Principles of Geology (6th edition and thereafter)
Theories As To The Nature Of Species, And Darwin On Natural Selection[*]
[261] Objections Urged Against the Theory of Transmutation and Lamarck's Replies--Mummies of Animals and Seeds of Plants from Egyptian Tombs Identical in Character with Species Now Living--Linnaeus Opinion that Species have been Constant Since Their Creation--Brocchi's Hypothesis of the Gradual Diminution of Vital Power in a Species-Whether If New Species are Created from Time to Time Their First Appearance Must Have Been Witnessed By The Naturalist--Geoffroy St. Hilaire and Lamarck on Rudimentary Organs--The Question of Species as Treated of in "Vestiges Of Creation."--Mr. Alfred Wallace on The Law Which has Regulated the Introduction of New Species--Mr. Darwin on Natural Selection and Mr. Wallace on The Same--Darwin's Origin of Species and the Change of Opinion Which it Effected--Dr. Hooker's Flora of Australia and His Views as to the Origin of Species by Variation.
Objections urged against the Theory of Transmutation and Lamarck's Replies.--The theory of the transmutation of species, considered in the last chapter, was received with some degree of favour by many naturalists, from their desire to dispense, as far as possible, with the repeated intervention of a First Cause, as often as geological monuments attest the successive appearance of new races of animals and plants, and the extinction of those preexisting. But, independently of a predisposition to account, if possible, for a series of changes in the organic world by the regular action of secondary causes, we have seen that in truth many perplexing difficulties present themselves to all who attempt to establish the reality and constancy of the specific character. And if once there appears ground for reasonable doubt, in regard to the constancy of species, the amount of transformation which they are capable of undergoing might seem to resolve itself into a mere question of the quantity of time [264] assigned to the past and future duration of animate existence.
The opponents of Lamarck objected to his arguments that 1 he could not adduce a single instance of the gradual conversion of any one species of animal or plant into another; and,l that in his appeal to the results obtained by the breeder and,d horticulturist, he had failed to show such a change in the structure and constitution of individuals descending from a common stock as might fairly entitle the new race to rank as a distinct species. It was conceded, for example, on all hands that the modifications produced in the different races of dogs exhibit the influence of man in the most striking point of view. These animals had been transported into every climate, and placed in every variety of circumstances: they had been made, as M. Dureau de la Malle observed, the servant, the companion, the guardian, and the intimate friend of man, and the power of a superior genius had had a wonderful influence not only on their forms, but on their manners and intelligence.[1] Different races have undergone remarkable changes in the quantity and colour of their clothing; the dogs of Guinea are almost naked, while those of the Arctic Circle are covered with a warm coat both of hair and wool, which enables them to bear the most intense cold without inconvenience. There are differences also of another kind no less remarkable, as in size, the length of their muzzles, and the convexity- of their foreheads. "The difference in stature," said Cuvier, "in some canine races as compared to others is as 1 to 5 in linear dimensions," making a difference of a hundredfold in volume.[2]
But, said the advocates of the immutability of species, if we look. for some essential changes, such as might serve as a foundation for the theory of Lamarck, respecting the growth of new organs and the gradual obliteration of others, we find nothing of the kind. In all the varieties of the dog, as Cuvier affirmed, the relation of the bones with each other remains essentially the same; the form of the teeth never changes in any perceptible degree, except that, in some [265] individuals, one additional false grinder occasionally appears, sometimes on the one side, and sometimes on the other.[3] The greatest departure from a common type--and it constitutes the maximum of variation as yet known in the animal kingdom,--is exemplified in those races of dogs which have a supernumerary toe on the hind foot with the corresponding tarsal bones; a variety analogous to one presented by six-fingered families of the human race.[4]
It was moreover urged, and of all objections this was the most serious, that however distinct were the various races of the dog, they could all breed freely together and produce fertile offspring, as was also the case with various domesticated birds, such as the common fowl, of which such marked varieties s had been obtained. In no instance had the mongrel offspring been shown to be habitually sterile, like the common mule]e or the offspring of the horse and ass, where the two parents belong to two undoubtedly distinct species.
When the controversy had been brought to this point, and the amount of possible variation of animals under domestication, and of plants under culture, was still under discussion, the followers of Lamarck sometimes lamented that no accurate descriptions, and figures of known species, had been handed down from the earliest periods of history, such as might have afforded data for comparing the condition of the same species, at two periods considerably remote. To this, however, the opponents of transmutation replied, that we are in a great measure independent of such evidence, since, by a singular accident, the priests of Egypt have bequeathed to us, in their cemeteries, that information which the museums and works of the Greek and Roman philosophers have failed to transmit.
