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THE two articles from the pen of Mr. Herbert Spencer upon 'The Factors of Organic Evolution,' which were published in this Review for the months of April and May 1886, have not, I think, attracted the attention they deserve. They appear to me to mark an epoch in the history of the Darwinian Hypothesis, and in the immense volume of literature which, all over the civilised world, has been concerned with the discussion of it. It will be my object here to explain the significance of the arguments and conclusions of Mr. Herbert Spencer in the articles referred to.

In order to estimate this significance we must go back to the state of speculation and of suggestion upon the subject which existed before the publication of the Origin of Species. It is of course well known, but I do not think it equally well remembered, that Charles Darwin did not start the idea that-somehow and in some way or other-all animal forms have been derived from pre-existing forms by way of natural birth and of ordinary generation. It is true that this idea was not generally accepted. On the contrary, it was generally ridiculed as a vain imagination, and even the few who were inclined to entertain it hardly defined it to themselves, and for the most part thought of it vaguely and in silence. It is, however, important to observe that this general idea, however indeterminate in shape, stands in very close connection with certain other ideas in respect to the actual structure of animal forms, which had come to be firmly established in the very definite science of comparative anatomy. How animal forms came to be as we now see them—this was a highly speculative question. But what these forms actually VOL. XXII.-No. 130.

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are, as compared with each other-this was purely a matter of dissection, of comparison, and of careful observation. Of course in thiskind of observation the mind has to bring with it something more than its mere optic apparatus. It must not only see bones and tissues, but it must think of them in their relations of likeness or of difference with each other in the same animal, and with each other when compared with similar bones and tissues in other animals. Hence come the scientific facts and doctrines of what are called Homologies.' These doctrines were at first regarded as extremely theoretical and almost transcendental, and it is true that in their finest and highest applications they have taxed, and do still tax, some perhaps of the highest intellectual powers. But it is well worthy of note that the principle involved in the doctrine of homologies is not at all difficult, nor is it in any way transcendental. On the contrary, it is universally and instinctively recognised by the rudest and most illiterate of mankind. Nobody however ignorant-no savage evencould fail to recognise the hand of a monkey as the part of that animal's body which corresponds to the hand of a man. But monkeys. graduate as to outward appearance through many kinds, such as the lemurs, into squirrel-like forms, and into ordinary quadrupeds. Therefore, although ordinary quadrupeds do not use their fore limbs, as men and monkeys both do, for the purpose of taking hold of things, but only for the purpose of progression, this difference in use makes no difference in the popular recognition of the correspondence between the fore limbs of all quadrupeds and the human arms. The same kind of recognition extends to all the principal members or organs of the body, both internal and external. The head of all the higher animals corresponds with our head. Their organs of sight, of hearing, and of taste, have all an obvious correspondence which is admitted and expressed in the common use of speech. But this recognition of an obvious fact involves the whole doctrine of homologies. It is a general conception which includes—as a whole includes its parts-every detail of the same kind of correspondence which has yet been discovered by the most laborious research. When we have recognised the fact of a correspondence between the forearms of all quadrupeds and those of all bipeds, there is nothing whatever that is new in principle when we discover further that these forearms, however unlike in shape or different in methods of use, are all made on one fundamental arrangement of corresponding bones. Some of them may be shortened and thickened, as in the mole; others of them may be lengthened and attenuated, as in the deer and all the swifter quadrupeds; some of them may be more widely separated from the rest, as in the bat, whilst others again may be closer packed, or actually glued and stuck together, as in all birds. Some of them may be reduced to mere rudiments, whilst others are so enormously exaggerated as at first sight to escape recognition. But on closer inspection, and on more

careful comparison, that recognition invariably comes. All this is perfectly natural and consistent with the first and more general recognition of correspondence which no anatomy was needed to suggest or to demonstrate. Such further and more detailed correspondences are merely a further following and application of the same general idea.

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Yes! But be it observed that the 'thing' we follow is an ideaa pure idea. The bones and organs in two different animals which we recognise as the same' in one sense, are certainly not the same in another sense. In their visible capacity as objective existences' they are not the same. What then is the sense in which we do recognise them as the same? It is in a purely intellectual sense-a sense in which we mentally recognise an arrangement or structure consisting of separate constituent parts, and so put together as to present to the mind a principle or a plan. The sameness which we see in two separate bones taken from two distinct creatures, is merely a sameness of relation to this principle or plan. Without reference to this plan, whether we are conscious of it or not, the alleged sameness would be pure nonsense.

