very wide spaces in the same quarter of the world ; but we are very far from having any right to conclude that this has invari- ably been the ease, and that large areas have invariably been affected by the same movements. Wlien two formations have been deposited in two regions during nearly, but not exactly, the same period, we should find in both, from the causes ex- plained in the foregoing paragraphs, the same general succes- sion in the forms of life ; but the species would not exactly correspond ; for there will have been a little jnore time in the one region than in the other for modification, extinction, and immigration. I suspect that cases of this nature occur in Europe. Mr. Prestwich, in his admirable memoirs on the eocene dejiosits of England and France, is able to draw a close general parallelism between the successive stages in the two countries ; but when he compares certain stages in England with those in France, although he finds in both a curious accordance in the numbers Chap. X. TIIIIOUGIIOUT THE WOULD. ,^07 of tlie species bclongiiijr to the same genera, yet the species thetiisehes diircr in a manner very difTicult to account for, con- sidcrinfamilies and families, some of which are supposed to have perished at dilVercnt periods, and some to have endured to the present da}'. By looking at the diagram we can sec that if many of the extinct forms, supposed to be embedded in the successive for- mations, were discovered at several jioints low down in the series, the three existing families on the uppermost line would 310 AFFINITIES OF EXTINX'T SrFX'IKS. Chap. X. be rendered less distinct from each otlior. If, for instance, the g'enera «', «V ^'"5 /*» ''^*» ^''^S ''^'^ Averc disinterred, these three families would be so closely linked together that they proba- bly M'ould have to be xniited into one great family, in nearly the same manner as has occurred -with ruminants and certain pach^'dtM-ms. Yet he who objected to call the extinct genera which thus linked the living genera of three families together, intermediate in character, would be justified, as they are inter- mediate, not directly, but only by a long and circuitous course through many "vndely-different forms. If many extinct forms were to be discovered above one of the middle horizontal lines or geological formations — for instance, above No. VI. — but none from beneatli this line, then only two of the families (those on the left hand, «'*, etc., and b^*, etc.) would have to be imited into one ; and there wovild remain two families, which would be less distinct from each other than they were before the discovery of the fossils. So, again, if the three families formed of eight genera («" to ni^*), on the uppermost line, be supposed to differ from each other by half a dozen impor- tant characters, then the families which existed at the peiiod marked VI. would certainly have differed from each other by a less numljer of characters ; for they would at this early stage of descent have diverged in a less degree from their common progenitor. Thus it comes that ancient and extinct genera are often in some slight degree intermediate in character be- tween their modilled descendants, or between their collateral relations. In Nature the case will be far more complicates! than is represented in the diagram ; for the groups "will have been more numerous, they will have endured for extremely unequal lengths of time, and Avill have been modified in various degrees. As we possess only the last volume of the geological record, and that in a very broken condition, Ave have no right to expect, except in rare cases, to fill up the wide intervals in the natural system, and thus imife distinct fiimilies or orders. All that we have a right to expect is, that those groups which have within knoA\ni geological periods undergone much modification, should in the older formations make some slight approach to each other ; so that the older members should differ less from each other in some of their characters than do the existing members of the same groups; and this, by the concurrent evidence of our best paleontologists, is frequently the case. Thus, on the thcorv of dcsrcnt witli modification, the main CujLT.X. AFFINITIES OF EXTINCT SPECIES. 31 1 facts with respect to the mutual affinities of the extinct forms of life to each other and to livinf^ forms, are explained in a satisfactory manner. And they are wholly inexplicable on any other view. On this same theory, it is evident that the fauna of any one great ])eriod in the earth's history will be intermediate in g'eneral character between that which preceded and that which succeeded it. Thus the species which lived at the sixth p^reat stage of descent in the diagram are the modified ofTsjjring of those which lived at the fifth stage, and are the parents of 1 hose which became still more modified at the seventh stage; hence they could hardly fail to be nearly intermediate in char- acter between the forms duration of each formation is, probably, short compared with the average duration of specidc forms ; that migration has played an important part in the first ap- pearance of now forms in any one area and fonnation ; that widely-ranging species are those which have varied most fre- quently, and have oftenest given rise to new species ; that va- rieties have at first been local ; and lastly, although each spe- cies must have j^asscd through numerous transitional stages, it is probable that the periods, during which each underwent modification, though many and long as measured l)y years, have been short in comparison with the periods during which each remained in an unchanged condition. These causes, taken conjointly, will, to a large extent, explain why — though we do find many links — we do not find interminable varieties, con- necting together all extinct and existing forms liy the finest graduated steps. It should also be constantly borne in mind that any linking varieties between two or more forms, which might be found, would be ranked, unless the whole chain could be perfectly restored, as so many new and disthict species ; for it is not pretended that we have any sure criterion by which species and varieties can be discriminated. H(! who rejt;cts these views on the imperfection of tlie geo- logical record, will rightly reject the whole theory. For he may ask in vain where are the numberless transitional links which must formerly have connected the closely-allied or rep- resentative species found in the successive stages of the same great formation ? lie may disbelieve in the immense intervals of time which have elapsed between our consecutivt; forma- tions ; he may overlook how important a part migration has tjlaycd, when the fonnations of any one gieat region alone, as that of Europe, are considen-d ; he may urge the apparent, but often falsely appanMit, sudden coming in of whole groups of species. He may ask where are the remains of those infinitely- 3J0 SUMMARY OF THE Chap. X. numerous orsscnlially Aniorlcan, though they may be all peculiar species. We may look back to past ages, as shown in tli(j last chapter, and we lind American types then prevailing on the American Continent and in the American seas. AVe see in these fact.s some deep organic bond, throughout space and time, over the same areas of land and water, and indej)endent of physical con- ditions. The naturalist must be dull who is not led to inquire •what this bond is. The bond is simply inheritance, that cause which alone, as far as we positive!}' know, produces organisms quite Hke each other, or, as we see in the case of varieties, nearly alike. The dissimilarity of the inhabitants of different regions may be at- trilnited to modification througli natural selection, and in a subordinate degree to the definite inlluence of different phys- ical conditions. The degree of dissimilarity will depend on the migration of the more dominant forms of life from one re- gion into another having been prevented more or less effectually, at periods more or less remote — oii the nature and number of the former inmiigrants — and on the action of the inhabitants on each other in leading to the preservation of different modifi- cations; the relation of organism to organisin in the struggle for life being, as I have already often remarked, the most im- portant of all relations. Thus the high importance of barriers comes into play by checking migration ; as does time for the slow process of modification througli natural selection. Widely- ranging species, abounding in individuals, which have already triumphed over many competitors in their own widelj'-extendcd homes, will have the best chance of seizing on new places, when they spread into new countries. In their new homes they will be exposed to new conditions, and will fre(iuently undergo further modification and improvement ; and thus they will be- come still further victorious, and will produce grou[is of modi- fied descendants. On this principle of inheritance with modili- eation, we can uiidci-stand how it is that sections of genera, whole genera, and even families, are confined to the same areas, as is so commonly and notoriously the case. I believe, as was remarked in the last chapter, in no law of necessary d(?velopment. As the variability of each species is an independent property, and will Ix; taken advantage of by natural selection, only so far as it jirolits each individual in its complex struggle for life, so the amount of modification in difierent species Avill be no uniform (juantity. If a number of species, after having long conipeteil witli each otlier in their 32G SINGLE CENTRES OF CREATION. Chap. XI. old home, were to mi^^atc in a body into a new and afterward isolated country, they Avould be little liable to modification ; for n('itli(>r niig-ration nor isolation in themselves can effect any tiling. These principles come into play only by bringin*:^ or- ganisms into new relations with each other, and in a lesser de- gree Avith the surrounding physical conditions. As we have seen in the last chai)tcr that some forms have retained nearly the same character from an enormously remote geological period, so certain s})ecies have migrated over vast spaces, and have not become greatly or at all modified. According to these views, it is obvious that the several species of the same genus, though inhabiting the most distant quarters of the world, must originally have proceeded from the same source, as they are descended from the same progenitor. In the case of those species, which have undergone during whole geological periods but little modification, there is not much difficulty in believing that they may have migrated from the same region ; for, during the vast geographical and climatid changes which have supervened since ancient times, almost any amount of migration is possible. But in many other cases, in Avhich we have reason to believe that the species of a genus have been produced within comparatively recent times, there is great difficulty on this head. It is also obvious that the individ- uals of the same species, though now inhabiting distant and isolated regions, must have proceeded from one spot, where their parents were lirst produced: for, as explained in the last chajiler, it is incredible that individuals identically the same should have been produced from parents specifically distinct. Single Centres of supposed Creation. — We are thus brought to the question which has been largely discussed by naturalists, namely, whether species have beeia created at one or more points of the earth's surface. Undoubtedly there are many cases of extreme difficulty in understanding how the same species could possibly have migrated from some one point to the several distant and isolated points where noAV found. Nevertheless the simplicity of the view that each species was first produced within a single region captivates the mind. lie Avho rejects it, rejects the vera causa of ordinary generation with subsequent migration, and calls in the agency of a miracle. It is univer- sally admitted that in most cases the area inhabited by a spe- cies is continuous ; and that when a plant or animal inhabits two points so distant from each other, or with an interval of such a nature, that the space could not be easily passed over CuAP. XI. SINGLE CENTRES OF CREATION. . 327 by mip^raliun, the fact is g'iv^cn as something njiiarkablc and exceptional. The incapacity of migrating across a wide sea is more clear in the case of terrestrial mannnals than perhaps with any other organic beings; and, accordingly, Ave iind no inex- plicable instances of the same mammals inhabiting distant points of the world. No geologist feels any dilliculty in Great Britain possessing tlu^ same (juadrupeds with the rest of Europe, for they were no doubt once united. But if the same species can be produced at two separate points, why do we not find a single mammal common to Europe and Australia or Soutlx America ? The conditions of life are nearly the same, so that a multitude of European animals and plants have become nat- uralized in America and Australia; and some of the aboriginal plants are identically the same at these distant points of the northern and southern hemispheres ? The answer, as I believe, is, that mammals have not been able to migrate, whereas some ]>lants, from their varied means of disp(>rsal, have migrated across the wide and broken interspaces. The great and striking influence of barriers of all kinds is intelligible only on the view that the great majority of species have been produced on one side, and have not been able to migrate to the opposite side. Some few families, many sub-families, very many genera, and a still greater number of sections of genera, are conlined to a sin- gle region ; and it has been oljserved by several naturalists that the most natural genera or those genera in whicii the species are most closely related to each other, are generally confined to the same countr}', or if they have a wide range that their range is continuous. What a strange anomaly it woidd be, if a di- rectly opposite rule were to prevail, when we go down one step lower in the series, namely, to the individuals of the same spe- cies, and these had not been, at least at first, confined to some one region ! Hence it seems to me, as it has to many other naturalists, that the view of each species having been produced in one area alone, and having subsequently migrated from that area as far as its powers of migration and subsistence under past and pres- ent conditions permitted, is the most jirobable. Undoubtedly many cases occur, in which we cannot explain how the same species could have jiassed from one point to the other. But tlic geographical and dimatal dianges, which have certainly occurred within recent geological times, nuist have rendered discontinuous the formerly conliinious range of many species. So that we are reduced to consider whether the exceptions to 328 • SINGLE CENTKES OF CKEATION. Cii.vr. XI. contiiuiity of rani^c arc so numerous and of so grave a nature, that we ought to give up the belief, rendered probable by general considerations, that each species has been produced within one area, and has migrated thence as far as it could. It ^vould be hopelessly tedious to discuss all the exceptional cases of the same species, now living- at distant and separated points; nor do I for a moment i)retend that any explanation could l)c olVercd of many instances. But after some jireliminary remarks, I will discuss a few of the most striking classes of facts ; namely, the existence of the same species on the summits of distant; mountain-ranges, and at distant points in the arctic and ant- arctic regions; and secondly (in the following chapter), tlic wide distribution of fresh-water productions ; and thirdly, the occurrence of the same terrestrial species on islands and on the main-land, though separated by hundreds of miles of open sea. If the existence of the same species at distant and isolated points of the earth's surface, can in many instances be ex- plained on the view of each species having migrated from a single birthplace; then, considering our ignorance Avith respect to former climatal and geographical changes, and to the various occasional means of transport, the belief that a single birthplace is the law, seems to me incomparably the safest. In discussing this subject, we shall be enabled at the same time to consider a point equally important for us, namely, whether the several species of a genus, which must on the iheorj- all be descended from a common progenitor, can have migrated, undergoing modification during their migration, from some one area. When most of the species inhabiting one re- gion are diflcrcnt from those of another region, but are closely allied or belong to the same genera, if in all such cases it can be shown that there probably has been at some former period migration from the one region to the other, our general view will be much strengthened ; for the explanation is obvious on the principle of descent with modification. A volcanic island, for instance, upheaved and fomied at the distance of a few hundreds of miles from a continent, would prol^ably receive from it in the course of time a few colonists, and their descendants, though modified, would still be related by inheritance to the inhabitants of that continent. Cases of this nature are common, and are, as we shall hereafter see, inexplicable on the tlicory of indi'pendent creation. This view of the relation of the species of one region to those of another, does not differ much from that advanced by Mr. "Wallace, who concludes that " every spc- Chap. XI. MEANS OF DISPERSAL. 