It had fortunately happened that the men of science who accompanied the French armies during their four years' occupation of Egypt, from 1797 to 1801, instead of employing their whole time, as so many preceding investigators had done, in exclusively collecting human mummies, had examined diligently and sent home great numbers of embalmed bodies of consecrated animals, such as the bull, the dog, [266] the cat, the ape, the ichneumon, the crocodile, and the
They who have never raised their conceptions of the import of Natural History beyond the admiration of beautiful objects or the exertion of skill in detecting specific differences would wonder at the enthusiasm expressed in Paris at the beginning of this century, amidst the din of arms and the stirring excitement of political events, in regard to these precious remains. In the official report, drawn up by the Professors of the Museum at Paris, on the value of the objects alluded to, the following passages might seem extravagant, unless we reflect how fully the reporters ((Cuvier, Lacépède, and Lamarck) appreciated the bearing of the facts thus brought to light on the past history of the globe.
"It seems," say they, "as if the superstition of the ancient Egyptians had been inspired by Nature, with a view of transmitting to after ages a monument of her history. That extraordinary and eccentric people, by embalming with 80 much care the brutes which were the objects of their stupid adoration, have left us, in their sacred grottos, cabinets of zoology almost complete. The climate has conspired with the art of embalming to preserve the bodies from corruption, and we can now assure ourselves by our own eyes what was the state of a great number of species three thousand years ago. We can scarcely restrain the transports of our imagination, on beholding thus preserved, with their minutest bones, with the smallest portions of their skin, and in every particular most perfectly recognisable, many an animal, which at Thebes or Memphis, 2,000 or 3,000 years ago, had its own priests and altars." [5]
Among the Egyptian mummies thus procured were not only those of numerous wild quadrupeds, birds, and reptiles; but, what was perhaps of still higher importance in helping to decide the great question under discussion, there were the mummies of domestic animals, among which those above mentioned, the bull, the dog, and the cat, were frequent Now, such was the conformity, says Cuvier, of the whole of these species and races to those now living, that there was [267] no more difference between them than between the human mummies and the embalmed bodies of men of the present day. Yet some of these animals have since that period been transported by man to almost every variety of climate, and forced to accommodate their habits to new circumstances as far as their nature would permit. The cat, for example, has been( n carried over the whole earth, and, within the last three centuries has been naturalised in every part of the New W orld from the cold regions of Canada to the tropical plains of Guiana; yet it has scarcely undergone any perceptible mutation, and is still the same animal which was held sacred by the Egyptians. Of the ox, undoubtedly, there are many very distinct races; but the bull Apis, which was led in solemn processions by the Egyptian priests, did not differ[er from some of those now living.
Nor was the evidence derived from the Egyptian monuments confined to the animal kingdom; the fruits, seeds, and other portions of twenty different plants, were faithfully preserved in the same manner; and among these the common wheat was procured by Delille, from closed vessels in the sepulchres of the kings, the grains of which retained not only their form, but even their colour; so effectual had l proved the process of embalming with bitumen in a dry and equable climate. No difference could be detected between this wheat and that which now grows in the East and elsewhere, and similar identifications were made in regard to many other plants.
In answer to the argument drawn from this class of facts Lamarck observed, that "the animals and plants referred to ha not experienced any modification in their specific charactersn because the climate, soil, and other conditions of life had not varied in the interval. But if," he went on to say," the physical geography, temperature, and other natural conditions of Egypt had altered as much as we know they have done in many countries in the course of geological periods, the same animals and plants would have deviated from their pristine types so widely as to rank as new and distinct species."[6] This reply, when we consider its date (about the year 1809 [268] may well lay claim to our admiration, as it evinced Lamarck's thorough conviction, that geological changes are brought about so slowly that the lapse of thirty or forty centuries is utterly insignificant in the history of a species. Nearly all the men of science of his day, even the great majority of geologists, entertained extremely narrow views in regard to the duration of those periods of the past of which they v were studying the archives. They were generally inclined to attribute all great changes of the earth's crust, and its inhabitants, to brief and violent catastrophes, against which Lamarck emphatically protested.[7] Yet neither he nor an) of his contemporaries could as yet form any conception of the number and real magnitude of the revolutions in the animate world with which paleontology has since made us familiar. In certain passages of his work he admitted that possibly the Paleotherium, Anoplotherium, and some other fossil genera of quadrupeds then recently described by Cuvier a occurring in tertiary strata near Paris, may have disappeared having, perhaps, been exterminated by the power of man. But in regard to smaller animals, especially those of the aquatic tribes, which could not have been the victims of human intervention, he sometimes expressed a doubt whether most of these may not still have their representatives surviving in regions unexplored by the naturalist. Being, aware, however, that the specific and generic forms of animals and plants preserved in the rocks are more unlike those now existing in proportion as they are more ancient, Lamarck expressed his belief that in those cases where the fossil animals could be identified with the living, the strata containing them must be very modern, their descendants not having had time to vary, except within extremely narrow limits.[8]
It was by this constant reference to time as an essential element even in the definition of a species, that the teaching of Lamarck differed from that of Linnaeus, Blumenbach, and C!uvier.
