The speculations of the supposit.i.tious Indian sage, though quite as sound as those of many a "Mosaic Geology" which sells exceedingly well, have no great value if we consider them by the light of modern science.
The waters are supposed to have originally covered up the whole globe; to have deposited the rocky ma.s.ses which compose its mountains by processes comparable to those which are now forming mud, sand, and shingle; and then to have gradually lowered their level, leaving the spoils of the animal and vegetable inhabitants embedded in the strata.
As the dry land appeared, certain of the aquatic animals are supposed to have taken to it, and to have become gradually adapted to terrestrial and aerial modes of existence. But if we regard the general tenor and style of the reasoning in relation to the state of knowledge of the day, two circ.u.mstances appear very well worthy of remark. The first, that De Maillet had a notion of the modifiability of living forms (though without any precise information on the subject), and how such modifiability might account for the origin of species; the second, that he very clearly apprehended the great modern geological doctrine, so strongly insisted upon by Hutton, and so ably and comprehensively expounded by Lyell, that we must look to existing causes for the explanation of past geological events. The following pa.s.sage of the preface indeed, in which De Maillet is supposed to speak of the Indian philosopher Telliamed, his _alter ego_, might have been written by the most philosophical uniformitarian of the present day.
"Ce qu"il y a d"etonnant, est que pour arriver a ces connoissances il semble avoir perverti l"ordre naturel, puisqu"au lieu de s"attacher d"abord a rechercher l"origine de notre globe il a commence par travailler a s"instruire de la nature. Mais a l"entendre, ce renvers.e.m.e.nt de l"ordre a ete pour lui l"effet d"un genie favorable qui l"a conduit pas a pas et comme par la main aux decouvertes les plus sublimes. C"est en decomposant la substance de ce globe par une anatomie exacte de toutes ses parties qu"il a premierement appris de quelles matieres il etait compose et quels arrangemens ces memes matieres observaient entre elles. Ces lumieres jointes a l"esprit de comparaison toujours necessaire a quiconque entreprend de percer les voiles dont la nature aime a se cacher, ont servi de guide a notre philosophe pour parvenir a des connoissances plus interessantes. Par la matiere et l"arrangement de ces compositions il pretend avoir reconnu quelle est la veritable origine de ce globe que nous habitons, comment et par qui il a ete forme."--(Pp. xix. xx.)
But De Maillet was before his age, and as could hardly fail to happen to one who speculated on a zoological and botanical question before Linnaeus, and on a physiological problem before Haller, he fell into great errors here and there; and hence, perhaps, the general neglect of his work. Robinet"s speculations are rather behind than in advance of those of De Maillet, and though Linnaeus may have played with the hypothesis of trans.m.u.tation, it obtained no serious support until Lamarck adopted it, and advocated it with great ability in his "Philosophie Zoologique."
Impelled towards the hypothesis of the trans.m.u.tation of species, partly by his general cosmological and geological views; partly by the conception of a graduated, though irregularly branching scale of being, which had arisen out of his profound study of plants and of the lower forms of animal life, Lamarck, whose general line of thought often closely resembles that of De Maillet, made a great advance upon the crude and merely speculative manner in which that writer deals with the question of the origin of living beings, by endeavouring to find physical causes competent to effect that change of one species into another which De Maillet had only supposed to occur. And Lamarck conceived that he had found in nature such causes, amply sufficient for the purpose in view. It is a physiological fact, he says, that organs are increased in size by action, atrophied by inaction; it is another physiological fact that modifications produced are transmissible to offspring. Change the actions of an animal, therefore, and you will change its structure, by increasing the development of the parts newly brought into use and by the diminution of those less used; but by altering the circ.u.mstances which surround it you will alter its actions, and hence, in the long run, change of circ.u.mstance must produce change of organization. All the species of animals, therefore, are in Lamarck"s view the result of the indirect action of changes of circ.u.mstance upon those primitive germs which he considered to have originally arisen, by spontaneous generation, within the waters of the globe. It is curious, however, that Lamarck should insist so strongly[64] as he has done, that circ.u.mstances never in any degree directly modify the form or the organization of animals, but only operate by changing their wants, and consequently their actions; for he thereby brings upon himself the obvious question, how, then, do plants, which cannot be said to have wants or actions, become modified? To this he replies, that they are modified by the changes in their nutritive processes, which are effected by changing circ.u.mstances; and it does not seem to have occurred to him that such changes might be as well supposed to take place among animals.