329 cics has come into existence coincident both in sjKice and time with a preexist ino- closely-allied species." And I now know that he attributes this coincidence to descent with modification. The discussion on " single and multiple centres of crea- tion " does not directly l)ear on another allied question — namely, whether all the individuals of the same species are descended from a single pair, or single hermaphrodite, or whether, as some authors suppose, from many individuals si- uuiltan<'()usly created. With organic beings which never inter- cross, if such exist, each species must be descended from a suc- cession of modified varieties, which have sujiplanted each other, but which have never blended with other individuals or varieties of the same species ; so that, at each successive stage of modification and improvement, all the individuals of the same variety will be descended from a single parent. IJut in the great majority of cases, namely, with all organisms which habitually unite for each birth, or which occasionally inter- cross, the individuals of the same species inhabiting the same area will be kept nearly uniform by intercrossing; so that many individuals will go on simultaneously changing, and the whole amount of modification at each stage will not be due to descent from a single parent. To illustrate what I mean : our English race-horses chfler from the horses of every other breed ; but they do not owe their difference and superiority to descent from any single pair, but to continued care in the selecting and training of many individuals during each generation. Before discussing the three classes of facts, which I have selected as presenting the greatest amount of difliculty on the theory of " single centres of creation," I must say a few words on the means of dispersal. Means of Dlq:)crsaL Sir C Lyell and other authors have ably treated this sub- ject. I can give here only the l^riefest abstract of the more important facts. Change of climate must have had a p(jwcr- ful influence on migration ; an impassable region when its climate was different from what it now is, may have been a high-road for migration; I shall, however, presently have to discuss this branch of the subject in some detail. Changes of level in the land must also have been highly influential : a narrow isthnuis now separates two marine faunas ; submerge it, or let it fonncrly have been submerged, and the two faunas 330 MEANS OF DISPERS^IL. Chap. XI. \vill now Ijlend or may fonnerly have blended : where the sea now extends, land may at former pciiods have connected islands or possibly even continents together, and thus have allowed terrestrial productions to pass from one to the other. No geologist will dispute that great mutations of level have occurred within the period of existing organisms. Edward Forbes insisted that all the islands in the Atlantic must have been recently connected with Europe or Africa, and Europe likewise with America. Other authors have thus hj'potheti- cally bridged over every ocean, and united almost every island to some main-land. If, indeed, the arguments used by Forbes are to be trusted, it must be admitted that scarcely a single island exists which has not recently been united to some conti- nent. This view cuts the Gordian knot of the dispersal of the same species to the most distant points, and removes many a dilhculty : but, to the best of my judgment, we are not author- ized in admitting such enormous geograplucal changes within the period of existing species. It seems to me that we have abundant evidence of great oscillations in the level of the land or sea ; but not of such vast changes in the position and exten- sion of our continents, as to have imited them within the recent period to each other and to the several intervening oceanic islands. I freely admit the former existence of many islands, now buried beneath the sea, which may have served as halting-places for plants and for many animals during their migration. In the coral-pi'oducing oceans such sunken islands are now marked by rings of coral or atolls standing over them. AVhcneverit is fully admitted, as no doubt it Avill some day be, that each species has proceeded from a single birthplace, and Avlien in the course of time we know something definite about the means of distribution, we shall be enabled to speculate with security on the former extension of the land. But I do not be- lieve that it Avill ever be proved that within the recent period most of our continents which now stand quite separate, have been continuously, or almost continuously, united with each oth- er, and Avith the many existing oceanic islands. Several facts in distribution — such as the great difference in the marine faunas on the opposite sides of almost every continent — the close re- lation of the tertiary inhabitants of several lands and even seas to their present inhabitants — the degree of affinity between the mammals inhabiting islands with those of the nearest con- tinent, being in ])art determined (as we shall hereafter see) by the depth of the intervening ocean — these and other such facts CiiAP. XI. MEANS OF DISPERSAL. 331 sceni to me oj^poscd to tlic julinissioii of such jirodigious geo- graphical revolutions -within the recent period, as are necessary on the view advanced b}^ Forbes and admitted by his many- followers. The nature and relative proportions of the inhab- itants of oceanic islands likewise seem to me opposed to the belief of their former continuity with continents. Nor does the almost universall}' volcanic composition of such islands iavor the admission that they are the wrecks of sunken conti- nents; if they had originally existed as continental mountain- ranges, some at least of the islands would ha^-e been formed, like other mountain-sinnmits, of granite, metamorphic schists, old fossiliferous and other rocks, instead of consisting of mere pih.'S of volcanic matter. I nmst now say a few words on -what are called accidental means, but which more properly should be called occasional means of distribution. 1 shall here coniine myself to plants. In botanical works, this or tliat plant is stated to be ill adapted for -wide dissemination ; but for transport across the sea the greater or less facilities may be said to be almost wholly unknown. Until I tried, -with JMr. ]5erkeley's aid, a few experiments, it -was not even known how far seeds could resist the injurious action of sea-water. To my surprise, I found that, out of 87 kinds, 6-4 germinated after an immersion of 28 days, and a few survived an inmiersion of 137 days. It deserves notice that certain orders Avere far more injured than others : nine Legu- minoste were tried, and, with one exception, they resisted the salt-water badly ; seven species of the allied orders, Hydro- phjdlaccw and Polemoniace:e were all killed by a month's im- mersion. For convenience' sake I chiefly tried small seeds, without the cajisule or fruit ; and, as all of these sank in a few (laj-s, they could not have been floated across wide spaces of th(^ sea, whether or not they were injured by the salt--water. Afterward I tried some larger fruits, capsules, etc., and some of these floated for a long time. It is well kncnvn what a dif- ference there is in the buoyancy of green aiid seasoned tim- ber; and it occurred to me that floods might wash down plants or branches, and that these might be dried on the banks, and then liy a fresh rise in the stream be washed into the sea. Hence I was led to dry stems and branches of 9-i ])lants with ripe fruit, and to plac(; them on sea-water. The majority sank quickly, but some, which while green floated for a very short time, when drietl floated nmch longer ; for instance, ripe hazel- nuts sank immediately, but when dried they floated for 90 332 MEANS OF DISPERSAL. Chap. XI. days, and afterward -wlioii planted tliey cremunatcd ; an aspar- a2:u.s-j)lant with ripe berries floated for ^3 days, when dried it floated for 85 (lays, and the seeds afterward germinated; the ripe seeds of llelosciadimn sank in two days, when dried they floated for above 90 days, and afterward germinated. Alto- gether, out of the 94 clried plants, 18 floated for above '28 days, and some of the 18 floated for a very much longer period. So that as -J;^- seeds germinated after an immersion of 28 days ; and as 1} ])lants with ripe fruit (but not all the same species as in the foregoing experiment) floated, after being dried, for above 28 days, as far as we may infer any thing from these scanty fticts, we may conclude that the seeds of ^^ plants of any country might be floated by sea-currents during 28 days, and would retain their power of germination. In Johnston's Physical Atlas, the average rate of the several Atlantic cur- rents is 33 miles per diem (some currents running at the rate of GO miles per diem) ; on this average, the seeds of -jJ^g- plants belonging to one country might be floated across 924 miles of sea to another country ; and when stranded, if blown to a favorable spot by an inland gale, they Avould germinate. Subsequently to my experiments, M. Martens tried similar ones, but in a much better manner, for he placed the seeds in a box in the actual sea, so that they were alternately wet and exposed to the air like really floating plants. He tried 98 seeds, mostly different from mine ; but he chose many large fruits and likewise seeds from plants which live near the sea; and this would have fiivored the average length of their flota- tion and of their resistance to the injurious action of the salt- water. On the other hand, he did not pre^^iously dry the plants or branches with the fruit ; and this, as we have seen, would have caused some of them to have floated much longer. The result was, that ^ of his seeds floated for 42 days, and were then capable of germination. But I do not doubt that plants exposed to the waves would float for a less time than tliose protected from violent movement as in our experiments. Therefore it would perhaps be safer to assume that the seeds of about -^/g- plants of a flora, after having been dried, could be floated across a space of sea 900 miles in Avidth, and would then germinate. The fact of the larger fruits often floating longer than the small, is interesting ; as plants with large seeds or fruit could hardly be transported by any other means ; and Alph. do Candollo has shown that such plants generally have restricted ranges. CiiAP. XI. MEANS OF DISPERSAL. 833 But seeds 111:13' l)c occasionally transported in another man- ner. Drift-timber is thrown up on most islands, even on those in the midst of the widest oceans; and the natives of the coral-islands in the Pacific procure stones for their tools, solely from the roots of drifted trees, these stones being a valualjle royal tax. I find, on examination, that, when irregularly- shaped stones are embedded in the roots of trees, small parcels of earth are very frequently enclosed in their interstices and behind them — so perfectly that not a particle could be washed away in the longest transport : out of one small portion of. earth thus completely enclosed by wood in an oak about 50 years old, three dicotyledonous plants germinated : I am cer- tain of the accuracy of this observation. Again, I can show that the carcasses of birds, when floating on the sea, sometimes escape being immediately devoured ; and seeds of many kinds in the crops of floating birds long retain their vitality : peas andvetches, for instance, are killed by even a few days' immer- sion in sea-water ; but some taken out of the crop of a pigeon, which had floated on artificial salt-water for 30 days, to my surprise nearly all germinated. LiWng birds can hardly fail to be highly-effective agents in the transportation of seeds. I could give many facts showing how frequently birds of many kinds are bloAvn by gales to vast distances across the ocean. We may safely assume that undei" such circumstances their rate of flight would often be 35 miles an hour ; and some authors have given a far higher estimat'C. 1 have never seen an instance of nutritious seeds passing through the intestines of a bird ; but hard seeds of fruit pass uninjured through even the digestive organs of a turkey. In the ctHirse of two months, I picked up in my garden 12 kinds of seeds, out of the excrement of small birds, and these seemed jierfect, and some of them, which were tried, germinated. But the following fact is more important : the crops of birds do iif)t secrete gastric juice, and do not, as I know by trial, injure in the least the germination of seeds ; now, after a bird has found and devoured a large supply of food, it is positively asserted that all the grains do not pass into the gizzard for twelve or even eighteen hours. A bird in this interval might easily be blown to tliS(^ islands had been partly stocked by ice-borne seeds, during the Glacial epoch. At my request Sir C. Lyell wrote to M. llartung to incjuire whether he had observed erratic bowlders on these islands, and he answered that lie had found large fragments of granite and other rocks, 330 MEANS OF DISPERSAL. Chap. XI. ■wliicli do not occur in the archipelago. Hence we may safely infer that icebergs formerly landed their rocky burdens on the shores of these niid-occau islands, and it is at least possible that they may have brought thither the seeds of northern plants. Considering that tliese several means of transport, and that other means, which without doubt remain to be discovered, have been in action year after year, for tens of thousands of years, it would, I think, be a marvellous fact if many plants had not thus become widely transported. These means of trans- port are sometimes called accidental, but this is not strictly correct : the currents of the sea are not accidental, nor is the direction of prevalent gales of wind. It should be observed that scarcely any means of transport would carry seeds for very great distances; for seeds do not retain their vitality Avhen exposed for a great length of time to the action of sea- water; nor could they be long carried in the crops or intes-« tines of birds. These means, however, would suffice for occa- sional transport across tracts of sea some hundred miles in breadth, or from island to island, or from a continent to a neighboring island, but not from one distant continent to another. The floras of distant continents would not by such means become mingled ; but would remain as distinct as tliey now are. The currents, from their course, would never bring seeds from North America to Britain, though they might and do bring seeds from the West Indies to our western shores, where, if not killed by their very long immersion in salt-water, they could not endure our climate. Almost every year, one or two land-birds are blown across the whole Atlantic Ocean, from North America to the western shores of Ireland and England ; but seeds could be transported by these rare wan- derers only by one means, namely, by dirt adhering to their feet or beaks, which is in itself a rare accident. Even in this case, how small the chance would be of a seed falling on fa- vorable soil, and coming to matuiity ! But it would be a great error to argue that because a well-stocked island, like Great Britain, has not, as far as is known (and it would be very diffi- cult to prove this), received within the last few centuries, through occasional means of transport, immigrants from Eu- lope or any other continent, that a poorly-storked island, though standing more remote from the main-land, would not receive colonists by similar means. Out of a hundred seeds or animals transported to an island, even if far less well CnxF. XI. DISPERSAL DURING THE GLACTAL PERIOD. 337 stocked than Britain, perhaps not more than one would be so well litted to its new home as to become naturalized. But this, as it seems to me, is no valid arf^ument against "what would be effected by occasional means of transport, during tlu; long lapse of geological time, while the island was being up- heaved, and before it had become fully stocked with inhabit- ants. On almost bare land, with few or no destructive insects or birds living there, nearly every seed which chanced to arrive, if fitted for the climate, would germinate and survive, Disjyersal durbuj the Glacial Period. The identity of many plants and animals, on mountain- gunnnits, separated from each other by hundreds of miles of lowlands, where Alpine species could not possibly exist, is one of llie most striking cases known of the same species living at distant points, without the apparent possibility of their having migrated from one point to the other. It is indeed a remark- able fact to see so many plants of the same species living on the snowy regions of the Alps or Pyrenees, and in the extreme jiorthern parts of Europe ; but it is far more remarkable that the plants on the White Mountains, in the United States of America, arc all the same with those of Labrador, and nearly all the sanie, as we hear from Asa Gray, with those on the loftiest mountains of Europe. Even as long ago as 1747, such facts led Gmelin to conclude that the same species must have Ijcen independently created at several distinct points ; and we might have remained in this same belief, had not Agassiz and others called vivid attention to the Glacial period, which, as we shall immediately see, affords a simple exfjlanation of these facts. We have evidence of almost every conceivable kind, organic and inorganic, that, within a very recent geological jx'riod, central Em-ope and North America suffered under an arctic climate. The ruins of a house burnt by lire do not tell their tale moni plainly than do the mountains of Scotland and Wales, with their scored flanks, polishi^d surfaces, and perched bowlders, of the icy streams with which their valleys W(M-e late- Iv filled. So grcatlv has the climate of Europe clianged, thai, in Northern Jtalv, gigantic moraines, left bv old glaciers, are now clothed by tlie vine and maize, 'J'hroughout a larg(^ part of the United States, erratic bowlders and scored rocks ])jainly reveal a former cold j)eriod, Tlie former influence of the glacial climate on the distribu- 15 338 DISPERSAL Cuat, Xi. lion of the inliabitiints of Europe, as cxj)lained hy Edward Forbes, is substantially as follows. But we shall follow the changes more readil}', by supposing a new glacial period slowly to come on, and then pass away, as formerly occurred. As the cold came on, and as each more southern zone became fitted for the inhabitants of the north, they would take the places of the former inhabitants of the temperate regions. The latter, at the same time, would travel farther and farther southward, unless thoy were stopped by bairiers, in which case they would perish. The mountains would become covered with snow and ice, and their former Alpine inhabitants would descend to the plains. By the time that the cold had reached its maximum, we should have an arctic fauna and flora, covering the central parts of Europe, as far south as the Alps and Pyrenees, and even stretching into Spain. The now temperate regions of the United States would likewise be covered by arctic plants and animals, and these would be nearly the same with those of Europe ; for the present cireumpolar inhabitants, which we sup- pose to have everywhere travelled southward, are remarkably uniform round the world. As the warmth returned, the arctic forms would retreat northward, closely followed up in their retreat by the produc- tions of the more temperate regions. And as the snow melted from the bases of the mountains, the arctic forms Avould seize on the cleared and thawed ground, always ascending, as the warmth increased and the snow still further disappeared, higher and higher, while their brethren were pursuing their northern journey. Hence, when the warmth had fully returned, the same species, which had lately lived together in a body on the European and North American lowlands, Avould again be found in the arctic regions of the Old and New Worlds, and on many isolated mountain-summits far distant from each other. Thus we can understalid the identity of many plants at points so immensely remote as on the mountains of the United States and of Europe. We can thus also understand the fact that the Alpine plants of each mountain-range are more espe- cially related to the arctic forms living due north or nearly due north of them : for the first migration when the cold came on, and the remigration on the returning warmth, would generally have been due south and north. The Alpine plants, for exam- ple, of Scotland, as remarked by Mr. II. C. Watson, and those of the Pyrenees, as remarked by Ramond, are more especially allied to the plants of northern Scandinavia ; those of the CiiAi'. XI. DUKING THE GLACIAL PERIOD. 