Linnaeus on species.--Linnaeus in one of his treatises had [269] said that classes and orders are the inventions of science, but species are the work of nature.[9] In another place he went so far as to declare that genera, like species, are primordial creations.[10]
Expressions may doubtless be found in some of his speculative essays, implying that he thought that some species at least were the daughters of time, "temporis filiae," and we shall see in Chap. XXXVII. that when a great number of closely allied species existed in the same region, he strongly suspected that they might be derived from other species-- possibly that they were hybrids, and had become so far permanent as to require to be treated as distinct species. But his deliberate opinion was contained in the following aphorism: "We reckon just so many species as there were forms created in the beginning.[11]" Blumenbach declared that "no general rule can be laid down for determining the distinctness of species, as there is no particular class of characters which can serve as a criterion. In each case we must be guided by analogy and probability."
In former editions of this work from 1832 to 1853, I did not venture to differ from the opinion of Linnaeus, that each species had remained from its origin such as we now see it, being variable, but only within certain fixed limits. The mystery in which the origin of each species was involved seemed to me no greater than that in which the beginning of all vital phenomena on the earth is shrouded. But I undertook to show that the gradual extinction of species one after another was part of the constant and regular course of nature, and must have been so throughout all geological time, because the climate, and the position of land and sea, and all the principal conditions of the organic and inorganic world, are always, and have been always, undergoing change. 1 pointed out how the struggle for existence among species, and the increase and spread of some of them, must tend to the extermination of others; and as these would disappear gradually and singly from the scene, I suggested [270] that probably the coming in of new species Would in like manner be successive, and that there was no geological sanction for the favourite doctrine of some theorists, that large assemblages of new forms had been ushered in at once to compensate for the sudden removal of many others from the scene.
Brocchi on the dying out of a species.--An Italian geologist Brocchi, the author in 1814 of an able work on the fossil shells of the Subapennine Hills, endeavoured to imagine some regular and constant law by which species might be made to disappear from the earth gradually and in succession. The death, he suggested, of a species might depend, like that of individuals, on certain peculiarities of constitution conferred upon them at their birth; and as the longevity of the one depends on a certain force of vitality, which, after a period, grows weaker and weaker, so the duration of the other may be governed by the quantity of prolific power bestowed upon the species which, after a season, may decline in energy, so that the fecundity and multiplication of individuals may be gradually lessened from century to century, "until that fatal term arrives when the embryo, incapable of extending and developing itself, abandons, almost at the instant of its formation, the slender principle of life by which it was scarcely animated,--and so all dies with it."[12] In opposition to this doctrine, I contended that there is no reason to suspect that the last individuals of a species of which the numbers are diminishing are physiologically deteriorated, or are in the least degree impaired in their prolific powers; for there are known causes in the animate and inanimate world which must in the course of ages annihilate species, however vigorous their powers of reproduction might remain. As the death of the last representatives of z species would be abrupt, I conjectured that the birth of new forms might be equally so, but as I had entire faith in the doctrine that what is now going on in the natura] world affords a true indication of what has been and will be, I assumed that the coming in of new species must be going On at about the same rate as the dying out of old ones; and I [271] therefore felt myself called upon to explain how the birth ofspecies could be always in progress, and yet the botanist and Zoologist remain wholly unconscious of the occurrence of dvents so wonderful, and to them of such transcendent interest rest.
Assuming that species were specially created from time to time to fill up the gaps to which the never-ceasing changes of the animate and inanimate world must give rise, I enquired what kind of evidence we had a right to expect of the origin of new forms of animals and plants in the course of the last twenty or thirty centuries. Ought we to have been as conscious of the fact as we are of the lessening of the numbers and the occasional extermination of particular species ? It was obviously, I remarked, more easy to) prove that a species, once numerously represented in a (riven district, had ceased to be, than that some other which did not pre-exist had made its appearance--assuming always that single stocks only of each animal and plant are originally!- created, and that individuals of new species do not sud{lenly start up in many different places at once. The latter hypothesis had already been considered by Linnaeus, and pronounced by him to be unphilosophical because quite unnecessary, since, as he observed, every animal or plant, even those which increase slowly, are capable in twenty or thirty generations of stocking a large part of the whole globe with their descendants.