When we have said that Lamarck felt that mere speculation was not the way to arrive at the origin of species, but that it was necessary in order to the establishment of any sound theory on the subject, to discover by observation or otherwise, some _vera causa_, competent to give rise to them; that he affirmed the true order of cla.s.sification to coincide with the order of their development one from another; that he insisted on the necessity of allowing sufficient time, very strongly; and that all the varieties of instinct and reason were traced back by him to the same cause as that which has given rise to species, we have enumerated his chief contributions to the advance of the question. On the other hand, from his ignorance of any power in nature competent to modify the structure of animals, except the development of parts, or atrophy of them, in consequence of a change of needs, Lamarck was led to attach infinitely greater weight than it deserves to this agency, and the absurdities into which he was led have met with deserved condemnation. Of the struggle for existence, on which as we shall see Mr. Darwin lays such great stress, he had no conception; indeed, he doubts whether there really are such things as extinct species, unless they be such large animals as may have met their death at the hands of man; and so little does he dream of there being any other destructive causes at work, that, in discussing the possible existence of fossil sh.e.l.ls, he asks, "Pourquoi d"ailleurs seroient-ils perdues des que l"homme n"a pu operer leur destruction?" ("Phil. Zool.," vol. i. p. 77).
Of the influence of selection Lamarck has as little notion, and he makes no use of the wonderful phenomena which are exhibited by domesticated animals, and ill.u.s.trate its powers. The vast influence of Cuvier was employed against the Lamarckian views, and as the untenability of some of his conclusions was easily shown, his doctrines sank under the opprobrium of scientific as well as of theological heterodoxy. Nor have the efforts made of late years to revive them, tended to re-establish their credit in the minds of sound thinkers acquainted with the facts of the case; indeed it may be doubted whether Lamarck has not suffered more from his friends than from his foes.
Two years ago, in fact, though we venture to question if even the strongest supporters of the special creation hypothesis had not, now and then, an uneasy consciousness that all was not right, their position seemed more impregnable than ever, if not by its own inherent strength, at any rate by the obvious failure of all the attempts which had been made to carry it. On the other hand, however much the few, who thought deeply on the question of species, might be repelled by the generally received dogmas, they saw no way of escaping from them, save by the adoption of suppositions, so little justified by experiment or by observation, as to be at least equally distasteful; The choice lay between two absurdities and a middle condition of uneasy scepticism; which last, however unpleasant and unsatisfactory, was obviously the only justifiable state of mind under the circ.u.mstances.
Such being the general ferment in the minds of naturalists, it is no wonder that they mustered strong in the rooms of the Linnaean Society, on the first of July of the year 1858, to hear two papers by authors living on opposite sides of the globe, working out their results independently, and yet professing to have discovered one and the same solution of all the problems connected with species. The one of these authors was an able naturalist, Mr. Wallace, who had been employed for some years in studying the productions of the islands of the Indian Archipelago, and who had forwarded a memoir embodying his views to Mr. Darwin for communication to the Linnaean Society. On perusing the essay Mr. Darwin was not a little surprised to find that it embodied some of the leading ideas of a great work which he had been preparing for twenty years, and parts of which, containing a development of the very same views, had been perused by his private friends fifteen or sixteen years before.
Perplexed in what manner to do full justice both to his friend and to himself, Mr. Darwin placed the matter in the hands of Dr. Hooker and Sir Charles Lyell, by whose advice he communicated a brief abstract of his own views to the Linnaean Society, at the same time that Mr. Wallace"s paper was read. Of that abstract, the work on the "Origin of Species" is an enlargement, but a complete statement of Mr. Darwin"s doctrine is looked for in the large and well-ill.u.s.trated work which he is said to be preparing for publication.[65]
The Darwinian hypothesis has the merit of being eminently simple and comprehensible in principle, and its essential positions may be stated in a very few words: all species have been produced by the development of varieties from common stocks, by the conversion of these, first into permanent races and then into new species, by the process of _natural selection_, which process is essentially identical with that artificial selection by which man has originated the races of domestic animals--the _struggle for existence_ taking the place of man, and exerting, in the case of natural selection, that selective action which he performs in artificial selection.
