作者：Romanes, George John
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The Scientific Evidences of Organic Evolution试读：
PREFACE.
Several months ago I published in the Fortnightly Review a lecture, which I had previously delivered at the Philosophical Institutions of Edinburgh and Birmingham, and which bore the above title．The late Mr．Darwin thought well of the epitome of his doctrine which the lecture presented, and urged me so strongly to republish it in a form which might admit of its being “spread broadcast over the land”, that I promised him to do so．In fulfilment of this promise, therefore—which I now regard as more binding than ever—I reproduce the essay in the “Nature Series”with such additions and alterations as appear to me, on second thoughts, to be desirable．The only object of the essay is that which is expressed in the opening paragraph.London, June 1, 1882.
Since this little Essay was published, it has been suggested to me that, in its mode of presenting the arguments in favour of Evolution, there is a similarity to that which has been adopted by Mr．Herbert Spencer in the third part of his Principles of Biology．I should therefore like to state, that while such similarity is no doubt in part due to the similarity of subject-matter, I think, upon reading again, after an interval of ten years, his admirable presentation of the evidence it may also in part be due to unconscious memory．This applies particularly to the headings of the chapters, which I find to be almost identical with those previously used by Mr．Spencer.G．J．R.INTRODUCTIONTHE SCIENTIFIC EVIDENCES OF ORGANIC EVOLUTION.
Although it is generally recognised that the Origin of Species has produced an effect both on the science and the philosophy of our age which is without a parallel in the history of thought, admirers of Mr．Darwin's genius are frequently surprised at the ignorance of his work which is displayed by many persons who can scarcely be said to belong to the uncultured classes．The reason of this ignorance is no doubt partly due to the busy life which many of our bread-winners are constrained to live; but it is also, I think, partly due to mere indolence．There are thousands of educated persons who, on coming home from their daily work, prefer reading literature of a less scientific character than that which is supplied by Mr．Darwin's works; and therefore it is that such persons feel these works to belong to a category of books which is to them a very large one—the books, namely, which never are, but always to be, read．Under these circumstances I have thought it desirable to supply a short digest of the Origin of Species, which any man, of however busy a life, or of however indolent a disposition, may find both time and energy to follow.
With the general aim of the present abstract being thus understood, I shall start at the beginning of my subject by very briefly describing the theory of natural selection．It is a matter of observable fact that all plants and animals are perpetually engaged in what Mr．Darwin calls a “struggle for existence.”That is to say, in every generation of every species a great many more individuals are born than can possibly survive; so that there is in consequence a perpetual battle for life going on among all the constituent individuals of any given generation．Now, in this struggle for existence, which individuals will be victorious and live? Assuredly those which are best fitted to live: the weakest and the least fitted to live will succumb and die, while the strongest and the best fitted to live will be triumphant and survive．Now it is this “survival of the fittest”that Mr．Darwin calls “natural selection.”Nature, so to speak, selects the best individuals out of each generation to live．And not only so, but as these favoured individuals transmit their favourable qualities to their offspring, according to the fixed laws of heredity, it follows that the individuals composing each successive generation have a general tendency to be better suited to their surroundings than were their forefathers．And this follows, not merely because in every generation it is only the flower of the race that is allowed to breed, but also because if in any generation some new and beneficial qualities happen to appear as slight variations from the ancestral type, these will be seized upon by natural selection and added, by transmission in subsequent generations, to the previously existing type．Thus the best idea of the whole process will be gained by comparing it with the closely analogous process whereby gardeners and cattlebreeders create their wonderful productions; for just as these men, by always selecting their best individuals to breed from, slowly but continuously improve their stock, so Nature, by a similar process of selection, slowly but continuously makes the various species of plants and animals better and better suited to the external conditions of their life.