So imperfect has the science of Natural History remained down to our own times, that, within the memory of persons now living, the numbers of known animals and plants have been doubled, or even quadrupled, in many classes. New and often conspicuous species are annually discovered in parts of the old continent, long inhabited by the most civilised nations. C:onscious, therefore, of the limited extent of our informaltion, we always infer, when such discoveries are made, that the beings in question had previously eluded our research; or had at least existed elsewhere, and only migrated at a recent period into the territories where we now find them. It is difficult to look forward to the time when we shall be be entitled to make any other hypothesis in regard [272] to all the marine tribes, and to by far the greater number of the terrestrial; such as birds, and insects, and a large portion of plants, especially those of the cryptogamous class, many of which possess such unlimited powers of diffusion as to be almost cosmopolitan in their range.
It may perhaps be said that if new species were suddenly called into being by special acts of creation, some forest tree or new quadruped ought to have been seen, for the first time, within the last ten or twenty centuries in the more populous parts of such countries as England or France In that case, the naturalist might have been able to de monstrate that no similar living form had before existed in the district.
Now, although this argument may seem plausible, its force will be found to depend entirely on the rate of fluctuation which we suppose to prevail in the animate world, and on the proportion which such conspicuous subjects of the animal and vegetable kingdoms bear to those which are less known and escape our notice. There are perhaps more than a million species of plants and animals, exclusive of the microscopic and infusory animalcules, now inhabiting the terraqueous globe; so that if only one of these were to become extinct annually, and one new one were to be every year called into being, much more than a million of years might be required to bring about a complete revolution in organic life.
I have never ventured to hazard any precise hypothesis as to the probable rate of change; but none will deny that when the annual birth and the annual death of one species on the globe was proposed as a mere speculation, this at least was to imagine no slight degree of instability in the animate creation. If we divide the surface of the earth into twenty regions of equal area, one of these might comprehend a space of land and water about equal in dimensions to Europe, and might contain a twentieth part of the million of species which may be assumed to exist in the animal kingdom. In this region one species only would, according to the rate of mortality before assumed, perish in twenty years, or only five out of fifty thousand in the course of a century. [273] But as a considerable proportion of the whole would belong to the aquatic classes, with which we have a very imperfect acquaintance, we must exclude them from our consideration; d if they constitute half of the entire number, then one species only might be lost in forty years among the terrestrial tribes Now the Mammalia, whether terrestrial or aquatic, bear so small a proportion to other classes of animals, forming less, perhaps, than one thousandth part of the whole, that if the longevity of species in the different orders were equal, a vast period must elapse before it would come to the turn of this conspicuous class to lose one of their number. If one species only of the whole animal kingdom died out in forty years, no more than one mammifer might disappear in 40,000 u years in a region of the dimensions of Europe.
It is easy, therefore, to see, that in a small portion of such an area, in countries, for example, of the size of England and France, periods of much greater duration must elapse before it would,uld be possible to authenticate the first appearance of one of the larger plants and animals, assuming the annual l)birth and death of one species to be the rate of vicissitude in the animate creation throughout the world. It would fo]low from the above considerations that if Lamarck was enlitled to plead insufficiency of time when challenged to bring forward a single case of transmutation, the advocates of special creation were equally entitled to say that if the introduction of new species goes on as slowly as the extinction of old ones, it could not be expected that they should have witnessed the first starting into being of a new animal or plant,lt,
Geoffroy St. Hilaire and Lamarck on rudimentary organs.-- The great majority of the best naturalists and geologists who succeeded Lamarck were content to believe with Humboldt that the origin of species was one of those mysteries which It was not given to natural science to penetrate. Omalius d'Halloy however, in his "Elements of Geology," which he published in 1831, and in six subsequent editions, taught that the species of animals now living were the descendants of progenitors which have left their fossil remains in the later Tertiary formations. I asked him in the year 1867, when [274] he was in his eighty-fourth year, by what facts and reasonings he had been led to entertain this view, and he told me that he owed his convictions on this head to the lectures of Geoffroy St. Hilaire, to which he had listened in the early part of this century at Paris. That great zoologist, he said, never lost an opportunity, when he spoke of the rudimentary organs found in so many animals, of pointing out their bearing on the theory of transmutation. According to him they were clearly the relics of parts which had been service able in some remote ancestor and had been reduced in size by disuse, and he rejected the idea as puerile that useless organs had been created for the sake of uniformity of plan.