The evidence brought forward by Mr. Darwin in support of his hypothesis is of three kinds. First, he endeavours to prove that species may be originated by selection; secondly, he attempts to show that natural causes are competent to exert selection; and thirdly, he tries to prove that the most remarkable and apparently anomalous phenomena exhibited by the distribution, development, and mutual relations of species, can be shown to be deducible from the general doctrine of their origin, which he propounds, combined with the known facts of geological change; and that, even if not all these phenomena are at present explicable by it, none are necessarily inconsistent with it.
There cannot be a doubt that the method of inquiry which Mr. Darwin has adopted is not only rigorously in accordance with the canons of scientific logic, but that it is the only adequate method. Critics exclusively trained in cla.s.sics or in mathematics, who have never determined a scientific fact in their lives by induction from experiment or observation, prate learnedly about Mr. Darwin"s method, which is not inductive enough, not Baconian enough, forsooth, for them. But even if practical acquaintance with the process of scientific investigation is denied them, they may learn, by the perusal of Mr. Mill"s admirable chapter "On the Deductive Method," that there are mult.i.tudes of scientific inquiries, in which the method of pure induction helps the investigator but a very little way.
"The mode of investigation" (says Mr. Mill) "which from the proved inapplicability of direct methods of observation and experiment remains to us as the main source of the knowledge we possess, or can acquire, respecting the conditions and laws of recurrence of the more complex phenomena, is called, in its most general expression, the deductive method, and consists of three operations: the first, one of direct induction; the second, of ratiocination; and the third, of verification."
Now, the conditions which have determined the existence of species are not only exceedingly complex, but, so far as the great majority of them are concerned, are necessarily beyond our cognisance. But what Mr.
Darwin has attempted to do is in exact accordance with the rule laid down by Mr. Mill; he has endeavoured to determine certain great facts inductively, by observation and experiment; he has then reasoned from the data thus furnished; and lastly, he has tested the validity of his ratiocination by comparing his deductions with the observed facts of nature. Inductively, Mr. Darwin endeavours to prove that species arise in a given way. Deductively, he desires to show that, if they arise in that way, the facts of distribution, development, cla.s.sification, &c., may be accounted for, _i.e._ may be deduced from their mode of origin, combined with admitted changes in physical geography and climate, during an indefinite period. And this explanation, or coincidence of observed with deduced facts, is, so far as it extends, a verification of the Darwinian view.
There is no fault to be found with Mr. Darwin"s method, then; but it is another question whether he has fulfilled all the conditions imposed by that method. Is it satisfactorily proved, in fact, that species may be originated by selection? that there is such a thing as natural selection? that none of the phenomena exhibited by species are inconsistent with the origin of species in this way? If these questions can be answered in the affirmative, Mr. Darwin"s view steps out of the ranks of hypotheses into those of proved theories; but so long as the evidence at present adduced falls short of enforcing that affirmation, so long, to our minds, must the new doctrine be content to remain among the former--an extremely valuable, and in the highest degree probable, doctrine, indeed the only extant hypothesis which is worth anything in a scientific point of view; but still a hypothesis, and not yet the theory of species.
After much consideration, and with a.s.suredly no bias against Mr.
Darwin"s views, it is our clear conviction that, as the evidence stands, it is not absolutely proven that a group of animals, having all the characters exhibited by species in nature, has ever been originated by selection, whether artificial or natural. Groups having the morphological character of species, distinct and permanent races in fact, have been so produced over and over again; but there is no positive evidence at present that any group of animals has, by variation and selective breeding, given rise to another group which was even in the least degree infertile with the first. Mr. Darwin is perfectly aware of this weak point, and brings forward a mult.i.tude of ingenious and important arguments to diminish the force of the objection. We admit the value of these arguments to their fullest extent; nay, we will go so far as to express our belief that experiments, conducted by a skilful physiologist, would very probably obtain the desired production of mutually more or less infertile breeds from a common stock, in a comparatively few years; but still, as the case stands at present, this "little rift within the lute" is not to be disguised nor overlooked.