Now, if this process of continuously adapting organisms to their environment takes place in nature at all, there is no reason why we should set any limits on the extent to which it is able to go up to the point at which a complete and perfect adaptation is achieved．Therefore we might suppose that all species would attain to this condition of perfect adjustment to their environment, and there remain fixed．And so undoubtedly they would, if the environment were itself unchanging．But forasmuch as the environment—or the sum total of the external conditions of life—of almost every organic type alters more or less from century to century (whether from astronomical, geological, and geographical changes, or from the immigrations and emigrations of other species living on contiguous geographical areas), it follows that the process of natural selection need never reach a terminal phase．And forasmuch as natural selection may thus continue, ad infinitum, slowly to alter a specific type in adaptation to a gradually changing environment, if in any case the alteration thus effected is sufficient in amount to lead naturalists to denote the specific type by some different name, it follows that natural selection has transmuted one specific type into another．And so the process is supposed to go on over all the countless species of plants and animals simultaneously—the world of organic types being thus regarded as in a state of perpetual, though gradual, flux.
Such, then, is the theory of natural selection, or survival of the fittest; and the first thing we have to notice with regard to it is, that it offers to our acceptance a scientific explanation of the numberless cases of apparent design which we everywhere meet with in organic nature．For all such cases of apparent design consist only in the adaptation which is shown by organisms to their environment, and it is obvious that the facts are covered by the theory of natural selection no less completely than they are covered by the theory of intelligent design．Perhaps it may be answered,—“The fact that these innumerable cases of adaptation may be accounted for by natural selection is no proof that they are not really due to intelligent design.”And, in truth, this is an objection which is often urged by minds—even highly cultured minds—which have not been accustomed to scientific modes of thought．I have heard an eminent professor tell his class that the many instances of adaptation which Mr．Darwin discovered and described as occurring in orchids, seemed to him to tell more in favour of contrivance than in favour of natural causes; and another eminent professor once wrote to me that although he had read the Origin of Species with care, he could see in it no evidence of natural selection which might not equally well be adduced in favour of intelligent design．But here we meet with a radical misconception of the whole logical attitude of science．For, be it observed, the exception in limine to the evidence which we are about to consider, does not question that natural selection may not be able to do all that Mr．Darwin ascribes to it: it merely objects to his interpretation of the facts, because it maintains that these facts might equally well be ascribed to intelligent design．And so undoubtedly they might, if we were all childish enough to rush into a supernatural explanation whenever a natural explanation is found sufficient to account for the facts．Once admit the glaringly illogical principle that we may assume the operation of higher causes where the operation of lower ones is sufficient to explain the observed phenomena, and all our science and all our philosophy are scattered to the winds．For the law of logic which Sir William Hamilton called the law of parsimony—or the law which forbids us to assume the operation of higher causes when lower ones are found sufficient to explain the observed effects—this law constitutes the only logical barrier between science and superstition．For it is manifest that it is always possible to give a hypothetical explanation of any phenomenon whatever, by referring it immediately to the intelligence of some supernatural agent; so that the only difference between the logic of science and the logic of superstition consists in science recognising a validity in the law of parsimony which superstition disregards．Therefore I have no hesitation in saying that this way of looking at the evidence in favour of natural selection is not a scientific or a reasonable way of looking at it, but a purely superstitious way．Let us take, for instance, as an illustration, a perfectly parallel case．When Kepler was unable to explain by any known causes the paths described by the planets, he resorted to a supernatural explanation, and supposed that every planet was guided in its movements by some presiding angel．But when Newton supplied a beautifully simple physical explanation, all persons with a scientific habit of mind at once abandoned the metaphysical explanation．Now, to be consistent, the above-mentioned professors, and all who think with them, ought still to adhere to Kepler's hypothesis in preference to Newton's explanation; for, excepting the law of parsimony, there is certainly no other logical objection to the statement that the movements of the planets afford as good evidence of the influence of guiding angels as they do of the influence of gravitation.