I may here remark that in my brief abstract of Lamarck's theory drawn up by me originally in 1832, and which for reasons explained in the last chapter (p. 248, note) I have now reprinted without alteration or addition, I omitted, when referring to what he had said on the impoverishment and final disappearance of organs by disuse, to cite many) examples which he gives in the "Philosophie Zoologique" in illustration of this principle. Among other facts the abortive teeth concealed in the jaws of some mammalia are mentioned, such teeth not being required because their food is swallowed without mastication. The discovery also by G. St. Hilaire of teeth in the foetus of a whale is alluded to, and the small size of the eyes in the mole, which makes scarcely any use of its organs of vision. Allusion is also made to the aquatic reptile called Proteus anguinus, inhabiting the waters of dark subterranean caverns, which retains only the vestiges or rudiments of eyes.[13]
The question of species as treated in the "Vestiges of Creation."-- But, speaking generally, it may be said that all the most influential teachers of geology, paleontology, zoology, and botany continued till near the middle of this century either to assume the independent creation and immutability of species, or carefully to avoid expressing any opinion on this important subject. In England the calm was first broken by the appearance in 1844 of a work entitled "The Vestiges of Creation," in which the anonymous author had gathered [275] together and presented to the public, with great clearness and skill the new facts brought to light in geology and the kindred sciences since the time of Lamarck in favour of the transmutation of species and their progressive development in time. He availed himself of the generalisations of paleontologists on the changes observable in the fossil fauna and flora of successive epochs of the past, showing that the structural affinity was greatest in those which stood nearest each other in position when the strata were arranged in chronological order, and that there had been a gradual approximation of the animate world as it changed from period to period to the state of things now represented by the living creation.
The embryological investigations of Tiedemann and others were referred to as being in harmony with the doctrine of transmutation the various phases of development through which a mammifer passes when in the foetal state representing in succession the likeness of a fish, reptile and bird, and lastly putting on the characters proper to the highest class of vertebrata. It was also suggested that these metamorphoses were comparable to the creative additions made in like chronological order to the organic world of past ages as revealed to us by the fossil remains preserved in the rocks. The arguments which lamarck and others had derived from rudimentary organs in favour of their views were re-stated and their validity emphatically insisted upon. The unity of plan exhibited by the whole organic creation fossil and recent, and the mutual affinities of all the different classes of the animal and vegetable kingdoms, were declared to be in harmony with the idea of new forms having proceeded from older ones by generation, species having been gradually modified by the influence of external conditions.
Lamarck had rendered his hypothesis very complete by embracing without any essential change the notions of Aristotle 2!I A as to spontaneous generation. The simplest
rudiments or germs of life were assumed to be always coming into being. This would account for the present abundance of species of the lowest grades of animal and vegetable existence in spite of the constant advance throughout past time of the [276] . organic creation towards a more perfect state. In his his,rer eagerness to supply the evidence which was wanting to confirm the reality of the working of this part of the plan of nature , the author of the "Vestiges" displayed an extraordinary want of philosophical caution. For he cited experiments which were supposed to prove that the action of a voltaic pile on a solution of potash could give origin to new species of insects. The careless way in which these experiments had been conducted contrasted in a striking manner with the extreme caution displayed by those who had been endeavouring to test the truth or falsehood of Harvey's dictum that "every living thing comes from an egg." The result of every increase in the power of the microscope had been to refute the theory of spontaneous generation, or at least to force the abettors of the old doctrine to take refuge in the obscure region of the infinitely minute. Distrust of the soundness of the author's judgment was also engendered by a suspicion that he was not practically versed in the study of any one department of natural knowledge. Every weak point, moreover, in this treatise was exposed with unsparing severity by critics who were impatient of the popularity it enjoyed, notwithstanding the adoption by the author of Lamarck's doctrine that Man was not only the last link of a long series of progressive developments but had been connected by descent with the inferior animals.