In the remainder of Mr. Darwin"s argument our own private ingenuity has not hitherto enabled us to pick holes of any great importance; and judging by what we hear and read, other adventurers in the same field do not seem to have been much more fortunate. It has been urged, for instance, that in his chapters on the struggle for existence and on natural selection, Mr. Darwin does not so much prove that natural selection does occur, as that it must occur; but, in fact, no other sort of demonstration is attainable. A race does not attract our attention in nature until it has, in all probability, existed for a considerable time, and then it is too late to inquire into the conditions of its origin. Again, it is said that there is no real a.n.a.logy between the selection which takes place under domestication, by human influence, and any operation which can be effected by nature, for man interferes intelligently. Reduced to its elements, this argument implies that an effect produced with trouble by an intelligent agent must, _a fortiori_ be more troublesome, if not impossible, to an unintelligent agent. Even putting aside the question whether nature, acting as she does according to definite and invariable laws, can be rightly called an unintelligent agent, such a position as this is wholly untenable. Mix salt and sand, and it shall puzzle the wisest of men with his mere natural appliances to separate all the grains of sand from all the grains of salt; but a shower of rain will effect the same object in ten minutes. And so while man may find it tax all his intelligence to separate any variety which arises, and to breed selectively from it, the destructive agencies incessantly at work in nature, if they find one variety to be more soluble in circ.u.mstances than the other, will inevitably in the long run eliminate it.
A frequent and a just objection to the Lamarckian hypothesis of the trans.m.u.tation of species is based upon the absence of transitional forms between many species. But against the Darwinian hypothesis this argument has no force. Indeed, one of the most valuable and suggestive parts of Mr. Darwin"s work is that in which he proves, that the frequent absence of transitions is a necessary consequence of his doctrine, and that the stock whence two or more species have sprung, need in no respect be intermediate between these species. If any two species have arisen from a common stock in the same way as the carrier and the pouter, say, have arisen from the rock-pigeon, then the common stock of these two species need be no more intermediate between the two than the rock-pigeon is between the carrier and pouter. Clearly appreciate the force of this a.n.a.logy, and all the arguments against the origin of species by selection, based on the absence of transitional forms, fall to the ground. And Mr. Darwin"s position might, we think, have been even stronger than it is if he had not embarra.s.sed himself with the aphorism, "_Natura non facit saltum_," which turns up so often in his pages. We believe, as we have said above, that nature does make jumps now and then, and a recognition of the fact is of no small importance in disposing of many minor objections to the doctrine of trans.m.u.tation.
But we must pause. The discussion of Mr. Darwin"s arguments in detail would lead us far beyond the limits within which we proposed, at starting, to confine this article. Our object has been attained if we have given an intelligible, however brief, account of the established facts connected with species, and of the relation of the explanation of those facts offered by Mr. Darwin to the theoretical views held by his predecessors and his contemporaries, and, above all, to the requirements of scientific logic. We have ventured to point out that it does not, as yet, satisfy all those requirements; but we do not hesitate to a.s.sert that it is as superior to any preceding or contemporary hypothesis, in the extent of observational and experimental basis on which it rests, in its rigorously scientific method, and in its power of explaining biological phenomena, as was the hypothesis of Copernicus to the speculations of Ptolemy. But the planetary orbits turned out to be not quite circular after all, and grand as was the service Copernicus rendered to science, Kepler and Newton had to come after him. What if the orbit of Darwinism should be a little too circular? what if species should offer residual phenomena here and there, not explicable by natural selection? Twenty years hence naturalists may be in a position to say whether this is, or is not, the case; but in either event they will owe the author of "The Origin of Species" an immense debt of grat.i.tude. We should leave a very wrong impression on the reader"s mind if we permitted him to suppose that the value of that work depends wholly on the ultimate justification of the theoretical views which it contains. On the contrary, if they were disproved to-morrow, the book would still be the best of its kind--the most compendious statement of well-sifted facts bearing on the doctrine of species that has ever appeared. The chapters on Variation, on the Struggle for Existence, on Instinct, on Hybridism, on the Imperfection of the Geological Record, on Geographical Distribution, have not only no equals, but, so far as our knowledge goes, no compet.i.tors, within the range of biological literature. And viewed as a whole, we do not believe that, since the publication of Von Baer"s Researches on Development, thirty years ago, any work has appeared calculated to exert so large an influence, not only on the future of Biology, but in extending the domination of Science over regions of thought into which she has, as yet, hardly penetrated.
FOOTNOTES:
[62] "On the Osteology of the Chimpanzees and Orangs." Transactions of the Zoological Society, 1858.