So much, then, for the absurdly illogical position that, granting the evidence in favour of natural selection and supernatural design to be equal and parallel, we should hesitate for one moment in our choice．But, of course, if the evidence is supposed not to be equal and parallel—i.e., if it is supposed that the theory of natural relation is not so competent a theory to explain the facts of adaptation as is that of intelligent design—then the objection is no longer the one that we are considering．It is quite another objection, and one which is not primâ facie absurd; it requires to be met by examining how far the theory of natural selection is able to explain the facts．Let us state the problem clearly.
Innumerable cases of adaptation of organisms to their environment are the observed facts for which an explanation is required．To supply this explanation two, and only two, hypotheses are in the field．Of these two hypotheses one is, intelligent design manifested in creation; and the other is, natural selection manifested during the countless ages of the past．Now it would be proof positive of intelligent design if it could be shown that all species of plants and animals were created—that is suddenly introduced into the complex conditions of their life; for it is quite inconceivable that any cause other than intelligence could be competent to adapt an organism to its environment suddenly．On the other hand, it would be proof presumptive of natural selection if it could be shown that one species becomes slowly transmuted into another—i.e., that one set of adaptations may be gradually transformed into another set of adaptations according as changing circumstances require．This would be proof presumptive of natural selection, because it would then become amply probable that natural selection might have brought about many, or most, of the cases of adaptations which we see; and if so, the law of parsimony excludes the rival hypothesis of intelligent design．Thus the whole question as between natural selection and supernatural design resolves itself into this—Were all the species of plants and animals separately created, or were they slowly evolved? For if they were specially created, the evidence of supernatural design remains unrefuted and irrefutable; whereas if they were slowly evolved, that evidence has been utterly and for ever destroyed．The doctrine of natural selection therefore depends for its validity on the doctrine of organic evolution; for if once the fact of organic evolution were established, no one would dispute that much of the adaptation was probably effected by natural selection．How much we cannot say—probably never shall be able to say; for even Mr．Darwin himself does not doubt that other causes besides that of natural selection have assisted in the modifying of specific types．For the sake of simplicity, however, I shall not go into this subject; but shall always speak of natural selection as the only cause of organic evolution．Let us, then, weigh the evidence in favour of organic evolution．If we find it wanting, we need have no complaints to make of natural theologians of to-day; but if we find it to be full measure, shaken together and running over, we ought to maintain that natural theologians can no longer adhere to the arguments of such writers as Paley, Bell, and Chalmers, without deliberately violating the only logical principle which separates science from fetishism.
To avoid misapprehension, however, I may here add that while Mr．Darwin's theory is thus in plain and direct contradiction to the theory of design, or system of teleology, as presented by the school of writers which I have named, I hold that Mr．Darwin's theory has no point of logical contact with the theory of design in the larger sense, that behind all secondary causes of a physical kind, there is a primary cause of a mental kind．Therefore throughout this essay I refer to design in the sense understood by the narrower forms of teleology, or as an immediate cause of the observed phenomena．Whether or not there is an ultimate cause of a psychical kind pervading all nature, a causa causarum which is the final raison d'être of the cosmos, this is another question which, as I have said, I take to present no point of logical contact with Mr．Darwin's theory, or, I may add, with any of the methods and results of natural science．The only position, therefore, which I here desire to render plain is that, if the doctrine of evolution is seen to be established by sufficient evidence, and therefore the causes which it sets forth are recognised as adequate to furnish a scientific explanation of the results observed, then the facts of organic nature necessarily fall into the same logical category, with reference to any question of design, as that of all or any other series of facts in the physical universe.
This being understood, I shall now proceed to render an epitome of the evidence in favour of organic evolution, and I shall do so by classifying the arguments in a way tending to show their distinct or independent character, and therefore calculated to display the additional force which they acquire from their cumulative nature.I.THE ARGUMENT FROM CLASSIFICATION.