339 United States to Labrador ; those of the mountains of Siberia to the arctic regions of that country. These views, f^rounded as they are 011 the perfectly well-ascertained occurrence of a former Glacial period, seem to me to explain in so satisfactory a manner the present distribution of the Alpine and arctic pro- ductions of Europe and .Vmcrica, that, when in other regions we lind the same species on distant mountain-summits, Ave may almost conclude, without other evidence, that a colder climate lV)rnierly permitted their migration across the intervening low- lands, now become too warm for their existence. As the arctic forms moved first southward and afterward backward to the north, in unison with the changing climate, they w ill not have been exposed during their long migrations to any great diversity of temperature ; and, as they all migrated in a body tog-ether, their mutual relations will not have been much disturlicd. Hence, in accordance with the principles in- culcated in tliis volume, these forms will not have been liabli> to much modification. But with the Alpine productions, left isolated fi-om the moment of the returning warmth, first at the bases and ultimately on the summits of the mountains, the case will have been somewhat different; for it is not likely that all the same arctic species will have been left on mountain-ranges far distant from each other, antl have survived there ever since; they will, also, in all probability, have become mingled Avitli ancient Alpine species, which must have existed on the moun- tains before tlie commencement of the Glacial epoch, and which during its coldest ])eriod will have been temporarily driven down to the plains ; they will, also, have been exposed to some- what different climatal influences. Their mutual relations will thus have been in some degree disturbed ; consequiMitlv they will have been liable to modification ; and this we find has been the case : for if we compare the present Alpine j)lants and ani- mals of the several great European mountain-ranges one with another, though many of the species remain idmitically the same, some exist as varieties, some as doubtful forms or sub-species, and some as certainly distinct yet closely-allied species repre- senting each other on the several ranges. In the foregoing illustration T have assumed that at the commencement of our imagin.ary Glacial period, the arctic pro- ductions were as uniform round the polar regions as they arc at the present day. But it is necessary also to include many sulvarctic and some few temperate forms, for some of these are the same on the lower mountain-slopes and on the plains of 340 DISPERSAL Chaj-. XI. North America and Europe ; and it may be asked how I ac- count for tliis degree of uniformity in the sub-arctic and temper- ate forms round tlie world, at the commencement of the real Glacial period. At the present day, the sub-arctic and northern temperate productions of the Old and New Worlds are sepa- rated from each other by the whole Atlantic Ocean and by the nortliern part of the Pacific. During the Glacial period, when the inhabitants of the Old and New Worlds lived farther south- ward than they do at present, they must have been still more completely separated from each other by wider spaces of ocean ; so that it may well be asked how the same species could have entered the two continents then so widely separated. The ex- planation, I believe, lies in the nature of the climate before the commencement of the Glacial period. At this, the newer Plio- cene period, the majority of the inhabitants of the world were specifically the same as now, and we have good reason to believe that the climate was warmer than at the present day. Hence we may suppose that the organisms which now live un- der latitude 60°, lived during the Pliocene jieriod farther north under the Polar Circle, in latitude CG°-G(° ; and that the pres- ent arctic productions then lived on the broken land still nearer to the pole. Now, if we look at a terrestrial globe, we see un- der the Polar Circle that there is almost continuous land from western Europe, through Siberia, to eastern America. And this ccHitinuity of the circumpolar land, with the consequent freedom imder a more favorable climate for intermigration, will account for the supposed uniformity of the sub-arctic and tem- perate productions of the Old and New Worlds, at a period anterior to the Glacial epoch. Believing, from reasons before alluded to, that our conti- nents have long remained in nearly the same relative position, though sul)jected to large but partial oscillations of level, I am strongly inclined to extend the above view, and to infer that during some still earlier and still vrarmer period, such as the older Pliocene period, a large number of the same plants and animals inhabited the almost continuous circumpolar land ; and that these plants and animals, both in the Old and New Worlds, began slowly to migrate southward as the climate be- came less warm, long before the commencement of the Glacial period. We now see, as I believe, their descendants, mostly in a modified condition, in the central parts of Europe and the United States. ( )n this A-iew we can understand tlie relation- shi]), ^^itll very little identity, l)etween the productions of Chap. XI. DURING TIIP: GLACIAL PEKIOD. 34I North America and Europe — a relationship whicli is highly romarkai)lc, considering the distance of the two areas, and tlieir separation hy tlie whole Atlantic Ocean. We can fur- ther understand the singular fact remarked on by several observers, that the productions of Europe and America during the later tertiary stages -were more closely related to each other than they are at the present time ; for during tliese warmer periods the northern parts of the Old and New Worlds will have been almost conthiuously united by land, serving as a bridge, since rendered impassable by cold, for the intermi- gration of their inhabitants. During the slowly-decreasing warmth of the Pliocene pe- riod, as soon as the species in common, which inliabitcd the New and 0!d Worlds, migrated south of the Polar Circle, they would be completely cut off from each other. This separation, as -far as the more temperate productions are concerned, must have taken place long ages ago. As the plants and animals migrated southward, they would become mingled in the one great region with the native American productions, and would have had to compete with them ; and, in the other great region, with those of the Old World. Consequently we have here every tiling favorable for much modification — for far more modification than with the Alpine productions, left isolated, within a much more recent period, on tlie several mountain- ranges and on the arctic lands of Europe and North America. Hence it has come that, when we compare the now living pro- ductions of the temperate regions of the New and Old Worlds, we find very few identical species (though Asa Gray has late- ly shown that more plants are identical than was formerly sup- j)Osed), but we find in every great class many forms, which some naturalists rank as geographical races, and others as dis- tinct species ; and a host of closely-allied or representative forms which are ranked by all naturalists as specifically dis- tinct. As on the land, so in the waters of the sea, a slow southern migration of a marine fauna, which, during the Pliocene or even a somewhat earlier pcried, was nearly imiform along the continuous shores of tlie Pohir Circle, will account, on tlie the- ory of modification, for many closely-allied forms now living in marine areas completely sundered. Thus, I tliink, we can imderstand the ])resence of some still existing and of some ter- tiary closely-allied fijrms on the eastern anti western shores of temperate North America ; and the still more striking fact of 342 alti:i:nate glacial tliuods chap. xl many clost'ly-allicd crustaceans (as described in Dana's admir- able work), of some fish and other marine animals, in the Med- iterranean and in the seas of Japan — these two areas being now completely separated by the breadth of a whole continent and by a wide space of ocean. These cases of close relationship in species either now or formerly inhabiting the seas on the eastern and western shores of North America, the Mediterranean, and Japan, and the tem- perate lands of North America and Europe, are inexplicable on the theory of creation. AVc cannot maintain that such spe- cies have been created alike, in correspondence with the neai'- ly similar physical conditions of the areas ; for if we compare, for instance, certain parts of South America with j^arts of South Africa or Australia, Ave see countries closely similar in all their j^hysical conditions, hut with inhabitants utterly dis- similar. Alternate Glacial Periods of the Korth and South. But we must return to our more immediate subject. I am convinced that Forbes's view may be largely extended. In Europe we meet with the plainest evidence of the Glacial pe- riod, from the Avcstcrn shores of Britain to the Ural range, and southward to the Pyrenees. We may infer, from the frozen mammals and nature of the mountain vegetation, that Siberia was similarly aflected. In the Lebanon, according to Dr. Hooker, perpetual snow formerly covered the central axis, and fed glaciers which rolled 4,000 feet down its valleys. Along the Himalaya, at points 900 miles apart, glaciers have left the marks of their former low descent ; and, in Sikkim, Dr. Hooker saw maize growing on gigantic ancient moraines. Southward of the Asiatic Continent, on the opposite side of the equator, we now know, from the excellent researches of Dr. J. Haast and Dr. Hector, that immense glaciers formerly descended to a low level in New Zealand ; and the same plants foimd by Dr. Hooker on wideh'-separated mountains in this island tell the same story of a former cold period. From facts commimi- catcd to me by tlie Rev. W. B. Clarke, it appears also that there are traces of former glacial action on the mountains of the southeastern corner of Australia. Looking to America: in the northern half, ice-born frag- ments of rock have been observed on the eastern side of the oontinent, as far south as kit. 3G°-37°, and on the shores of the Cjur. XI. OF THE NORTH AND SOUTH. 343 Pncific, where tlic climate is now so difiercnt, as far south as lat 4G''. Erratic bowlders have, also, been noticed on the Rocky Mountains. In the Cordillera of South America, nearly under the equator, glaciers once extended far Ix'low their pres- ent level. In central Chili I examined a vast mound of detri- tus with grciit bowlders, crossing the Portillo valle}', which there can hardly be a doubt once formed a huge moraine ; and Mr. D. Forbes informs me that he found in various parts of the Cordillera, from lat, 13° to 30° S., at about the height of 12,000 feet, deeply-furrowed rocks, resembling those with which he was familiar in Norway, and likewise great masses of detritus, including grooved pebbles. Along this whole space of the Cordillera true glaciers do not now exist even at nmcli more considerable heights. Farther south on both sides of the continent, from lat. 41° to the southernmost extremity, we have the clearest evidence of former glacial action, in nu- merous immense bowlders transported far from their parent source. From these several facts, namely, from the glacial action having extended all round the northern and southern hemis- pheres— from tlic period having been in a geological sense re- cent in both hemispheres — from its having lasted in both dur- ing a great length of time, as may be inferred from the amount of work effected — and lastly from glaciers having recently de- scended to a low level along the whole line of the Cordillera, it formerly appeared to me that we could not avoid the conclusion that the temperature of the whole world had been simultane- ously lowered during the Glacial period. But now Mr. CroU, in a series of admirable memoirs, lias attempted to show that a glacial condition of climate is the result of various physical causes, brought into operation by an increase in the eccentricity of the earth's orlnt. All these causes tend toward the same end ; but the most powerful appears to be the influence of the oocentricity of the orbit upon oceanic ciuTcnts. It follows, from Mr. Croll's researches, that cold periods regularly recur every ten or fifteen thousand years; but that at much longer inter- vals the cold, owing to certain contingencies, is extremely se- vere, and lasts for a great length of time. Mr. Cn)ll believes that the last great Glacial period occurred about 2-40,000 years ago, and endured with slight alterations of climate for about 160,000 years. With respect to more ancient Glacial periods, several geologists arc convinced from direct evidence that such occurred during the Miocene and Eocene fonnations, not to "344 ALTERNATE GLACIAL TEKIODS Chap. XI. mention still more ancient formations. But in relation to our present subject, the most important result arrived at by Mr, Croll is, that whenever the northern hemisphere passes through a cold period, the temperature of the southern hemisphere is actually raisctl, with the winters rendered much milder, chiefly through changes in the direction of the ocean-currents. So, conversely, it is with the northern hemisphere, when the south- ern passes through a glacial period. These conclusions have, as we shall immediately see, a most important bearing on geo- graphical distribution ; but I will first give the facts which de- mand an explanation. In South America, Dr. Hooker has shown that, besides many closely-allied species, between forty and fifty of the flowering plants of Terra del Fuego, forming no inconsiderable part of its scanty flora, are common to North America and Europe, enormously remote as these areas in opposite hemispheres are from each other. On the lofty mountains of equatorial America a host of peculiar species belonging to European genera occur. On the Organ Mountains of Brazil, some few temperate Euro- pean, some Antarctic, and some Andean genera were found by Gardner, which do not exist in the low intervening hot countries. On the Silla of Caraccas the illustrious Humboldt long ago found species belonging to genera characteristic of the Cordillera. In Africa, several forms characteristic of Europe and some few representatives of the flora of the Cape of Good Hope oc- cur on the Mountains of Abj'ssinia. At the Cape of Good Hope a very few European species, believed not to have been introduced by man, and on the mountains several representa- tive European forms are found, which have not been discovered in the intertrojDical parts of Africa. Dr. Hooker has also lately shown that several of the plants living on the upper parts of the lofty island of Fernando Po and on the neighboring Cam- eroon Mountains, in the Gulf of Guinea, are closely related to those on the Mountains of Abyssinia, and likewise to those of Icinperate P^urope. It now also appears, as I hear froni Dr. Hooker, tliat some of these same temperate plants have been discovered by the R;'v. R. T. Lowe on the mountains of the Cape de V^erde Islands. This extension of the same temperate forms, almost under the equator, across the whole continent of Africa and to the mountains of the Cape de Verde archipelago, is one of the most astonishing facts ever recorded in the dis' tribution of plants. CnAP. XI. OF THE NORTH AND SOUTH. 