[63] Colonel Humphreys" statements are exceedingly explicit on this point:--"When an Ancon ewe is impregnated by a common ram the increase resembles wholly either the ewe or the ram. The increase of the common ewe impregnated by an Ancon ram follows entirely the one or the other, without blending any of the distinguishing and essential peculiarities of both. Frequent instances have happened where common ewes have had twins by Ancon rams, when one exhibited the complete marks and features of the ewe, the other of the ram. The contrast has been rendered singularly striking, when one short-legged and one long-legged lamb, produced at a birth, have been seen sucking the dam at the same time."--Philosophical Transactions, 1813, Pt. I. pp. 89, 90.
[64] See Phil. Zoologique, vol. i. p. 222, _et seq._
[65] The reader will remember that Huxley was writing in 1860.
XIV
THE DARWINIAN HYPOTHESIS.
DARWIN ON THE ORIGIN OF SPECIES
There is a growing immensity in the speculations of science to which no human thing or thought at this day is comparable. Apart from the results which science brings us home and securely harvests, there is an expansive force and lat.i.tude in its tentative efforts, which lifts us out of ourselves and transfigures our mortality. We may have a preference for moral themes, like the Homeric sage, who had seen and known much:--
"Cities of men And manners, climates, councils, governments;"
yet we must end by confessing that
"The windy ways of men Are but dust which rises up And is lightly laid again,"
in comparison with the work of nature, to which science testifies, but which has no boundaries in time or s.p.a.ce to which science can approximate.
There is something altogether out of the reach of science, and yet the compa.s.s of science is practically illimitable. Hence it is that from time to time we are startled and perplexed by theories which have no parallel in the contracted moral world; for the generalizations of science sweep on in ever-widening circles, and more aspiring flights, though a limitless creation. While astronomy, with its telescope, ranges beyond the known stars, and physiology, with its microscope, is subdividing infinite minutiae, we may expect that our historic centuries may be treated as inadequate counters in the history of the planet on which we are placed. We must expect new conceptions of the nature and relations of its denizens, as science acquires the materials for fresh generalizations; nor have we occasion for alarms if a highly advanced knowledge, like that of the eminent Naturalist before us, confronts us with an hypothesis as vast as it is novel. This hypothesis may or may not be sustainable hereafter; it may give way to something else, and higher science may reverse what science has here built up with so much skill and patience, but its sufficiency must be tried by the tests of science _alone_, if we are to maintain our position as the heirs of Bacon and the acquitters of Galileo. We must weigh this hypothesis strictly in the controversy which is coming, by the only tests which are appropriate, and by no others whatsoever.
The hypothesis to which we point, and of which the present work of Mr.
Darwin is but the preliminary outline, may be stated in his own language as follows:--"_Species originated by means of natural selection, or through the preservation of the favoured races in the struggle for life_." To render this thesis intelligible, it is necessary to interpret its terms. In the first place, what is a species? The question is a simple one, but the right answer to it is hard to find, even if we appeal to those who should know most about it. It is all those animals or plants which have descended from a single pair of parents; it is the smallest distinctly definable group of living organisms; it is an eternal and immutable ent.i.ty; it is a mere abstraction of the human intellect having no existence in nature. Such are a few of the significations attached to this simple word which may be culled from authoritative sources; and if, leaving terms and theoretical subtleties aside, we turn to facts and endeavour to gather a meaning for ourselves, by studying the things to which, in practice, the name of species is applied, it profits us little. For practice varies as much as theory.
Let the botanist or the zoologist examine and describe the productions of a country, and one will pretty certainly disagree with the other as to the number, limits, and definitions of the species into which he groups the very same things. In these islands we are in the habit of regarding mankind as of one species, but a fortnight"s steam will land us in a country where divines and savans, for once in agreement, vie with one another in loudness of a.s.sertion, if not in cogency of proof, that men are of different species; and, more particularly, that the species negro is so distinct from our own that the Ten Commandments have actually no reference to him. Even in the calm region of entomology, where, if anywhere in this sinful world, pa.s.sion and prejudice should fail to stir the mind, one learned coleopterist will fill ten attractive volumes with descriptions of species of beetles, nine-tenths of which are immediately declared by his brother beetle-mongers to be no species at all.