Darwin and Wallace on the origin of species.--The next important effort to determine the manner in which new species may have originated was made in 1855 by Mr. Alfred Wallace in the "Annals of Natural History,"[14] in an essay entitled "On the Law which has regulated the Introduction of New Species." The opinions enounced in this paper carried with them the authority of one who was well versed in several departments of natural history, especially ornithology and entomology. He had first explored during four years, conjointly with Mr. H. W. Bates, the valley of the river Amazons, and the neighbouring equatorial parts of South America, their expedition having been [277] expressly undertaken to collect facts "towards solving the problem of the origin of species."[15] Mr. Wallace had afterwards, spent many years in studying the zoology of the Malay Archipelago, devoting his attention especially to the birds and insects; and the result of his experience, aided by the information obtained from geological writers, was summed up in the following proposition, "that every species has come into existence coincident both in space and time with a preexisting closely allied species."[16] Mr. Darwin,[17] when referring subsequently to this paper in his "Origin of Species," has stated that he knew from correspondence with Mr. Wallace that the cause to which he attributed the coincidence here alluded to was no other than "generation with modification," or, in other words, the "closely allied antitype' was the parent stock from which the new form had been derived by variation. All the most telling arguments which Lamarck had brought forward, and those drawn from various sources which the "Vestiges" had superadded, in favour of species being the result of indefinite modification, instead of special creation, were briefly and ably summed up by Mr. Wallace; but it was clear that the evidence which had most powerfully influenced his mind, was that derived from his own experience of the geographical distribution of species, and especially of birds and insects.
In geography, he remarked, a genus or species rarely occurs in two very distant localities without being also found in the intermediate space; so in geology the life of a genus or species is not interrupted, no species having come into existence twice, or having been renewed after having once died out. For the manner in which the gradual extinction of species had been brought about and was still in progress Atr. Wallace referred to my chapter on that subject in the "Principles of Geology," confining his speculations to the manner in which new forms were introduced from time to time to replace those which were lost.
Meanwhile Mr. Charles Darwin, well known by his "Voyage in the Beagle," and various works on Geology, had been for [278] many years busily engaged in collecting materials for a great work on the origin of species; having made for that purpose a vast series of original observations and experiments on do mesticated animals and cultivated plants, and having reflected profoundly on those problems in geology and biology which were calculated to throw most light on that question For eighteen years these researches had all been pointing to the same conclusion, namely, that the species now living had been derived by variation and generation from those which had pre-existed, and these again from others of still older date. Several of his MS. volumes on this subject had been read by Dr. Hooker as long ago as 1844, and how long the ever accumulating store of facts and reasonings might have remained unknown to the general public, had no one else attempted to work out the same problem, it is impossible to say. But at length Mr. Darwin received a communication, dated February 1858, from Mr. Wallace, then residing at Ter nate in the Malay Archipelago, entitled "On the Tendency of Varieties to depart indefinitely from the Original Type."
The Author requested Mr. Darwin to show this essay to me should he think it sufficiently novel and interesting It was brought to me by Dr. Hooker, who remarked how complete was the coincidence of Mr. Wallace's new views and those contained in one of the chapters of Mr. Darwin's unpublished work. Accordingly, he suggested that it would be unfair to let Mr. Wallace's essay go to press unaccompanied by the older memoir on the same subject. Although, therefore, Mr. Darwin was willing to waive his claim to priority, the two papers were read on the same evening to the Linnaean Society and published in their Proceedings for 1858. The title of the chapter extracted from Mr. Darwin's MS. ran as follows: "On the Tendency of Species to form Varieties, and on the Perpetuation of Species and Varieties by Natural Means of Selection."
Already in the previous year, September 1857, Mr. Darwin had sent to Professor Asa Gray, the celebrated American botanist, a brief sketch of his forthcoming treatise on what he then termed "Natural Selection." This letter, also printed by the Linnaean Society together with the papers above alluded [279] to, contained an outline of the leading features of his theory of selection as since explained, showing how new races were formed by the breeder, and how analogous results might or must occur in nature under changed conditions in the animate and inanimate world. Reference was made in the s;same letter to the law of human population first enunciated by Malthus, or the tendency in man to increase in a geometrical ratio, while the means of subsistence cannot be made to augment in the same ratio. We were reminded that in some countries the human population has doubled in twenty-five years, and would have multiplied faster if food could have been supplied. In like manner every animal and plant is capable of increasing so rapidly, that if it were unchecked by other species, it would soon occupy the greater part of the habitable globe; but in the general struggle for life few only of those which are born into the world can obtain subsistence and arrive at maturity. In any given species those alone survive which have some advantage over others, and this is often determined by a slight peculiarity capable in a severe competition of turning the scale in their favour. Notwithstanding the resemblance to each other and to their parents of all the individuals of the same family, no two of them are exactly alike. The breeder chooses out from among the varieties presented to him those best suited to his purpose, and the divergence from the original stock is more and more increased by breeding in each successive generation from individuals which possess the desired characters in the most marked degree. In this manner Mr. Darwin suggests that as the surrounding conditions in the organic and inorganic world slowly alter in the course of geological periods, new races which are more in harmony with the altered state of things must be formed in a state of nature, and must often supplant the parent type.