345 On the Himalaya, and on the isolated mountain-ranges of the Peninsula of India, on the heights of Cejlon, and on the volcanic cones of Java, many plants occur, cither identicall}' the same or representing each other, and at the same time repre- senting plants of Europe, not found in the intervening hot low- lands. A list of the genera of plants collected on the loftier peaks of Java raises a picture of a collection made on a hillock in Europe ! Still more striking is the fact that pecidiar south- ern Australian forms are represented by certain j)lants grow- ing on the summits of the mountains of Borneo. Some of these Australian forms, as I hear from Dr. Hooker, extend along the heights of the peninsula of Malacca, and are thinly scattered on the one hand over India, and on the other hand as far nortli as Japan. On the southern mountains of Australia, Dr. F. Muller has discovered several European species ; other species, not intro- duced by man, occur on the lowlands; and a long list can be given, as I am informed by Dr. Hooker, of European genera, found in Australia, but not in the intermediate torrid regions. In the admirable " Introduction to the Flora of New Zealand," by Dr. Hooker, analogous and strilving facts are given in re- gard to the ])lants of that large island. Hence we see that certain plants growing on the more lofty mountains of the tropics in all parts of the world, and on the temperate plains of the north and south, are either the same identical species or varieties of the same species. It should, however, be observed that these plants are not strictly Arctic forms ; for, as Mr. H. V. Watson has remarked, " in receding from polar toward equatorial latitudes, the Alpine or mountain floras really be- come less and less Arctic." Besides these identical and closely- allied forms, many species inhabiting the same widely-sundered areas behjng to genera not now found in the intermediate trop- ical lowlands. These brief remarks a]iply to plants alone ; but some few analogous facts could be givi'U in regard to terrestrial animals. In marine productions, similar cases likewise occur; as an ex- ample, I may quote a statement by the highest authority, Prof. Dana, that ''it is certainly a wonderful fact that New Zealand should have a closer reseml)lanc«; in its Crustacea to Great Ijiitain, its anlijiode, than to anv otiier part of the world." Sir J. Kiclianlson, also, speaks of the reappearance on the shores of New Zealand, Tasnianiii, etc., of northern forms of fish. Dr. Hooker informs me that twenty-five species of Algaj 346 ALTERNATE GLACIAL I'EKIODS CirAr. XI. arc common to Ncn' Zealand and to Europe, Inti Iiave not beer found in the inlennediate tropical seas. From the foregoing^ facts, namely, the presence of temperate forms on the highlands across the whole of equatorial iVfrica, and along the Peninsula of India to Ceylon and the Malay archi- pelago, and in a less ■\vell-raarked mann(T across the wide ex- panse of tropical South America, it appears almost certain that at some former period, no doubt during the most severe part of the Glacial period, the lowlands of these great continents were everyAvhere tenanted under the equator by a considerable number of temperate forms. At tliis ])criod the equatorial cli- mate at the level of the sea was probably about the same with that now^ experienced at the height of from five to six thousand feet vmder the same latitudes, or perhaps even rather cooler. During this, the coldest period, the lowlands under the equator must have been clothed with a mingled tropical and temperate vegetation, like that described by Hooker as growing luxuri- antly at the height of from four to five thousand feet on the lower slopes of the Himalaya, but with perhaps a still greater preponderance of temperate forms. So, again, on the moun- tainous island of Fernando Po, in the Gulf of Guinea, Mr. Mann found temperate European forms beginning to appear at the height of about five thousand feet. On the mountains of Panama, at the height of only two thousand feet. Dr. Seemann found the vegetation like that of Mexico, " Avith forms of the torrid zone harmoniously blended with those of the temperate." Now let us see Avhether Mr. Croll's conclusion, that Avhen the northern hemisphere sufTcred fi-om the extreme cold of the great Glacial period, the southern hemisphere Avas actually Avarmer, throAvs any clear light on the present apparently inex- ])licable distribution of A^arious organisms in the temperate jiarts of both hemispheres, and on mountains of the tropics. The Glacial period as measured by ^-ears, 7uust have been very long; and, Avhcn avc remember OA'cr Avhat vast spaces some naturalized plants and animals have spread Avithin a fcAV cen- turies, this period Avill haAC been ample for any amoimt of mi- gration. As the cold became more and more intense, Ave knoAV that Arctic forms iuA'aded the temperate regions ; and, from the facts just given, there can hardly be a doubt that some of the more A'igorous, dominant, and widest-spreading temperate forms actually then iuA'aded the equatorial loAvlands. The inhabitants of these lowlands Avould at the same time migrate to the tropi- Ciil and sub-tropical regions of the south, for the southern hemi- CiiAr. XI. OF THE NOKTII AND SOUTH. 347 splicre was at this period warmer. On the decline of the Ghi- cial ]>eriod, as both liemis])heres gradually recovered their former temperatures, the northern temperate forms, livins; on the lowlands under tlie equator, would be driven to their former homes or be destroyed, being replaced by the equatorial forms returning from the south. Some, howeveK, of the northern temperate forms would almost certainly ascend any adjoining highland, where, if sufficiently lofty, they would long survive, like the Arctic forms on the mountains of Europe. They might survive, even if the climate was not perfectly fitted for them, for the change of temperature must have been very slow, and plants undoubtedly possess a certain capacity for acclimatiza- tion, as slio\\Ti by their transmitting to their offspring different constitutional powers of resisting heat and cold. In the regular course of events the southern hemisphere would be subjected to a severe Glacial period, with the northern hemisphere rendered warmer ; and then the southern temperate forms would in their turn invade the equatorial lowlands. The northern forms which had before been left on the mountains would now descend and mingle Avith the southern forms. These latter, Avhen the warmth returned, Avould return to their former homes, leaving some few species on the mountains, and carrying southward with them some of the northern temper- ate forms which had descended from their mountain fastnesses. Thus, we should have some few species identically the same in the northern and southern temperate zones and on the moun- tains of the intermediate tropical regions. But the species left during a long time on these mountains, or in opposite hemi- spheres, would have to compete Avith many new forms, and would be exposed to somewhat clifTerent physical conditions ; hence tlicy would be eminently lialjle to modification, and would generally now exist as varieties or as representative s]>c- cies ; and this is the case. We must, also, bear in mind the occurrence in both hemispheres of former Glacial jieriods ; for these will account, in accordance with the same principles, for the many quite distinct species inhabiting the same widely- separated areas, and belonging to genera not now found in the intermediate torrid zones. It is a remarkaljlc fact, strongly insisted on by Hooker in regard to America, and by Alph. de Candolle in regard to Aus- tralia, that inany more identical or now slightly-modified spe- cies have migrated from the north to the south, than in a re- versed direction. We see, however, a few southern forms on 348 ALTERNATE GLACIAL PERIODS Chap. XI. tlie mountains of Borneo and Abyssinia. I suspect that this preponderant niij^ration from the north to t!ie south is due to the f^rcatcr extent of hmd in the north, and to tlie noi'tbeni forms liaviuii;' existed in their own homes in g-reater numbers, and havini^ eonsccfucntly been advanced throu<>;h natural selec- tion and comjM'tition to a liigher stage of perfection, or domi- nating jKnver, tliiui the soutliern forms. And thus, when the two sets became commingled in the equatorial regions, during the alternations of the Glacial periods, the northern forms were the more powerful and were able to hold their places on the mountains, and afterward to migrate southward with the south- ern forms ; but not so the southern in regard to the northern forms. In the same manner we see, at the present day, that very many European productions cover the groimd in La Plata, New Zealand, and to a lesser degree in Australia, and have beaten the natives ; whereas extremely few southern forms have become naturalized in any part of the northern hemisphere, though hides, wool, and other objects likely to carry seeds, have been largely imported into Europe during the last two or three ccnturietj from La Plata, and during the last thirty or forty years from Australia. The Ncilgherric Mountains in India, however, offer a part ial exception ; for here, as I hear from Dr. Hooker, Australian forms are rapidly sowing themselves and becoming naturalized. Before the last great Glacial period, no doubt the intertropical mountains Avere stocked with en- demic Alpine forms ; but these have almost everywhere yielded to the more dominant forms, generated in the larger areas and more efficient workshops of the north. In many islands the native pi'oductions are nearly equalled, or even outnumbered, by those which have become naturalized ; and this is the llrst stage toward their extinction. Mountains are islands on the land, and their inhabitants have yielded to those produced with- in the larger areas of the north, just in the same way as the inhabitants of real islands have everywhere yielded and are still yielding to continental forms naturalizetl through man's agency. The same principles apply to the distribution of terrestrial animals and of marine productions, in the northern and south- ern temperate zones, and on the intertropical mountains. When during the height of the Glacial period the ocean-cur- rents Avere Avidely diiferent to Avhat thev noAV are, some of the inhabitants of the temperate seas might have reached the equa- tor ; of these a fcAV would perhaps at once be able to migrate CiFAP. XI. OF THE NORTH AND SOUTH. 349 southward, by kpcpiiiG^ to the cooler currents, Avhile others might remain and survive in the cooler depths, until the south- ern hemisphere was in its turn subjected to a glacial climate and permitted of their lurtlier progress ; in nearly the same manner as, accortling to Forbes, isolated spaces inhabited by Arctic productions exist to the present day in the deeper parts of the temperate seas. I am far from supposing that all dilhculties in regard to the distribution and ailinities of the identical and allied spe- cies, which now live so widely separated in the north and south, and sometimes on the intermediate mountain-ranges, are removed on the views above given. The exact lines of mi- gration cannot be indicated. We cannot say why certain spe- cies and not others have migrated ; why certain species have been modified and have given rise to new forms, while others liave remained unaltered. We cannot hope to explain such facts, until we can say why one species and not another be- comes naturalized l^y man's agency in a foreign land ; Avhy one species ranges twice or thrice as far, and is twice or thrice as common, as another species within their own homes. Various special difficulties also remain to be solved ; for in- stance, the occurrence, as shown by Dr. Hooker, of tiie same plants at points so enormously remote as Kerguelen Land, New Zealand, and Fuegia; but icebergs, as suggested by Ly- cll, may have l)een concerned in tlieir dispersal. The existence, at these and other distant points of the southern hemisphere, of species, which, though distinct, belong to genera exclusively confined to the south, is a more remarkable case. Some of these species are so distinct, that we cannot suppose that there has been time since the commencement of the last Glacial period for their migration and subsequent modification to the necessary degree. The facts seem to me to indicate that dis- tinct species belonging to the same genera have migrated in radiating lines from a common centre ; and I am inclined to look in tiie southern, as in the northern heniisphere, to a former and warmer ])erio(l, before the commencement of the Ghicial period, when the Antarctic lands, now covere^l with ice, sup- ported a liighly pecuhar and isolated flora. It may be suspected that, before this flora was exterminated during the last Glacial epoch, a few forms had been already widely dispersed to vari- ous points of the southern hemisphere l)y occasional means of transport, and by the aid, as halting-places, of now sunken islands. Thus tlie southern shores of America, Australia, and 350 ALTERNATE GLACIAL PERIODS. Cuap. XI. New Zealand, niio^'ht have become slightly tinted by the same peculiar forms of life. Sir C. Lycll in a striking' passage has speculated, in lan- guage almost identical Avith mine, on the effects of great alter- nations of climate throughout the world on geographical dis- tribution. And we have uow seen that Mr. Oroll's conclusion, that successive Glacial periods in the one hemisphere coin- cided with warmer periods in the opposite hemisphere, to- gether with the admission of the slow modification of species, explains a multitude of facts in the distribution of the same and of the allied forms of life in all parts of the globe. The living waters have flowed during certain periods from the north and afterward from the south, and in both cases have reached the equator ; but the stream of life has flowed with greater force from the north than in the opposite direction, and has consequently more freely inundated the south. As the tide leaves its drift in horizontal lines, rising higher on the shores where the tide rises highest, so have the Hving waters left their living drift on our mountain-summits, in a line gently rising from the Arctic lowlands to a great altitude under the equator. The various beings thus left stranded may be com- pared with savage races of man, driven up and surviving in the mountain-fastnesses of almost every land, Avhich serve as a record, full of interest to us, of the former inhabitants of the surroundinjr lowlands. Chat. XII. FRESH-WATER PRODUCTIONS. 851 CHAPTER XII. GEOGnAPiiicAL DisTKiBUTiox — Continued. Distribution of Prcsh-wnfcr Productions— On the Inhabitants of Ocranic IrJlands— Absence of 15atrucl)ian8 and of Terrestrial Mammals— On the Relation of tlie In- tuibitants of I^landx to those of the nearest Main-land— On Colouizaticm from tho nearest Source with subsequent Modification— Summary of the last and present Chapter. Fresh-water Productions. As lakes and river-sj'stenis arc separated from each other by barriers of land, it might have been thought that fresh- water productions would not have ranged widely within the same country, and, as the sea is apparently a still more for- midable barrier, that they never would have extended to dis- tant countries. But the case is exactly the reverse. Not only have many fresh-water species, belonging to quite different classes, an enormous range, but allied species prevail in a re- markable manner throughout the world. I well remember, when first collecting in the fresh waters of Brazil, feeling much surprise at the similarity of the fresh-water insects, shells, etc., and at the dissimilarity of the surrounding terrestrial beings, compared with those of Britain. But this power in fresh- water productions of ranging wide- ly, though so imcxpected, can, I think, in most cases be ex- plaiiiod by their having become fitted, in a manner highly useful to them, for short and frequent migrations from pond to pond, or from stream to stream ; and liability to wide disper- sal would follow from this capacity as an almost necessary con- seipience. We can here consider only a few cases. In regard to fish, I believe that the same species never occur in the fresh waters of distant continents. But on the same continent the species often range widely and almost capriciously ; for two river-systems will have some fish in common and some different. A few facts seem to favor the possibility of their occasi(mal transport by accidental means ; like that of the live 352 FEESII-WATER PRODUCTIONS. Ciixr. XII. fish not rarely dropped hy Avhirlwinds in India, and the vitality t)f their ova when removed from the Avater. ]3ut I am inclined to attribute the dispersal of frcsh-Avater iish mainly to changes in the level of the land within the recent period having caused rivers to flow into each other. Instances, also, coidd be given of this having occniTcd during floods, without any change of level. The wide dilTerence of the fish on the opposite sides of continuous mountain-ranges, -which from an early period must have completely prevented the inosculation of the river- systems, seems to lead to the same conclusion. "With respect to allied fresh-water fish occurring at very distant points of the world, no doubt there arc many cases which cannot at present be explained : but some fresh- Avater fish belong to very ancient fonns, and in such cases there will have been ample time for great geograpliical changes, and consequently time and means for such migration. In the second place, salt-water fish can Avith care be sloAvly accustomed to Ha'c in fresh Avater; and, according to Valenciennes, there is hardly a single group of Avhich all the members are confined exclusively to fresh Avatcr, so that a marine species of a frcsh-Avater group might travel far along the shores of the sea, and subsequently liecome modi- fied and adapted to the fresh Avaters of a distant land. Some species of fresh-Avater shells haA'e A'crA' Avide ranges, and allied species Avhich, on our theory, are descended from a common parent, and must haA'C proceeded from a single source, prevail throughout the Avorld. Their distribution at first per- plexed me much, as their OA'a are not likely to be transported by birds, and are immediately killed by sea-Avater, as are the adults. I could not even understand hoAV some naturalized species have spread rapidly throughout the same country. But tAvo facts, Avhich I have observed — and no doubt many others remain to be observed — throAV some light on this subject. ^^'hen a duck suddenly emerges from a pond coA'cred Avith duck-Avced, I have tAvicc seen these little plants adhering to its back ; and it has happened to me, in remoAing a little duck-AVoed from one aquarium to another, that I have quite un- intentionally stocked the one Avith fresh-water shells from the other. 