The truth is that the number of distinguishable living creatures almost surpa.s.ses imagination. At least a hundred thousand such kinds of insects alone have been described and may be identified in collections, and the number of separable kinds of living things is under estimated at half a million. Seeing that most of these obvious kinds have their accidental varieties, and that they often shade into others by imperceptible degrees, it may well be imagined that the task of distinguishing between what is permanent and what fleeting, what is a species and what a mere variety, is sufficiently formidable.
But is it not possible to apply a test whereby a true species may be known from a mere variety? Is there no criterion of species? Great authorities affirm that there is--that the unions of members of the same species are always fertile, while those of distinct species are either sterile, or their offspring, called hybrids, are so. It is affirmed not only that this is an experimental fact, but that it is a provision for the preservation of the purity of species. Such a criterion as this would be invaluable; but, unfortunately, not only is it not obvious how to apply it in the great majority of cases in which its aid is needed, but its general validity is stoutly denied. The Hon. and Rev. Mr.
Herbert, a most trustworthy authority, not only a.s.serts as the result of his own observations and experiments that many hybrids are quite as fertile as the parent species, but he goes so far as to a.s.sert that the particular plant _Crinum capense_ is much more fertile when crossed by a distinct species than when fertilised by its proper pollen! On the other hand the famous Gaertner, though he took the greatest pains to cross the primrose and cowslip, succeeded only once or twice in several years; and yet it is a well-established fact that the primrose and the cowslip are only varieties of the same kind of plant. Again, such cases as the following are well established. The female of species A if crossed with the male of species B is fertile, but if the female of B is crossed with the male of A, she remains barren. Facts of this kind destroy the value of the supposed criterion.
If, weary of the endless difficulties involved in the determination of species, the investigator, contenting himself with the rough practical distinction of separable kinds, endeavours to study them as they occur in nature--to ascertain their relations to the conditions which surround them, their mutual harmonies and discordances of structure, the bond of union of their parts and their past history, he finds himself, according to the received notions, in a mighty maze, and with, at most, the dimmest adumbration of a plan. If he starts with any one clear conviction, it is that every part of a living creature is cunningly adapted to some special use in its life. Has not his Paley told him that that seemingly useless organ, the spleen, is beautifully adjusted as so much packing between the other organs? And yet, at the outset of his studies, he finds that no adaptive reason whatsoever can be given for one-half of the peculiarities of vegetable structure; he also discovers rudimentary teeth, which are never used, in the gums of the young calf and in those of the foetal whale; insects which never bite have rudimental jaws, and others which never fly have rudimental wings; naturally blind creatures have rudimental eyes; and the halt have rudimentary limbs. So, again, no animal or plant puts on its perfect form at once, but all have to start from the same point, however various the course which each has to pursue. Not only men and horses, and cats and dogs, lobsters and beetles, periwinkles and mussels, but even the very sponges and animalcules commence their existence under forms which are essentially undistinguishable; and this is true of all the infinite variety of plants. Nay, more, all living beings march side by side along the high road of development, and separate the later the more like they are; like people leaving church, who all go down the aisle, but having reached the door some turn into the parsonage, others go down the village, and others part only in the next parish. A man in his development runs for a little while parallel with, though never pa.s.sing through, the form of the meanest worm, then travels for a s.p.a.ce beside the fish, then journeys along with the bird and the reptile for his fellow travellers; and only at last, after a brief companionship with the highest of the four-footed and four-handed world, rises into the dignity of pure manhood. No competent thinker of the present day dreams of explaining these indubitable facts by the notion of the existence of unknown and undiscoverable adaptations to purpose. And we would remind those who, ignorant of the facts, must be moved by authority, that no one has a.s.serted the incompetence of the doctrine of final causes, in its application to physiology and anatomy, more strongly than our own eminent anatomist, Professor Owen, who, speaking of such cases, says (_On the Nature of Limbs_, pp. 39, 40): "I think it will be obvious that the principle of final adaptations fails to satisfy all the conditions of the problem."