Although this law of natural selection constituted one only of the grounds on which Mr. Darwin relied for establishing his views as to the origin of species by variation, yet it formed so original and prominent a part of his theory that the fact of Mr. Wallace having independently thought out the same principle and illustrated it by singularly analogous [280] examples, is remarkable. It raises at the same time a strong presumption in favour of the truth of the doctrine. Both writers referred to the number of the feathered tribe which perish annually. "Very few birds," says Mr. Wallace, "produce less than two young ones each year, while many have six, eight, or ten; and if we suppose that each pair produce young only four times in their life, each would at this rate increase in fifteen years to nearly ten millions, whereas we have no reason to believe that the number of the birds of any country increases at all in fifteen or even in 150 years. It is evident, therefore, that each year an immense number of birds must perish, as many in fact as are born; and as on the lowest calculation the progeny are each year twice as numerous as their parents, it follows that whatever be the average number of individuals existing in any given country, twice that number must perish annually.
"Large broods are superfluous: on the average all above one become food for hawks and kites, wild cats and weazels, or perish of cold and hunger as winter comes on."[18] The most remarkable instance of an immense bird population is that of the passenger pigeon of the United States, "which lays only one or at most two eggs, and is said to rear generally but one young one. Why is this bird so extraordinarily abundant, while others producing two or three times as many young are much less plentiful? The explanation is not difficult. The food most congenial to this species, and on which it thrives best, is abundantly distributed over a very extensive region, offering such differences of soil and climate, that in one part or another of the area the supply never fails. The bird is capable of very rapid and long-continued flight, so that it can pass without fatigue over the whole of the district it inhabits, and as soon as the supply of food begins to fail in one place is able to discover a fresh feeding-ground. This example strikingly shows us that the procuring a constant supply of wholesome food is almost the sole condition requisite for ensuring the rapid increase of a given species, since neither the limited fecundity, nor the unrestrained [281] attacks of birds of prey and of man, are here sufficient to check it."[19]
When pointing out how every variation from the typical form of a species gives an advantage to some individuals Over others, Mr. Wallace shows that even a change of colour, by rendering certain animals more or less distinguishable, affects their safety- He also observes that in a state of nature, a race better fitted for changed conditions would never revert to the form which it had displaced; although in the case of domesticated animals allowed to run wild or become "feral," they must, to a certain extent, recover the character which they had lost during their subjugation to man, for reasons which will be explained in Chapter XXXVII. The essay concluded with some judicious criticisms on Lamarck's notion that animals may by their own efforts promote the development of some of their organs, or even acquire new ones. "Changes," says Mr. Wallace, "have been brought about, not by the volition of the creatures themselves, but by the survival of varieties which had the greatest facilities of obtaining food. The giraffe did not acquire its long neck by desiring to reach the foliage of lofty trees and by constantly stretching out its neck for that purpose, but varieties which occurred with a longer neck than usual had an advantage over their shorter-necked companions, and, on the first scarcity of food, were enabled to survive them.[20]
After the publication of the detached chapter of his book in the Linnaean Proceedings, Mr. Darwin was persuaded by his friends that he ought no longer to withhold from the world the result of his investigations on the nature and origin of species, and his theory of Natural Selection. Great was the sensation produced in the scientific world by the appearance of the abridged and condensed statement of his views comprised in his work entitled "On the Origin of Species by means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life." From the hour of its appearance it gave, as Professor Huxley truly said,"a [282] new direction to biological speculation," for even where it failed to make proselytes, it gave a shock to old and time honoured opinions from which they have never since recovered. It effected this not merely by the manner in which it explained how new races and species might be formed by Natural Selection, but also by showing that, if we assume this principle, much light is thrown on many very distinct and otherwise unconnected classes of phenomena, both in the present condition and past history of the organic world.