I shall first take the argument from classification．Naturalists find that all species of plants and animals present among themselves structural affinities．According as these structural affinities are more or less pronounced, the various species are classified under genera, orders, families, classes, sub-kingdoms, and kingdoms．Now in such a classification it is found impossible to place all the species in a linear series, according to the grade of their organization．For instance, we cannot say that a wolf is more highly organized than a fox or a jackal; we can only say that the specific points wherein it differs from these animals are without significance as proving the one type to be more highly organized than the others．But of course in many cases, and especially in the cases of the larger divisions, it is often possible to say—The members in this division are more highly organized than are the members in that division．Our system of classification therefore may be likened to a tree, in which a short trunk may be taken to represent the lowest organisms which cannot properly be termed either plants or animals．This short trunk soon separates into two large trunks, one of which represents the vegetable and the other the animal kingdom．Each of these trunks then gives off large branches signifying classes, and these give off smaller, but more numerous branches, signifying families, which ramify again into orders, genera, and finally into the leaves, which may be taken to represent species．Now, in such a representative tree of life, the height of any branch from the ground may be taken to indicate the grade of organization which the leaves, or species, present; so that, if we picture to ourselves such a tree, we will understand that while there is a general advance of organization from below upwards, there are numberless slight variations in this respect between leaves growing even on the same branch; but in a still greater number of cases, leaves growing on the same branch are growing on the same level—that is, although they represent different species, it cannot be said that one is more highly organized than the other．Now, this tree-like arrangement of specific organisms in nature is an arrangement for which Mr．Darwin is not responsible．I mean that the framing of this natural classification has been the work of naturalists for centuries past; and although they did not know what they were doing, it is now evident to evolutionists that they were tracing the lines of genetic relationship．For, be it observed, a scientific or natural classification differs very much from a popular or hap-hazard classification, and the difference consists in this, that while a popular classification is framed with exclusive reference to the external appearance of organisms, a scientific classification is made with reference to the whole structure．A whale, for instance, is often thought to be a fish, because it resembles a fish in form and habits; whereas dissection shows that it is beyond all comparison more unlike a fish than it is like a horse or a man．This is, of course, an extreme case; but it was cases such as this that first led naturalists to see that there are resemblances between organisms much more deep and important than appear upon the surface; and consequently, that if a natural classification was possible at all, it must be made with reference to these deeper resemblances．Of course, it took time to perceive this distinction between fundamental and superficial resemblances．I remember once reading a very comical disquisition in one of Buffon's works on the question as to whether or not a crocodile was to be classified as an insect; and the instructive feature in the disquisition was this, that although a crocodile differs from an insect as regards every conceivable particular of its internal anatomy, no allusion at all is made to this fact, while the whole discussion is made to turn on the hardness of the external casing of a crocodile resembling the hardness of the external casing of a beetle; and when at last Buffon decides that, on the whole, a crocodile had better not be classified as an insect, the only reason given is, that as a crocodile is so very large an animal, it would make “altogether too terrible an insect.”
But now, when at last it came to be recognised that internal anatomy rather than external appearance was to be taken as a guide to classification, the question was, What features in the internal anatomy are to take precedence over the other features? And this question it was not hard to answer．A porpoise, for instance, has a large number of teeth, and in this feature resembles most fish, while it differs from all mammals．But it also gives suck to its young, and in this feature it differs from all fish, while it resembles all mammals．Now, looking at those two features alone, should we say that a porpoise ought to be classed as a fish or as a mammal? Assuredly as a mammal, and for this reason: The number of teeth is a very variable feature both in fish and in mammals, whereas the giving of suck is an invariable feature among mammals, and occurs nowhere else in the animal kingdom．This, of course, is purposely chosen as a very simple illustration; but it exemplifies the general fact that the guiding principle of scientific classification is the comparing of organism with organism, with the view of seeing which of the constituent organs are of the most invariable occurrence, and therefore of the most typical signification.
Now, since the days of Linnæus this principle has been carefully followed, and it is by its aid that the tree-like system of classification has been established．No one, even long before Darwin's days, ever dreamed of doubting that this system is in reality, what it always has been in name, a natural system．What, then, is the inference we are
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