13ut another agency is perhaps more eftectual : I sus- pended a duck's feet in an aquarium, Avhere many ova of fresh- Avater shells Avere hatching ; and I found that numbers of the extremely minute and just-hatched shells craAvled on the feet, and clung to them so firmly that, Avhen taken out of the Avater, they could not be jarred off, though at a someA\hat more ad- CuAr. XII. FRESn-WATER PRODUCTIONS. 353 vanced a;^c ihoy would voluntarily drop olT. These just- hatched moUusks, thoui^h aquatic in their nature, survived on the duck's feet, in damp air, from twelve to twenty hours ; and in this lenp^tli of time a duck or heron mij^ht (ly at least six or seven hundred miles, and, if blown across the sea to an oceanic island or to. any other distant point, would be sure to alight on a pool or rivulet. Sir Charles Lyell informs me that a Dj'tiscus has been cau2;ht with an Ancylus (a fresh-water shell like a limpet) firmly adhering- to it ; and a Avater-beetle of the same family, a Colymbetes, once flew on board the " Beagle," when forty-five miles distant from the nearest land : how much farther it might have been blown with a favoring gale no one can tell. With respect to plants, it has long been known what enormous ranges many fresh-water, and even marsh-species, have, both over continents and to the most remote oceanic isFands, This is strikingly shown, as remarked by Alph. de Candolle, in large groups of terrestrial plants, ^vluc]l have only a very few aquatic members ; for these latter seem immediate- ly to acquire, as if in consequence, a very wide range. I think favorable means of dispersal explain this fact. I have be- fore mentioned that earth occasionally, though rarely, adheres in some quantity to the feet and beaks of birds. Wading- birds, which frequent the muddy edges of ponds, if suddenly flushed, would be the most likely to have muddy feet. Birds of this order, I can show, arc the greatest wanderers, and are occasionally found on the most remote and barren islands in the open ocean ; they would not be likely to alight on the sur- face of the sea, so that the dirt would not be washed off their feet ; and, when making land, they would be sure to fly to their natural fresh-water haunts. I do not believe that botanists an; aware how charged the mud of ponds is with seeds : I have tried several little experiments, l)ut will here give only the most striking case : I took in February three table-spoon- fuls of mud from three different points, beneath water, on the edge of a little pond: this mud when dry weighed only GJ ounces ; I kcjit it covered up in my study for six months, pull- ing up and CHjunting each plant as it grew ; the plants were f)f many kinds, and were altogether 537 in number; and yet the viscid mud was all contiiined in a breakfast cup ! Consid- ering these facts, I think it would be an inexplic;d)l(> circum- stance if water-birds did not transport the seeds of the same fivsh-water plants to unstocked ponds and streams, situated at 364 FRESU-WATEK PRODUCTIONS. Chap, XII. very distant points. The same ag'ency may lia\'C come into play with tlic ed for by su]iposed ditVerences in their ])hysical condi- tions; but this explanation is not a little doubtful. Facility 358 ABSENCL OF TERKESTEIAL MAMMALS Chap. XII. of inimioTalion seoins to liavc been fully as impoptant as the nature of the conditions. JIany remarkable little facts could bo given with respect to the inhabit:iuts of oceanic islands. For instance, in certain islands not tenanted Iiy a sing-le mammal, some of the endemic plants have beautifully hooked seeds; and few relations arc more manifest than that hooked seeds are adapted for trans- portal in the wool or fin- of quadi-upeds. But a hooked seed might be carried to an island by other means ; and the plant then becoming modified Avould form an endemic species, Avhich might still retain the hooks, Avhich would not form a more use- less appendage than the shrivelled wings imder the soldered elytra of many insular beetles. Again, islands often possess trees or bushes belonging to orders elsewhere including only herbaceous species ; now trees, as Alph. de Candolle has shown, generally have, whatever the cause may be, confined ranges. Hence trees Avould be little likely to reach distant oceanic islands ; and an herbaceous plant, which had no chance of suc- cessfully competing Avitli the many fully-developed trees grow- ing on a continent, might, when established on an island, gain an advantage by growing taller and taller and overtopping the other herbaceous plants. In this case, natural selection would tend to add to the stature of the plant, to whatever order it belonged, and thus convert it first into a bush and then into a tree. Absence of Batrachians and Terrestrial j[Tammals on Oce- anic Islands. "With resjiect to the absence of whole orders of animals on oceanic islands, I5ory St. Vincent long ago remarked that Ba- trachians (frogs, toads, newts) are never found on any of the many islands with which the great oceans are studded. I have taken pains to verify this assertion, and have found it strictly true, with the exception of New Zealand, of the Andaman Islands, and perhaps of the Salomon Islands. But I have al- ready remarked that it is doubtful whether New Zealand ought to be classed as an oceanic island ; and this is still more doubt- ful with respect to the Andaman and Salomon groups. This general absence of frogs, toads, and newts, on so many oceanic islands cannot be accounted for by their physical conditions; indeed, it seems that islands are peculiarly well fitted for these animals: for frogs have been introduced into Madeira, the Chap. XII. ON OCEANIC ISLANDS. 359 Azores, and Mauritius, and liavo multiplied so as to become a nuisance. But, as these animals ami their spawn are known to be immediately killed hy sea-wat<^r, there wovdd be great dilfi- culty in their transportal across the sea, and therefore we can sec why they do not exist on any oceanic island. But why, on tlie theory of creation, they should not have been created there, it would be very dillicult to explain. Mammals ofler another and similar case. I have carefully searched the oldest voyao-es, and as yet I have not found a single instance, free frt)m doubt, of a terrestrial mammal (ex- cluding domesticated animals kept by the natives) inhabiting' an island situated above 300 miles from a continent or great continental island ; and many islands situated at a much less distance are equally l)arren. The Falkland Islands, which are inhabited by a wolf-like fox, come nearest to an exception ; but tlws group cannot be considered as oceanic, as it lies on a bank in connection with the main-land at the distance of about 280 miles ; moreover, icebergs formerly brought bowlders to its western shores, and they may have formerly transported foxes, as now frequently happens in the arctic regions. Yet it cannot be said that small islands will not support at least small mam- mals, for they occur in many parts of the world on very small islands, when lying close to a continent; and hardly an island can be named on which oin- smaller quadrupeds have not be- come naturalized and greatly nuiltiplicd. It capnot be said, on the ordinary view of creation, tliat there has not been time for the creation of mammals; many volcanic islands are sufTi- ciently ancient, as shown l)y the stupendous degradation which they have suffered, and by their tertiary strata : there has also bcHMi time for the production of endemic species belonging to other classes ; and on continents it is known that mammals ap- pear and disappear at a quicker rate than any other and lower animals. Although terrestrial mammals do not occur on oce- anic islands, aerial mammals do occur on almost every island. New Zealand possesses two bats found nowhere else in the ■world : Norfolk Island, the Viti Archipelago, the Bonin Islands, the Caroline and Mariannt" Archipelagoes, and Mauritius, all possess their jieculiar bats. N\ hy, it may be asked, has the supposed creative force ])roduced bats and no other mam- mals on remote islands ? On my view this (juestion can easily be answered; for no terrestrial mammal can be transported across a wide space of sea, but bats can fly across. Bats have been seen wandering by d:iy far over the Atlantic Ocean; and 3 GO ABSENCE OF TERKESTKIAL MAMMALS Chap. XII. two North American species either ref^ularly or occasionally visit Bermuda, at the distance of GOO miles from the main-land. I hear from Mr. Tomes, who has specially studied this family, that many species have enormous ranj^es, and are found on con- tinents and on far-distant islands. Hence we have only to sup- pose that such wandering species have been modified in their new homes in relation to their new position, and we can under- stand the presence of endemic bats on oceanic islands, with the absence of all other terrestrial mammals. Another interesting relation exists, namely, between the depth of the sea separating islands from each other or from the nearest continents, and the degree of affinity of their mamma- lian inhabitants. Mr. Windsor Earl has made some striking observations on this head, since greatly extended by Mr. Wal- lace's admirable researches, in regard to the great Malay Ar- chipelago, which is traversed near Celebes by a space of deep ocean, and tliis separates two widely-distinct mammalian fau- nas. On either side the islands stand on a moderately shallow submarine bank, and these are inhabited by the same or by very closely-allied quadrupeds. I have not as yet had time to follow uj") this subject in all quarters of the world ; but, as far as I have gone, the relation generally holds good. For in- stance, Britain is separated by a shallow channel from Europe, and the mammals are the same on both sides ; and so it is with all the islands near the shores of Australia. The West-Indian Islands, on the other hand, stand on a deeply-submerged bank, nearly 1,000 fathoms in depth, and here we find American forms, but the species and even the genera arc quite distinct. As the amount of modification which animals of all kinds un- dergo partly depends on the lapse of time, and as islands sep- arated from each other or from the main-land by shallow chan- nels are more likely to have been continuously united within a recent period than islands separated by deeper channels, we can imdcrstand how it is that a relation exists l)etween the depth of the sea separating two mammalian fiiimas, and the de- gree of their affinity — a relation which is quite inexplicable on the theory of independent acts of creation. The foregoing statements in regard to the inhabitants of oceanic islands — namelv, the fewness of the species, with a large proportion consisting of endemic forms — the members of certain groups, and not of other groups in the same class, having hoen modified — the absence of certain whole orders, as of batrachians and of terrestrial mammals, notwithstanding the CuAP. XII. ON OCEANIC ISLANDS. 301 presence of aerial bats — the singular proportions of certain orders of plants — herbaceous forms having been develoi)cd into trees, etc., seem to me to accord better with the belief in the cfiiciency of occasional means of transport, carried on during a long course of time, than Avith the belief in the former connec- tion of all oceanic islands with the nearest continent ; for, on this latter view, it is probable that the various classes would have immigrated more uniformly, and, from the species having entered in a body, their mutual relations would not have been much distiu-bed, and consequently tliey would have been modi- fied citlier not at all or in a more equal manner. I do not deny that there arc many and serious difficulties in understanding how many of the inhabitants of the more remote ishxnds, whether still retaining the same specific form or subsequently modified, have reached their present homes. Hut the probability of islands having existed as halting-places, of which not a wreck now remains, must not he overlooked. T will specify one such dillicult case. Almost all oceanic islands, even the most isolated and smallest, are inhabited l)y land-shells, generally by endemic species, but sometimes l)y species found elsewhere — striking instances of which have been given by Dr. A. A. Gould in relation to the Pacific. Kow, it is notorious that land-shells are easily killed by sea- water; their eggs, at least such as I have tried, sink in it and are killed. Yet there must l)e, according to our view, some luiknown, but occasionally eflicient, means for their trauspor- tal. ^Vould the just-hatched young sometimes adhere to the fe(>t of l)irds roosting on the ground, and thus get transported? It occurred to me that land-shells, when hj'bcrnating, and hav- ing a nuMnljranous dia]ihragm over the mouth of the shell, might be floated in chinks of drifted tunl)er across moderately wide arms of the sea. And I found that several species in this state withstood uninjured an immersion in sea-water during seven days : one shell, the Helix pomatia, after having been thus treated and again hybernating, was put into sea-water for twenty da\s, and ])erfectly recovered. During tliis length of time tlie shell might have been carried, by a marine current of average sv.iftness, to a distance of GGO geogra])liical miles. As this Helix has a thick calcareous operculum, I removed it, and, when it had formed a new membranous one, I again im- mersed it for fourteen days in se:irwater, and again it recov- ered and crawled away. Baron Aucapitaine has recently tried similar experiments: he placed 100 land-shells, belonging to IG 3C2 RELATIONS OF THE INHABITANTS OF Chap. XII. ten species, in :i ])ox pierced with holes, and immersed it for a fortnight in the sea. Out of the hundred shells, twenty-seven recovered. The presence of an ojierculum seems to have been of importance, as out of twelve sj^ecimcns of Cydostoma cle- gans, which is thus furnished, eleven revived. It is remarkable, seeing how well with me the Helix pomatia resisted the salt- water, that not one of fifty-four specimens, l^elonging to four species of Helix tried by Aueapitaine, recovered. It is, how- ever, not at all probable that land-shells have often been thus transported ; the feet of birds is a more probable method. On the Relations of the Inhabitants of Islaiids to those oftJie nearest Main-land. The most striking and important fact for us is the affinity of the species which inhabit islands to those of the nearest main-land, without being actually the same. Numerous in- stances could be given. The Galapagos Archipelago, situated under the equator, lies at the distance of between 500 and GOO miles from the shores of Soutli America. Here almost every jiroduct of the land and the water l)ears the unmistakable stamp of the American Continent. There are twenty-six land- birds, and twenty-one, or, perhaps, twenty-three, of these arc )"anked as distinct species, and are supposed to have been created here ; yet the close affinity of most of these birds to American species in every character, in their habits, gestures, and tones of A^oice, was manifest. So it is with the other ani- mals, and with a large proportion of the plants, as shown by Dr. Hooker in his admirable Flora of this archipelago. The naturalist, looking at the inhabitants of these volcanic islands in the Pacific, distant several hundred miles from the continent, yet feels that he is standing on American land. Why should this be so ? why should the species which are supposed to have been created in the Galapagos Archipelago, and nowhere else, bear so plain a stamp of affinity to those created in Amer- ica ? There is nothing in the conditions of life, in the geologi- cal nature of the islands, in their height or climate, or in the proportions in which the several classes are associated to- gether, which closely resembles the conditions of the South American coast : in fact, there is a considerable dissimilarity in all the foregoing respects. On the other hand, there is a considerable degree of resemblance in the volcanic nature of the soil, in the climate, height, and size of the islands, be- Chap. XII. ISLANDS TO THOSE OF THE MAIN-LANJ). 