Hooker on and selection and the formation of species in the vegetable world.--The abandonment of the old received doctrine of the immutability of species" was accelerated in England by the appearance, in the same year (1859), of Dr. Hooker's essay on the Flora of Australia. In several of his previous writings this eminent botanist had said all that could be said in support of the "constancy of the specific character in the vegetable world." He had been freely discussing for fifteen years with Mr. Darwin, all the facts and arguments which they could bring to bear on this question, but he stated in his Introduction, that until the views of his friend and those of Mr. Wallace in favour of Natural Selection had been made known, he scarcely felt himself at liberty frankly to declare how far, as a botanist, he was prepared to go in the same direction. He had been occupied for more than twenty years in the study of plants of various parts of the world, arctic, temperate, and tropical, insular and continental. He had personally explored the floras of several of these regions, had described and classified thousands of species, and was well known to unite caution with boldness in his philosophical speculations. From his new essay the general public learnt, not without surprise, how little the most experienced botanists are agreed amongst themselves as to the limits of species, and to what an. extent these limits are a mere matter of opinion, even amongst those who believe that species have remained unchanged since their creation, and will remain immutable as long as they continue on the globe. As conspicuous evidence of this he stated that the number of known [283] species of flowering plants is assumed by some to be under 80,000, and by others over 150,000.[21]
Dr. Hooker showed that in proportion as we study the same plant under varied conditions and in distant regions, it becomes more and more difficult to define its precise specific characters; also that in the flora of every country there are some groups of species which are apparently unvarying, others which on the contrary run so much one into another that the whole group may be regarded as a continuous series of varieties between the terms of which no hiatus exists such as might allow of the intercalation of any intermediate variety. The genera Rubus, Rosa, Salis, and Saxifraga afford conspicuous examples of these unstable forms; Veronica, Campanula, and Lobelia of comparatively stable ones. At the same time he points out in accordance with Mr. Darwin's theory how the extinction of a certain number of the intermediate races by destroying the transitional links would facilitate the classification of the remaining species, and hints that we may be indebted to such extinction in past times for whatever facility we now enjoy of resolving plants into distinct species, genera, and orders. "The mutual relations," he observes "of the plants of each groat botanical province, and in fact, of the world generally, are just such as would have resu]ted if variation had gone on operating throughout indefinite periods, in the same manner as we see it act in a limited number of centuries, so as gradually to give rise in the course of time to the most widely divergent forms."
When we reflect that this statement was made after a study of the characters and geographical distribution of tens of thousands of species, we feel disposed at once to declare that a theory which is in harmony with so many facts must be true; but if so, we have to enquire how it happens that so many naturalists, of undoubted ability and knowledge, have always held and still believe that species have been constant from the beginning. In reference to this question, Dr. Hooker admits that species are realities and may be treated as if they were permanent and immutable; for the forms and characters at least of the great majority of them, may be [284] faithfully transmitted through thousands of generations) and may have remained constant within the range of our experience. "But our experience," he remarks, "is so limited that it will not account for a single fact in the present geographical distribution, or origin of any one species of plant no, for the amount of variation it has undergone, nor will it indicate the time- when it first appeared nor the form it had when created."
[22] [*] Charles Lyell (1872), Principles of Geology, or the Modern Changes of the Earth and its Inhabitants Considered as Illustrative of Geology, 11th ed. (NY: D. Appleton and Co.), ch. 35, pp.
[1] Dureau de la Malle, Ann. des Sci. Nat., tom. xxi. p. 53, Sept. 1830.
[2] Cuvier, Discours Prélim., p. 128.
[3] Disc. Prél. p. 129, sixth edition.
[4] Ibid.
[5] Ann. du Museum d'Hist. Nat. tom. i. p. 234. 1802.
[6] Phil. Zool. pp. 70-71.
[7] Phil. Zool. p. 80.
[8] Ibid. chap. iii., De l'Espèce p. 79.
[9] 'Classis et Ordo est sapientiae, Species naturae opus.'
[10] "Gemus omen est naturale, in primordio tale creatum," &c. Phil. Bot. SS159. See also ibid. SS162
[11] "Totidem numeramus species quot in principio formae sunt creatae."
[12] Brocchi, Conch. Foss. Subap., tome i. 1814.
[13] Phil. Zool. tom. i. p. 240, where other examples are also given.
[14] Series 2, vol. xvi. Republished in s "Contributions to the Theory of Selection,' p. 1.
[15] Bate's Preface to his Naturalist on the River Amazons. xvi. p. 186.
[16] Annals of Nat, Hist. ser. 2, vol. xvi, p. 186
[17] Ist ed. p. 355; 4th cd. p. 424.
[18] Journ. of Linnaean Soc., vol. iii. p. 55. 1858. Contributions to Natural Selection, p. 30.
[19] Journ. of Linnaean Soc., vol. iii. P 55. 1868. "Contributions to Natural Selection,' p. 42
[20] Journ. of Linnaean Soc. p. 61. "Contributions to Natural Selection,' p. 42
[21] Flora of Tasmania, p. iii.
[22] Hooker, Flora of Tasmania.