But, if the doctrine of final causes will not help us to comprehend the anomalies of living structure, the principle of adaptation must surely lead us to understand why certain living beings are found in certain regions of the world and not in others. The palm, as we know, will not grow in our climate, nor the oak in Greenland. The white bear cannot live where the tiger thrives, nor _vice versa_, and the more the natural habits of animal and vegetable species are examined, the more do they seem, on the whole, limited to particular provinces. But when we look into the facts established by the study of the geographical distribution of animals and plants it seems utterly hopeless to attempt to understand the strange and apparently capricious relations which they exhibit. One would be inclined to suppose _a priori_ that every country must be naturally peopled by those animals that are fittest to live and thrive in it. And yet how, on this hypothesis, are we to account for the absence of cattle in the Pampas of South America when those parts of the New World were discovered? It is not that they were unfit for cattle, for millions of cattle now run wild there; and the like holds good of Australia and New Zealand. It is a curious circ.u.mstance, in fact, that the animals and plants of the Northern Hemisphere are not only as well adapted to live in the Southern Hemisphere as its own autochthones, but are in many cases absolutely better adapted, and so overrun and extirpate the aborigines. Clearly, therefore, the species which naturally inhabit a country are not necessarily the best adapted to its climate and other conditions. The inhabitants of islands are often distinct from any other known species of animal or plants (witness our recent examples from the work of Sir Emerson Tennent, on Ceylon), and yet they have almost always a sort of general family resemblance to the animals and plants of the nearest mainland. On the other hand, there is hardly a species of fish, sh.e.l.l, or crab common to the opposite sides of the narrow isthmus of Panama. Wherever we look, then, living nature offers us riddles of difficult solution, if we suppose that what we see is all that can be known of it.
But our knowledge of life is not confined to the existing world.
Whatever their minor differences, geologists are agreed as to the vast thickness of the acc.u.mulated strata which compose the visible part of our earth, and the inconceivable immensity of the time of whose lapse they are the imperfect, but the only accessible witnesses. Now, throughout the greater part of this long series of stratified rocks are scattered, sometimes very abundantly, mult.i.tudes of organic remains, the fossilised exuviae of animals and plants which lived and died while the mud of which the rocks are formed was yet soft ooze, and could receive and bury them. It would be a great error to suppose that these organic remains were fragmentary relics. Our museums exhibit fossil sh.e.l.ls of immeasurable antiquity, as perfect as the day they were formed, whole skeletons without a limb disturbed--nay, the changed flesh, the developing embryos, and even the very footsteps of primaeval organisms.
Thus the naturalist finds in the bowels of the earth species as well defined as, and in some groups of animals more numerous than, those that breathe the upper air. But, singularly enough, the majority of these entombed species are wholly distinct from those that now live. Nor is this unlikeness without its rule and order. As a broad fact, the further we go back in time the less the buried species are like existing forms; and the further apart the sets of extinct creatures are the less they are like one another. In other words, there has been a regular succession of living beings, each younger set being in a very broad and general sense somewhat more like those which now live.
It was once supposed that this succession had been the result of vast successive catastrophes, destructions, and re-creations _en ma.s.se_; but catastrophes are now almost eliminated from geological, or at least paleontological speculation; and it is admitted on all hands that the seeming breaks in the chain of being are not absolute, but only relative to our imperfect knowledge; that species have replaced species, not in a.s.semblages, but one by one; and that, if it were possible to have all the phenomena of the past presented to us, the convenient epochs and formations of the geologist, though having a certain distinctness, would fade into one another with limits as undefinable as those of the distinct and yet separable colours of the solar spectrum.
Such is a brief summary of the main truths which have been established concerning species. Are these truths ultimate and irresolvable facts, or are their complexities and perplexities the mere expressions of a higher law?
A large number of persons practically a.s.sume the former position to be correct. They believe that the writer of the Pentateuch was empowered and commissioned to teach us scientific as well as other truth, that the account we find there of the creation of living things is simply and literally correct, and that anything which seems to contradict it is, by the nature of the case, false. All the phenomena which have been detailed are, on this view, the immediate product of a creative fiat and consequently are out of the domain of science altogether.
Whether this view prove ultimately to be true or false, it is, at any rate, not at present supported by what is commonly regarded as logical proof, even if it be capable of discussion by reason; and hence we consider ourselves at liberty to pa.s.s it by, and to turn to those views which profess to rest on a scientific basis only, and therefore admit of being argued to their consequences. And we do this with the less hesitation as it so happens that those persons who are practically conversant with the facts of the case (plainly a considerable advantage) have always thought fit to range themselves under the latter category.