303 twccn the Galap.'ifjos aiul Capo dc Verde Arcliipclaii-oes : but Avhat an entire and absolute difTorence in their inlialjitants ! The inliabitants of the Cape de Verde Islands are related to those of Africa, like those of the Galapagos to America. Facts such as these admit of no sort of explanation on the ordinary view of independent creation ; whereas, on the view, here maintained, it is obvious that the Galapagos Islands would be likely to receive colonists, whether by occasional means of transport or by formerly continuous land, fi'om Amer- ica ; the Cape de Verde Islands from Africa : and that such colonists would be liable to modification — the principle of in- heritance still betraving their original birthplace. Many analogous facts could be given : indeed, it is an al- most universal rule that the endemic productions of islands are related to those of the nearest continent, or of the nearest' island. The exceptions are few, and most of them can be explained. Thus, although Kerguelen Land stands nearer to Africa than to America, the plants are related, and that very closely, as we know from Dr. Hooker's accoimt, to those of America : but, on the view that this island has been mainly stocked by seeds brought with earth and stones on icel)ergs, drifted by the prevailing currents, this anomaly disappears. New Zealand in its endemic plants is much more closely re- lated to Australia, tlie nearest main-land, than to any other region: and this is what might have been ex])ccted ; but it is also plainly related to South America, which, although the next nearest continent, is so enormously remote, that the fact becomes an ancjinaly. But this difFiculty almost disajipears on the view that New Zealand, South America, and tlie other southern lands, have been partially stocked from a nearly in- termediate though distant point, namely, from the antaictic islands, when they were clothed with vegetation, during a warmer tertiary period, before the commencement of the last Glacial period. The allinity, which, though feeble, I am assured by Dr. Hooker is real, between the flora of the S(juth- western corner of Australia and of the Ca]ie of Good Ilojie, is a far more remarkable case ; but this ailinity is ccmlined to the ])lan(s, and will, no doubt, be some day explained. The same law which has determined the relati(mshi|) be- tween the inhabitants of islands and the nearest main-land i.s sometimes displayed on a small scale, but in a most interesting manner, within tlie limits of the same archipelago. Thus each separate island of the Galapagos Archipelago is ti-nanted, and 564 EELATIONS OF TlIK INHABITANTS OF Chap. XII. the fact is a iiiarvcllous ono, by distinct species; but these species arc related in a very much closer manner to each other than to the inhabitants of any other quarter of the world. Tliis is what mii^ht have been expected, for islands situated so near each other would almost necessarily receive immiung birds hatched, than can possibly be reared ; and we may infer that the mocking-tlirush l)eculiar to Charles Island is at least as well fitted for its home as is the species pecuhar to Chatham Island. Sir C. Lyell and Mr. Wollaston have communicated to me a remarkable fact bearing on this subject ; namely, that Madeira and the adjoining islet of Porto Santo possess many distinct but representative species of land-shells, some of which live in crevices of stone; and, although large quantities of stone are annuallv transported from Porto Santo to Madeira, yet this latter island has not be- come coloniz(vl by the Porto Santo species : nevertheless, both islands have been colunized by European land-shells, which no 3G6 KELATIONS OF THE INIIABITAXTS OF Ciia.-. XII. doubt liad some advantalains and mountains, of the forests, marshes, and deserts, are linked together in so mysteri- ous a manner, and are likewise linked to the extinct beings which fonnerly inhabited the same continent. Bearing in mind that the mutual relation of organism to organism is of the high- est importance, we can see why two areas liaving nearly the same phj'sical conditions sliould often be inhabited by very different forms of life ; for, according to the length of time which has elapsed since the colonists entered one of the regions, or both; according to the nature of the communication which allowed certain forms and not others to enter, either in greater or lesser numbers; according or not as those which entered happened to come into more or less direct coinpetiti(m with each other and with the aborigines ; and according as the im- migrants were capable of varying more or less rapidly, there would ensue in the two or more regions, indejiendently of their phj-sical conditions, infinitcly-diver.sified conditions of life — there would be an almost endless amount of organic action and reaction — and we should lind, as we. do find, some groups of beings greatly, and some only slightly, modified — some devel- oped in great force, some existing in scanty numbers — in the several great geographical provinces of the world. On these same principles we can undc^rstand, as I have en- deavored to show, why oceanic islands should have few in- habitants, but of these a great number should be endemic or 370 SUMMAKY OF THE LAST Ciiai-. XII. peculiar; aiiration, one group of beings, even Avithin the same class, should have all its species peculiar, and another group should have all its species the same Avith those in other quarters of the world. We can see why "whole groups of organisms, as batrachians and terres- trial mammals, should be absent from oceanic islands, Avhilc the most isolated islands should possess their own peculiar species of aerial mammals or bats. We can see why there should be some relation between the presence, in islands, of mammals, in a more or less modified condition, and the depth of the sea be- tween such islands and the main-land. We can clearly see why all the inhabitants of an archipelag-o, though specifically distinct on the several islets, should be closely related to each other, and likewise be related, but less closely, to those of the nearest ccnitinent or other source whence immigrants might have been derived. AVe can sec why, in two areas, however distant from each other, where very closely-allied or representative species exist, there should almost always exist some identical species. As the late Edward Forbes often insisted, there is a striking parallelism in the laws of life throughout time and space: the laws governing the successioni of fonns in past times being nearly the same with those governing at the present time the differences in difTerent areas. We see this in many facts, llie endurance of each species and group of species is continuous in time ; for the exceptions to the rule are so few, that they may be fairly attributed to our not having as yet discovered in an intermediate dei")Osit certain forms which are absent in it, but which occur both above and below : so in space, it certainly is the general rule that the area inhabited by a single species, or by a grouji of species, is continuous, and the exceptions, which are not rare, may, as I have attempted to show, be accounted for by former migrations mider difTerent circumstances, or through occasional means of transport, or by the species having become extinct in the intermediate tracts. Both in time and space, species and groups of species have their points of maxi- mum development. Groups of species, living during the same j)eri(jd of time, or living jvitliin the same area, are often char- acterized by trifling features in common, as of sculpture or color. In looking to the long succession of past ages, as in now looking to distant provinces throughout the world, we find that species in certain classes differ little from each other, while others in a different class, or only in a different family of the same order, differ greatly from each other. In both time CiiAP. XII. AND PRESENT CHAPTER. 37I and space the lowly-orn^auized members of each class generally change less than the highly-organized ; but there are in botli cases marked exceptions to the rule. According to our theory these several relations throughout time and space arc intelli- gible ; for, whether we look to the forms of life which have changed during successive ages, or to those which have changed after having migrated into distant quarters, in both cases the forms witliin each class arc connected by the same bond of or- dinary generation ; and in both cases the laws of variation have been the same, and modifications have been accumulated by the same means of natural selection. 37S CLASSIFICATION. Cuap. XIII. CHAPTER XIII. MUTUAL AFFINITIES OF ORGANIC BEINGS: MOEPHOLOGY : EM- BRYOLOGY: RUDIMENTARY ORGANS. Classifiriition. Groups subordinate fn Group? — Natural System— Rnlcs ;ind Difficiiltica in Chissilio.'UiDii. explained on tlio Theory of Descent with Moditicalion — Classi- fication of Vnrieliew— Descent alwavH used in Classification — Analofrical or Adap- tive Characters — Affinities, General. Complex, and liadiatincj— Extinction sepa- rates and defines Groups — Morpliolou'V- between members of the same Class, be- tween parts of the same Individual — Embryoloiry. Laws of. explained by Vari.i- tions not supervening at an early Age, and being inherited at a corresponding Age — Kudimeutary Organs ; their Origin explainuU — Suinniar3'. Classijtcation. From a very remote period in the history of the world or- ganic beings have resembled each other iu descending degrees, so that they can be classed in groups under groups, litis classiftcation is not arbitrary, like the grouping of the stars in constellations. The existence of groups would have been of simple signification, if one group had been exclusively fitted to inhtibit the land, and another tlie water; one to feed on flesh, another on vegetable matter, and so on ; but the case is widely different in Nature ; for it is notorious how commonly members of even the same sub-group have dilfercnt habits. In the second :ind fourth chapters, on Variation and on Natural Selection, I have attempted to show that within each country it is the Avidel^'-ranging, the much-dilfused and common, that is, the dominant sj^ccies belonging to the larger genera in each class, which vary most. The varieties, or incipient species, thus pro- duced, ultimately become converted into new and distinct spe- cies ; and these, on the principle of inheritance, tend to pro duce other new and dominant species. Consequently, the groups which are now large, and which generally include many dominant sjiecies, tend to go on increasing in size. I further attempted to show that, from the varying descendants of each species t tying to occupy as many and as different places as pos- Chap. XIII. CLASSIFICATION. 373 siblc in the economy of Nature, there is a constant tendency in their characters to diverge. This latter conclusion is supported by observing tlio great diversity of forms which in any small area come into the closest competition, and by certain facts in naturalization. I attempted also to show that there is a constant tendency in the forms -which arc increasing in number and diverging in character, to supplant and exterminate the preceding, less divergent and less improved forms. I request the reader to turn to the diagram illustrating the action, as formerly ex- ])lained, of these several jmnciples ; and he will see that the inevitable result is, that tlie modilied descendants proceeding from one progenitor become broken up into groups subordi- nate to groups. In the diagram each letter on the uppermost line may represent a genus including sevei'al species ; and the whole of tlie genera along this upper line form together one class, for all arc descended from one ancient parent, and, con- sequentl}', have inherited something in common. But the three genera on the left hand have, on this same principle, nnich in common, and form a sul>family, distinct from that in- cluding the next two genera on the right hand, w^hich diverged from a common parent at the fifth stage of descent. These five genera have also much, though less, in common ; and they form a faniilv distinct from that including the three genera still fartlier to tlie right liand, which diverged at a still earlier period. And all these genera, descended from (A), form an order distinct from the genera descended from (1). So tliat we here have many species descended from a single progenitor groujied into genera ; and the genera in sub-families, families, and orders, all in one great class. Thus, the grand fact of the natural suliordination of all organic beings in groups under groups, which, from its familiarity^, does not always sulliciently strike us, is in my judgment explained. No doubt organic lieings, like all other objects, can be classed in many ways, either artificially by single characters or more naturally by a number of characters. We know, for instance, that minerals and the elemental substances can be thus aiTanged ; in this case there is of course no relation to genealogical succession, and no cause can at present be assigned for tlicir falling into group.s. But with organic beings the case is different, and the view above given explains their natural an-angement in group under gr(jup ; and no other explanation has ever hccn at- tcm])lc(l. r,74 CLASSIFICATION. Chap. XIII Naturalists, as we have seen, try to arraiioc the species, genera, and families in each class, on what is called the Natu- ral System. But -what is meant by this system ? Some authors look at it merely as a scheme for arranginj^ tog'ethcr those llvina^ objects which are most alike, and for separating those which are most inilike ; or as an artificial means for enunciating, as briefly as possible, general propositions — that is, by one sentence to give the characters common, for instance, to all mammals, by another those common to all carnivora, by another those conunon to the dog-genus, and then, by adding a single sentence, a full description is given of each kind of dog. The ingenuity and utility of this system are indispu- table. But many naturalists think that something more is meant ])y the Natural System ; they believe that it reveals the plan of the Creator ; but, unless it be specified Avhether order in time or space, or both, or what else is meant by the plan of the Creator, it seems to me that nothing is thus added to our knowledge. Such expressions as that fiimous one by Lin- nieus, and which wc often meet with in a more or less con- cealed form, that the characters do not make the genus, but that the genus gives the characters, seem to imply that some- thing more is included in our classification than mere resem- blance. I believe that something more is included, and that propin(juity of descent — the only known cause of the similarity of organic beings — is the bond, hidden as it is by various de- grees of modification, which is partially revealed to us by our classifications. Let us now consider the rules followed in classification, and the difficulties which are encountered, on the view that classification either gives some imknown plan of creation, or is simply a scheme for enunciating general propositions and of jilacing together the forms most like each other. It might have been thought (and Avas in ancient times thought) that those parts of the structure which determined the habits of life, and the general place of each being in the economy of Nature, would be of very high importance in classification. Nothing can be more false. No one regards the external similarity of a mouse to a shrew, of a dugong to a whale, of a whale to a fish, as of any importance. These reseml)lances, though so intimate- ly connected with the whole life of the being, are ranked as merely " adaptive or analogical characters ; " but to the con- sideration of these resemblances we shall recur. It may even be given as a general rul(>, that the less nny part of the organi- CriAr. XIII. CLASSIFICATION. 375 zation is concerned with special habits, the more important it becomes for chissilication. As an instance — Owen, in speak- inf];' of the dufjonp:, says : " The generative or<^ans, Ijein^r those wiiich are most remotely related to the habits and food of an animal, I have always regarded as alTordinti; very clear indica- tions of its true airmities. We are least likely in the modifi- cations of these organs to mistake a merely adaptive for an essential character." With plants, how remarkable it is that the organs of vegetation, on which tlieir nutrition and life de- ]icnd, are of little signification ; whereas the organs of repro- duction, with their product tlie seed and embr^'o, are of ])ara- mount importance ! So, again, in formerly discussing morpho- logical dilleronces M'hich are not jihysiologically important, we have seen that they are often of the highest service in classifi- cation. Tliis depends on their constancy througliout many allied groups ; and the constancy depends chiefly on any shght deviations of structure in such parts not having been pre- served and accumulated by natural selection, wliich acts only on useful characters. That tlie mere jihysiological importance of an organ does not determine its classificatory value, is almost proved by the fact that in allied groups, in which the same organ, as w^e have every reason to suppose, has nearly the same physiologi- cal value, its classificatory value is widely difltM-ent. No naturalist can have worked at aily group without being struck with this fact ; and it has been fully acknowledged in the writings of almost every author. It will suffice to quote the highest aulhorily, Kobert ]}ro'\\Ti, who, in speaking of certain organs in the Proteacere, says, their generic importance, " like that of all their parts, not only in this, Init, as I apprehend, in every natural familv, is verv unequal, and in some cases seems to be entirely lost." Again, in another work, he says, the genera of the Connaracea? " differ in having one or more ovaria, in the existence or absence of albumen, in the imbri- cate or valvular rcstivation. Any one of these characters singly is fref|uently of more than generic importance, though here even when all taken together they a])pear insufficient to separate Cnestis from Connarus." To give an example among insects: in one great division of the Ilymenoptera, the antenna?, as Westwood has remarked, arc most constant in structure ; in another division they differ nuich, and the differences are of (juitc subordinate vahie in classification ; yet no one will say that the antennif in these two divisions of the 37G CLASSIFICATION. CriAr. XIII. same order are of unequal jilijsiological importance. Any number of instances could be given of the varying importance for classification of the same important organ williin the same group of beings. Agahi, no one Avill say that rudimentary or atrophied or- gans are of high physiological or ^ital importance ; ^-et, un- doubtedly, organs in this condition are often of high value in classification. No one will dispute that the rudimentary teeth in the upper jaws of young ruminants, and certain rudimentary bones of the leg, are highly serviceable in exhibiting the close affinity between Ruminants and Pachyderms. Robert Brown has strongly insisted on the fact that the position of the rudi- mentary florets is of the highest importance in the classification of the Grasses. Numerous instances could be given of characters derived Ij-om parts which must be considered of Acry trifling physiolo- gical importance, but which are universally admitted as highly serviceable in the definition of Avhole groups. For instance, whether or not there is an open passage from the nostrils to the mouth, the only character, according to Owen, which abso- lutely disting-uishcs fishes and reptiles — the inflection of the angle of the jaws in ^larsupials — the manner in which the wings of insects are folded — mere color in certain Alga; — mere pubescence on parts of the tlower in grasses — the nature of the dermal covering, as hair or feathers, in the Vertebrata. If the Ornithorhyncluis had been covered with feathers instead of hair, this external and trifling character would have been considered by naturalists as an important aid in determining the degree of affinity of this strange creature to birds. The importance, for classilication, of trifling characters, mainly depends on their being correlated Avith several other characters of more or less importance. The value indeed of an aggregate of characters is very evident in natural history. Hence, as has often been remarked, a species may depart from its allies in several characters, both of high physiological im- portance and of almost universal prevalence, and A'ct leave us in no doubt where it sliould be ranked. Hence, also, it has been found that a classification founded on any single charac- ter, however important that may be, has always failed ; for no part of the organization is invariably constant. The impor- tance of an aggregate of characters, even when none are im- portant, alone explains the aphorism by Linuiens, naTuely, that the characters do not give the genus, but the genus gives the CiiAP. .