Now, this plan, although as much a fact of nature as any one of the bones which can be weighed and measured, does not represent or suggest any physical cause to account for it. The physical cause which has produced the plan of structure is a separate question altogether. And there is another question, also quite separate, touching the reason for the structure with reference to the ends it serves. This last question is obviously the highest of all. That is to say, it is the question asked by the highest of our own rational faculties which can be concerned in such investigations. And it is remarkable that this highest question touching the use of an organ is much more easily answered than the lower one touching the origin or the physical cause. If weight has to be supported above the ground, and bodies having weight are to be moved along the ground, it becomes perfectly intelligible to our rational faculties why animals should have a skeleton of jointed bones. In like manner, if external matter must be taken in and assimilated, it is equally intelligible why all animals should have the organs needed for each step of this process-mouths to catch and chew, and stomachs to digest. If the circulating fluids require oxygen, it is similarly intelligible why a special apparatus such as the lung or gills should be supplied for this end. And as all these necessities apply to every animal it becomes the most intelligible of all facts that the special organs for meeting them are identical in principle and in plan among them all, and even in form and pattern among very many. But again, although this explains the end or the result, it does not explain the means. It explains the functions discharged by the several organs, and the similarity of them, but it does not explain

how they have been made, or how they have arisen. Yet this question of How is one which can never be silenced by any amount of satisfaction given to the question Why. It is a question which may indeed be postponed because of its seeming to be so vain and hopeless, or because there is an answer and an explanation which may be silently accepted or assumed as sufficient without much reflection. Thus the question how bone corresponds with bone in two individuals of the same species, in two men, or in two monkeys, or in two dogs, involves exactly the same difficulties as the like correspondence between the bones of separate individuals of separate species. But in the case of the likeness between individuals of the same species the question is, as it were, postponed by an answer which is accepted as sufficient. That answer is summed up in the word 'inheritance.' It is the property of all living things to transmit their own structure to their offspring, which are mere repetitions of themselves. The real causes and nature of this property of reproduction are, indeed, inscrutable to us. But so likewise is the nature and cause of the force of gravitation. In both cases an ultimate and familiar fact is accepted by us as all that we can reach, although not by any means all that we can desire to know. This accordingly is the explanation which satisfies us of the physical causation by means of which sameness of structure arises in all individuals of the same species. They are like each other simply because they are all children of like parents, and it is an axiom that things that are equal to the same thing must be equal to one another. But a moment's consideration will convince us that inheritance does not represent a true physical cause for the first beginnings or ultimate origin of any animal structure, but only for its preservation and continuation in the world, assuming as a fact its previous existence with inherent reproductive powers. It takes us back to no beginning except the beginning of the individual. It says nothing whatever of the beginning of the whole series.

Nothing? Yes-nothing on the assumption that each species is a separate and distinct series by itself. But this assumption is fundamental. If a species does not constitute such a series-if its specific characters are not constant, not immutable, but liable to flux and change-if a rock pigeon may give birth to a stockdove, or to a turtle-dove, or to a ringdove-or if this kind of transition and passage can be brought about gradually by small and insensible changes -then the whole question is altered, and inheritance may represent a true physical cause accounting for the preservation of the distinctions which we are accustomed to think of as constituting species, and accounting also for the likenesses, identities, or homologies of structure which run through different species quite as clearly as through different individuals.

The moment this idea is suggested, it takes firm hold of the

speculative mind. When we really try to face the only alternative idea, that each distinct kind of wild pigeon has been originally a separate creation from inorganic matter, the conception seems to be, if not actually absurd, at least to be very difficult of acceptance. I mention the case of pigeons not at all because of the hideous monstrosities and deformities which man produces in this beautiful family of birds by artificial breeding. Far too much has been made of these. They are not new species, nor are they the least like new species. I mention the pigeons because it is a family of birds widely diffused over the world, with an immense variety of wild species, all seeming to be permanently distinct, and yet all possessing so many characters in common that everybody, however ignorant of ornithology as a science, would in a moment recognise every one of those species as a kind of pigeon. There are other families of birds equally rich in species, and equally distinctive from all other groups-as for example the hawks, falcons, and eagles, the ducks, the seagulls, the humming birds, the parrots, and many others. The specific differences, although perfectly distinct, and, so far as human history extends, permanent, are yet so small in comparison with the general features which are common to them all, that the theory of separate creation for each kind must strike us as increasingly improbable the more we think of it.

But all thinking on such matters must be strictly disciplined; else it will never ripen into knowledge. Thinking about them comes of necessity out of the suggestiveness of things; and the suggestiveness of nature arises out of the fact that our own minds belong to it, and that our faculties have an innate tendency to arrange all external facts according to some rational or intelligible order. But as these faculties are themselves various, so do they concern themselves with various kinds of order. There are three questions which we instinctively ask in respect to all natural things-What, How, and Why. I do not say this is the order in which we always ask them— the order of historical and natural precedence. But it is the order of dignity as regards the faculties concerned. Simple likenesses between creatures with their related differences of form, or of colour, or of habits, are recognised by the simplest of our faculties. These answer to the question What--in what points the creatures we see are alike, and in what they are distinct. Hardly separable in fact, although clearly separable in idea, comes that other kind of order or arrangement which is concerned with function, or with use; and this is an order which concerns the higher question Why, and the faculties which recognise the reason of things, as distinguished from the mere description of them as to form. As a matter of fact and of historical precedence, this is the kind of order which men have earliest observed and thought of. Limbs have been recognised first as those parts of the creature which serve for the use of walking, or of

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