\III. CLASSIFICATION. 377 characters ; for this seems foun Jed on an .appreciation of many trillinp;' points of resemblance, too slif^ht to be delinetl. Cer- tain plants, belongiiii^ to the Mali)igliiacene, bear perfect and degraded fio\vcrs; in the hitter, as A. do Jussicuhas remarked, " the grc'iter nuinl)er of the characters proper to the species, to the genus, to tlie family, to the class, disappear, and thus laugh at our classification." But when Aspicarpa produced in France, during several years, only degraded flowers, departing so wonderfully in a number of the most important points of structure from the proper type of the order, yet M. Richard sagaciously saw, as Jussieu observes, that this genus should still be retained among the MalpighiacefO. This case seems to me well to illustrate the spirit of our classification. Practically, Avhen naturalists are at work, they do not trouble themselves about the physiological value of the charac- ters which they use in defining a group or in allocating any particular species. If they find a character nearly uniform, and common to a great number of forms, and not common to others, they use it as one of high value ; if common to some lesser number, they use it as of subordinate value. This prin- ciple has been broadly confessed by some naturalists to be the true one; and by none more clearly than by that excellent botanist, Aug. St. Hilaire. If certain characters are always found correlated with others, though no apparent bond of con- nection can be discovered between them, especial value is set on them. As in most groups of animals, important organs, such as those fc^r propelling the blood, or for aerating it, or those for propagating the race, arc foimd nearly uniform, they are considered as highly sernceable in classification; but in some groups of anim:ds all these, the most important vital organs, are found to ofler characters of quite subordinate value. Thus, as Fritz MllUer has lately remarked, in the same group of crustaceans, ('ypridina is furnished Avith a heart, while in two closely-allied genera, namely, C^-pris and Cytherea, there is no such organ : one species of Cypridina has well-developed branchi.'C, while another species is destitute of them. Wo can see why characters derived from the embrj^o should be of equal importance with those derived from the adult, for a natural classification of course includes all ages. But it is by no means ol)vious, on the ordinary view, why the structure of the embryo should be more important for this purpose than that of the adult, which alone plays its full part in the econo- my of Nature. Yet it has been strongly urged by those great 378 CLASSIFICATION. Chap. XIII. naturalists, iMiliic Edwards and Ag'assiz, tliat embryoloijifal characters ai o the most important of all ; and this doctrhie has very g'encrally been admitted as true. Nevertheless, their im- portaTicc has sometimes been exaggerated; in order to show tlii.s, Fritz Miillcr arranged by the aid of such characters the great class of crustaceans, and the arrangement did not prove a natural one. But there can be no douljt that characters de- rived from tlie embryo are generally of the highest value, not only Avith animals but Avith plants. Thus the two main divis- ions of flowering plants are founded on differences in the em- bryo— on the number and position of the cotyledons, and on the mode of development of the plumule and radicle. Wc shall immediately see why these characters possess so high a value in classification, namely, from the natural system being genealogical in its arrangement. Our classifications are often plainly influenced by chains of aflinities. Nothing can be easier than to define a number of chanicters common to all birds ; but in the case of crustaceans, such definition has hitherto been found impossible. There are crustaceans at the opposite ends of the series, which have hardly a character in common ; yet the species at both ends, from being plainly allied to others, and these to others, and so onward, can be recognized as unequivocally belonging to this, and to no other class of the Articulata. Geographical distribution has often been used, though per- haps not quite logically, in classification, more especially in very largo groups of closely-allied forms. Temminck insists on the utility or even necessity of this practice in certain groups of birds ; and it has been followed by several entomol- ogists and botanists. Finally, wutli respect to the comparative value of the vari- ous groups of species, such as orders, sub-orders, families, sub- families, and genera, they seem to be, at least at present, almost arbitrary. Several of the best botanists, such as Mr. IJontham and others, have strongly insisted on their arbitrary value. Instances could be given among plants and insects, of :i group of forms, first ranked by ])ractised naturalists as only a genus, and then raised to the rank of a sub-family or family ; and this has been done, not because further research has de- tected important structural differences, at first overlooked, but because nmnerous allied species, with slightly-different grades of difference, have been subsequently discovered. All the foregoing rules and aids and difBcultics in classifi- Chap. XIII. CLASSIFICATION. 3-79 cation arc explained, if I do not greatly deceive myself, on the view that the Natural System is founded on descent Avitli modification; that the characters which naturalists consider as showing true allinity between any two or more species, are those which have been inherited from a connnon parent, all true classification being genealogical ; that community of descent is the hidden bond Avhich naturalists have been unconsciously seeking, and not some unknown plan of creation, or the enun- ciation of general propositions, and the mere putting together and separating objects more or less alike. But I must explain my meaning more fully. I believe that the arret)} rjcment of the groujis within each class, in due sub- ordination and relation to each other, must be strictly genea- logical in order to be natural ; but that the amount of diflerence in the several branches or groups, though allied in the same (h'grce in blood to their common progenitor, may diller greatly, In'ing duo to the different degrees of modification Avhich they have luidergone ; and this is expressed by the forms being ranked under different genera, families, sections, or orders. The reader will best understand what is meant, if he will take the trouble to refer to the diagram in the fourth chapter. We will sup]iose the letters A to L to rcpi'csent during the Silurian epoch allied genera, descended from some still earlier forms. In three of these genera (A, F, and I) the species have trans- mitted modified descendants to the present day, represented l)y the fifteen genera («'* to s'*) on the uppermost horizontal line. Now all these modified descendants from a single species, are related in blood or descent to the same degree ; they may metaphorically be called cousins to the same millionth degTce ; y(}t tiiey differ widely and in diU'ercnt degrees from each other. The forms descended from A, now broken up into two or three families, constitute a distinct order from those descended from I, also broken up into two families. Nor can the existing sjie- cies, desecnded from A, be ranked in the same genus with the parent A; or those from I, with the parent I. But the exist- ing geiuis f" may be supposed to have been but slightly modi- tied ; and it will then rank with tlie parent-genus F ; just as some few still living organisms belong to Silurian genera. So Ihat the amount or value of the differences between these or- ganic beings which arc all related to each other in the same degree in blood, has come to be widely different. Nevertheless, their genealogi(Ml arranffcment remains strictly true, not only at tlie present time, but at each successive period of descent. 380 CLASSIFICATION. Cnxr. XIII. All the modified descendants from A will have inherited some- thing' in common from their common parent, as will all the de- scendants from I ; so will it be with each subordinate branch of descendants, at each successive stage. If, however, we suppose any descendant of A or of I to have been so much modified as to have lost all traces of its parentage, in this case, its place in the natural system will likev>dse be lost — as seems to have oc- curred with some few existing organisms. All the descendants of the genus F, along its whole line of descent,' are supposed to have been but little modified, and they form a single genus But this genus, tliough much isolated, will stiil occupy its projv er intermediate position. This natural arrangement is shown in the diagram as far as is possible on paper, but in much too simple a manner. If a branching diagram had not been used, and only the names of the groups had been written in a linear series, it would have been still less possible to have given a natural arrangement ; and it is notoriously not possible to rep- resent in a series, on a flat surface, tlic aflinities which we dis- cover in Nature among the beings of the same group. Thus, on the view which I hold, the natural system is genealogical in its arrangement, like a i^edigrcc ; but the amount of modifi- cation which the different groups have undergone has to be expressed by ranking them und(?r different so-called genera, sub-families, families, sections, orders, and classes. It may be worth while to illustrate this view of classification, by taking the case of languages. If we possessed a perfect jiedigree of mankind, a genealogical arrangement of the races of man would afford the best classification of the various lan- guages now spoken throughout the world ; and if all extinct lan- guages, and all intermediate and slowly-changing dialects, had to be included, such an arrangement would be the only pos- sible one. Yet it might be that some ancient languages had altered very little and had given rise to few new languages, while others had altered much, o\\'ing to the spreading, isola- tion, and state of civilization of the several co-descended races, and liad thus given rise to many new dialects and languages. The various degrees of difference between the languages of tlie same stock would have to be expressed by groups subordi- nate to groups ; but the proper or even only possible arrange- ment would still be genealogical ; and this would be strictly natimal, as it would connect together all languages, extinct and recent, by the closest affmitics, and would give the filiation and origin of each tongue. Chap. XIII. CLASSIFICATION. 381 In confirmation of this view, let us glance at the classifica- tion of varieties, which are believed or known to be descended i'roin a sinu:le sjiecies. These are grouped under the species, witli the sub-variclies under the varieties ; and in some cases, as with domestic pigeons, several otlier grades of dilVerence are requisite. Nearly the same rules are followed as in classifj-ing species. Authors have insisted on the necessity of arranging varieties on a natural instead of an artificial system ; we are cautioned, for instance, not to class two varieties of the pine- apple together, merely because their fruit, though the most important part, happens to be nearly identical; no one puts llie Swedish and conunon turnijjs together, though the escu- h'nt and thickened stems are so similar. Whatever part is found to be most constant, is used in classing varieties : thus the great agriculturist Marsiiall says the liorns are very useful loE this purpose with cattle, because they are less variable than tlie shape or color of the body, etc. ; whereas with sheep the horns are nuicli less serviceable, because less constant. In classing varieties, I apprehend that if we had a real pedigree, a genealogical classification would be universally preferred ; and it has been attempted in some cases. For we might feel sure, wlicther there had been more or less modification, the ])rinciple of inheritance would keep the forms together which were alHed in the greatest number of points. In tundjler ])igeons, tliougli some of the sul>varieties difler in the impor- tant character of having a longer beak, yet all are kept togeth- er, from having the common habit of tumbling; but the shoit- faccd breed has nearly or quite lost this haljit; nevertheless, without any thought on the sul)ject, these tumblers arc kept in the same group, because allied in blood and alike in some other respects. With species in a state of nature, every naturalist has in fact brought descent into his classification ; for lie includes in his lowest grade, that of the species, the two sexes ; and how enormously these sometimes dilh^r in the most important char- acters, is known to every naturalist : scarcely a single fact can be predicated in common of the adult males and hermajihro- • lites of certain cirripedes, and yet no one dreams of sejiarating ihem. As soon as the three Orchidean forms, Monachanthus, -Myanthus, and Catasetum, which had jireviously been ranked as three distinct genera, were known to bo sometimes pro- duced on the same plant, they were inunediately considered as varieties; and now 1 have been able to show that they are the 382 CLASSIFICATION. Chap. XIII. male, female, and hermaphrodite forms of the same species. The naturalist ineludes as one species the various larval stages of the same individual, however much they may differ from each other and from the adult, as Avell as the so-called alter- nate generations of Stcenslrup, which can only in a technical sense be considered as the same individual. He includes monsters and varieties, not from their partial resemblance to the parent-form, but because they are descended from it. As descent has universally been used in classing together the individuals of the same species, though the males and fe- males and larvfc are sometimes extremely different ; and as it has been nscd in classing varieties which have undergone a certain, and sometimes a considerable amount of moditication, may not this same element of descent have been unconsciously used in grouping species under genera, and genera imder higher groujis, all under the so-called natural system? I be- lieve it has been unconsciously used; and thus only can I im- derstand the several rules and guides which have been fol- lowed by our best systematists. We have no written pedi- grees ; Ave have to make out community of descent by resem- blances of any kind. Therefore Ave choose those characters Avhich, as far as Ave can jucige, are the least likely to Ikia'C been modified in relation to the conditions of life to Avhich each spe- cies has been recently exposed. Rudimentary structures on this vicAV are as good as, or CA'en sometimes better than, other ]")arts of the organization. We care not hoAV trilling a charac- tcn- may be — let it be the mere inflection of the angle of the jaAv, the manner in Avhich an insect's Aving is folded, Avhet'licr the skin be covered b}' hair or feathers— if it prevail thn^ugh- out many and different species, especially those haA'ing very different habits of life, it assumes high A'alue ; for Ave can ac- coimt for its presence in so many forms Avith such different habits, only by inheritance from a common ]iareut. A\^e may err in this resjiect in regard to single points of structure, but Avhen scA'eral characters, let them be eA'cr so trifling, concur throughout a large group of beings haA'ing different habits, aa'C may feel almost sure, on tlieory of descent, that these charac- ters have been inlierited from a common ancestor. And aa-g knoAV that such correlated or aggregated charactei's haA'e es jiecial value in classification. We can understand Avhy a sjiecies or a group of sjiecies may depart, in several of its most important characteristics, from its allies, and yet be safely classed Avith them. This may Chap. XIII. ANALOGICAL RESEMBLANCES, 3S3 be safely doiip, aiul is often done, as long as a sufficient num- ber of characters, let them be ever so unimportant, betrays the hidden bond of community of descent. Let two forms have not a single character in conunon, yet if these extreme forms are connected together by a chain of intermediate groups, we may at once infer their community of descent, and we put them all into the same class. As we find organs of high physiologiortl importance — those which serv'C to preserve life under the most diverse conditions of existence — are generally the most constant, we attach especial value to them ; but if these same organs, in another group or section of a group, are found to differ much, we at once value them less in our classi- fication. We sliall presently see why embryological charac- ters are of such high classiticatpry importance. Geographical distribution may sometimes be brought usefully into play in classing large genera, because all the species of the same genus, inlial)iting any distinct and isolated region, are in all probability descended from the same ]>arents. jinalof/lcal Resemhlanccs. — We can understand, on the above views, the very important distinction between real af- finities and analogical or adaptive resemblances. Lamarck first called attention to this distinction, and he has been ably followed by Macleay and others. The resemblance in the shape of the body and in the fin-like anterior limbs, between the dugong, which is a pachydermatous animal, and the whale, and between both these mammals and fishes, is analo- gical. Among insects there are innumerable instances: tlujs Linnicus, misled by external appearances, actually classed an ho;noi)terous insect as a moth. We see something of the same kind even in our domestic varieties, as in the thickened stems of the common and Swedish turnips. The resemblance of the greyhound and race-horse is hardly more fanciful than the analogies whicli have been drawn by some authors between widely-distinct animals. On my view of characters being of real importance for classification, only in so far as they reveal descent, we can clearlv understand why analogical or adaj^tive characters, although of the utmost importance to the welfare of tlie being, are almost valueless to the systematists. For aniiMiils belonging to two most distinc-t lines of descent may readily have become adapted to similar conditions, and thus have assumed a close external resemblance ; but such resem- blances will not reveal — will rather tend to conceal their blood-rclalionsliip. We can thus also understand the appar- 384 ANALOGICAL KESEMBLANCES. Chap. XIIL cut paradox, that the very same characters are analogical when one class or one order is compared -with another, but give true aflniities Avhen the members of the same class or or- der are compared together: thus, the shape of the body and iin-likc Hmbs are only analogical "when whales are compared AviLli fishes, being adaptations in both classes for swimming through the water; but the shape of the body and fin-like limbs serve as characters exhibiting true aflinity between the several members of the whale family ; for these cetaceans agree in so many characters, great and small, that we cannot doubt that they have Hiherited their general shape of body and structure of limbs from a common ancestor. So it is with fishes. The most remarkable case of analogical resemblance ever recorded, though not dependent on adaptation to similar con- ditions of life, is that given by Mr. Bates with respect to cer- tain butterflies in the Amazonian region closely mimicking other kinds. This excellent observer shows that in a district where, for instance, an Ithomia abounds in gaudy swarms, another butterfly, namely, a Leptalis, is often found mingled in the same flock, and so closely resembles the Ithomia in every shade and stripe of color, and even in the shape of its wings, that jNIr. Bates, with his eyes sharpened by collecting during eleven years, was, though always on his guard, con- tinually deceived. When the mockers and the mocked are caught and comjiared, they are found to be totally different in essen- tial structure, and to belong, not only to distinct genera, but often to distinct families. Had this mimicry occurred in only one or two instances, it might have been passed over as a strange coincidence. But, if we proceed from a district where one Leptalis imitates an Ithomia, another mocking and mocked species belonging to the same genera, equally close in their re- semblance, will be found. Altogether no less than ten genera are enumerated, which include species that iinitate other but- terflies. The mockers and mocked alwaj's inhabit the same region ; we never find an imitator living remote from the form which it imitates. The mockers are almost invariably rare insects ; the mocked in almost every case aboimd in swanns. In the same district in which a species of Lcjitalis closely imitates an Ithomia, there are sometimes other Lejn- doptera mimicking the same Ithomia ; so that in the same place, species of three genera of butterflies and even a moth are found nil closely resembling a butterfly belonging to a fourth Chap. XIII. ANALOGICAL EESEMBLANCES. 385 p^emis. It deserves especial notice that many of the mimick- inn^ forms of the Lcptalis, as ■well as of the mimicked forms, can be shown hy a g^raduated series to be merely varieties of the same species ; while others are undoubtedly distinct spe- cies. But why, it may be asked, are certain Jbrms treated as tlie mimicked and othei's as tlic mimickers ? Mr. Bates satis- factorily answers this question, by showing);' that the form Avliicli is imitated keeps the usual dress of the f^roup to which it lielongs, while the counterfeiters have changed their dress and do not resemble their nearest allies. ^\'e are next led to inquire what reason can possibly be as- signed for certain butterflies and moths so often assuming the dress of another and quite distinct form ; why, to the perplexity of naturalists, has Nature condescended to the tricks of the stage? ]Mr. Bates has, no doubt, hit on the true explanation. The mocked forms, wliich always aboimd in numbers, nuist habitually escape destruction to a large extent, otherwise they could not exist in such swarms ; and Mr. Bates never saw them preyed on l)y birds and certain large insects which at- tack other butterllies. He has good reason to believe that this innnunity is owing to a peculiar and offensive odor which they emit. The mocking forms, on the other hand, that in- habit the same district, are comparatively rare, and belong to lare groups ; hence they must suffer habitually from some danger, A^r otherwise, from the number of eggs laid bv all but- terflies, they would in three or four generations swarm over the wliole country. Now if a member of one of these persecuted and rare groups were to assume a dress so like that of a well- ])rotected species that it continually deceived the practised eyes of an entom(ilogist, it would often deceive predacious birds and insects, and thus escape much destruction. Mr. Bates may almost be said to have actually witnessed the process by which the mimickers have come so closely to resemble the mimicked ; for he found that some of the forms of Lcptalis which mimic so many other butterflies, varied in an extreme degree. In one district several varieties occurred, and of these one alone resembled, to a certain extent, the conunon Ithomia of the same district. In another district there were two or three varieties, one of which was much commoner than the others, and this closely mocked another form of Ithomia. From facts^of this nature, Mr. Bates concludes that the I^op- talis first varies ; and when a variety hajipens to resemble in some degree any common butterfly inhabiting the same district, 17 386 AFFINITIES CONNECTING Cuap. XIII. tliis variety, from its rcseinl)lancc to a flourishing aiul little-per- secuted kind, has a better chance of escaping' destruction from predacious birds and insects, and is consequently oftener pre- served ; " the less ];)erfect degrees of resemblance being gener- ation after generation eliminated, and only the others left to ]')ropagate their kind." So that here we have an excellent il- lustration of natural selection. Mr, Wallace has recently described several equally striking cases of mimicry in the Lepidoptera of the Malay Archipelago, and other instances could be given with other orders of insects. Mr. Wallace has also described one case of mimicry among birds, but we have no such cases Avith the larger quadrupeds. Tlie much greater fi-equency of mimicry with insects than with other animals, is probably the consequence of their small size ; insects cannot defend themselves, excepting indeed the kinds that sting, and I have never heard of an instance of these mocking other insects, though they are mocked ; insects can- not escape by flight from the larger animals ; hence they are reduced, like most weak creatures, to trickery and dissimula- tion. But to return to more ordinary cases of analogical resem- l:)lance : as members of distinct classes have often been adapted by successive slight modifications to live under nearly similar circumstances — to inhabit, for instance, the three elements of land, air, and water — we can perhaps understand how it is that a numerical parallelism has sometimes been observed between the sub-groups in distinct classes. A naturalist, struck by a ])arallelism of this nature in any one class, by arbitrarily rais- ing or sinking the value of the groups in other classes (and all our experience shows that their valuation is as j'et arbitrary) could easily extend the parallelism over a wide range ; and thus the septenary, quinary, quaternary, and ternary classifica- tions have probably arisen. 0)1 tlic N'ature of the Affinities connecting Organic be- ings.— As the modified descendants of dominant species, be- longing to the larger genera, tend to inherit the advantages which made the groups to which they belong large aud their parents dominant, they are almost sure to spread widely, and to seize on more and more places in the economy of Nature. The larger and more dominant groups within each class thus ttMid to go on increasing in size ; and they consecjjiently sup- j>lant many smaller and feebler groups. Thus wc can account for the fact that a]l organisms, recent and extinct, are included Chai-. XIII. ORGANIC BEINGS. 337 under a few p^rcat orders, and under still fewer classes. As sho\vinher frroups are in number, and how widely they are spread throuf^hout the world, the fact is strik- in;i-, that the discovery of Australia has not added an insect l)(>l()n<2,"iiij^ to a new class ; and that in the vegetable kingdom, as I learn fi-om Dr. Hooker, it has added only two or three families of small size. In the chapter on Geological Succession I attempted to show, on the principle of each group having generally diverged nuich in character during the long-C(jntinued process of modifi- cation, how it is that the more ancient forms of life often pre- sent characters in some degree intermediate between existing groups. Some few of these old and intermediate forms hav- ing transmitted to the present day descendants but little modi- lied, constitute our so-called osculant or aberrant species. The more aberrant any form is, the greater must be the number of connecting forms Avhicli have been exterminated and utterly lost. And we hav(! some evidence of aberrant groups having suffered severely from extinction, for they are almost always represented by extremely f(jw species ; and such species as do occur are generally very distinct from each other, which again implies extinction. The genera Ornithorhynchus and Lepi- (losiren, for example, woulil not have been less aberrant had each been represented by a dozen species instead of by a single one, or by one; or two. We can, I think, account for this fact only by looking at aberrant groups as forms which have been con([uered by more successful competitors, with a few mem- bers still preserved under unusually favorable conditions. Mr. Waterhouse has remarked that, when a member be- longing to one group of animals exhibits an alTinity to a cpiite distinct group, this alVmity in most cases is general and not special : thus, according to Mr. Waterhouse, of all Rodents, the bizcacha is most nearly related to Marsupials; but in the points in which it ajiproaches this order, its relations are gen- eral, and not to any one marsupial species more than to an- other. As \]ui jjoints of allinity are believed to be real and not merely adaptive, they nnist be due, in accordance with our view, to inheritance from a conunon ])rogenitor. Therefore we must su])j)ose either that all Rodents, including the bizcacha, branched olV from some ancient Marsupial, which will naturallv have been more or less intermediate in character with respect to id! existing Marsupials; or that both Rodents and Marsu- jjials branched olV iVom a cominou progenitor, and that both 388 AFFINITIES CONNECTING Chap. XIII. g^roups have since undergone miicli modification in divergent directions. On citlicr view we must suppose that the bizcacha has retained, by inheritance, more of the character of its an- cient progenitor than have other Kodents ; and therefore it will not be specially rchited to any one existing Marsupial, but indirectly to all or nearly all Marsupials, from having partially retained the character of their conimon progenitor, or of some earl}"^ member of the group. On the other hand, of all Mar- suj>ials, as Mr. Waterhousc has remarked, the Phascolomys resembles most nearly, not any one species, but the general order of Rodents. In this case, however, it may be strongly suspected that the resemblance is only analogical, owing to the Phascolomys having become adapted to habits like those of a Rodent. The elder De Candolle has made nearly similar observations on the general nature of the affinities of distinct families of plants. On the principle of the multiplication and gradual diver gence in character of the species descended from a common progenitor, together with their retention by inheritance of some characters in common, we can understand the excessively complex and radiating affinities by which all the members of the same family or higher group are connected together. For the common progenitor of a whole family, now broken up by extinction into distinct groups and sub-groups, will have trans- mitted some of its characters, modified in various ways and degrees, to all the species ; and they will consequently be related to each other by circuitous lines of affinity of. various lengths (as may be seen in the diagram so often referred to), mounting up through many predecessors. As it is difficult to show the blood-relationship between the numerous kindred of any ancient and noble family even by the aid of a genealogical irco, and almost impossible to do so without this aid, we can understand the extraordinary difficulty which naturalists have experienced in describing, without the aid of a diagram, the various affinities which tliey perceive between the many hving and extinct members of the same great natural class. Extinction, as we have seen in the fourth chapter, has played an imjiortant part in defining and M-idening the inter- vals between the several groups in each class. AVe may thus account for the distinctness of whole classes from each other — for instance, of birds from all other vertebrate animals — by the belief that many ancient forms of life have been utterly lost, through which the earlv progenitors of l)irds were formerly CiiAi-. Xlll. ORGANIC BEINGS. 339 connected with the early progenitors of the other and at that time less diirercntiated vertebrate classes. Tlicre has been less complete extinction of the forms of life which once con- nected fishes with batrachians. There has been still less in some other classes, as with the Crustacea, for here the most wonderfully diverse forms are still linked together hy a long and only partially-broken chain of allinities. Extinction has only se})arated the groups : it has by no means made them ; for, if every form which has ever lived on this earth were sud- denly to reappear, tliough it would l)e cjuite impossi])le to give (Iclinitions by which each group could be distinguished, still a natural classification, or at least a natural arrangement, Avould l)c j)ossible, AVe shall see this by turning to the diagram: tlie letters, A to L, may represent eleven Silurian genera, some of which have produced large groups of modified descendants, Avjtli every link in each branch and sub-branch still alive ; and the links not greater than those between the finest varieties. In this case it would be quite im{)ossible to give definitions by which the several members of the several groups could be dis- tinguished from their more immediate parents and descendants. Yet the arrangement in the diagram would still hold good and would be natural ; for, on the principle of inheritance, all the forms descended, for instance, front A, Avould have something in common. In a tree we can distinguisli this or that branch, though at the actual fork the two unite and blend together. AV'e could not, as I have said, define the several groups ; but we could pick out t\'pes, or forms, representing most of the characters of each group, whether large or small, and thus give a general idea of the value of the differences between them. This is what Ave should be driven to, if we were over to suc- c(^ed in collecting all the foi-ms in any one class which have lived througliout all time and sjiace. Assuredly we shall never succeed in making so perfect a colh^ction : nevertheless, in cer- tain classes, we arc tending toward this end; and Milne Ed- wards has lately insisted, in an able paper, on the high impor- tance of looking to types, whetlier or not we can separate and define the groups to which such types belong. Finally, we have seen that natural selection, which results from the struggle for existence, and which almost inevitably leads to extinct ion and divergence of character in the descend- ants from one dominant ]ia rent-species, explains that great and universal feature in tlie atlinities of all organic beings, namely, their suboiilination in group under group. We use the clc- 3'JO MORPHOLOGY. Chap. XIII. ment of ilrst'cnt in chissint!^ the in(livi