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The Relations Between Zoology and Palæontology

Nature volume 5, pages 3435 | Download Citation



MY distinguished predecessor, the late Prof. E. Forbes, appears to have been the first who undertook the systematic study of marine zoology with reference to the distribution of marine animals in space and in time. After making himself well acquainted with the fauna of the British seas to the depth of about 200 fathoms by dredging, and by enlisting the active co-operation of many friends, among whom we find MacAndrew, Barlee, Gwyn Jeffreys, William Thompson, and many others, entering enthusiastically into the new field of natural history inquiry; in the year 1841, Forbes joined Captain Graves, who was at that time in command of the Mediterranean Survey as naturalist. During about eighteen months he studied writh the utmost care the conditions of the Ægean and its shores, and conducted upwards of 100 dredging operations at depths varying from 1 to 130 fathoms. In 1843 he communicated to the Cork meeting of the British Association an elaborate report on the mollasca and radiata of the Ægen Sea, and on their distribution as bearing on geology. Three years later, in 1846, he published in the first volume of the “Memoirs of the Geological Survey of Great Britain,” a most valuable memoir upon the connection between the existing Fauna and Flora of the British Isles and the geological changes which have affected their area, especially during the epoch of the northern drift. In the year 1859 appeared the “Natural History of the European Seas,” by the late Prof. Edward Forbes, edited and continued by Robert Godwin-Austen. In the first hundred pages of this little book Forbes gives a general outline of some of the more important of his views with regard to the distribution of marine forms. The remainder of the book is a continuation by his friend Mr. Godwin-Austen, for before it was finished an early death had cut short the career of the most accomplished and original naturalist of his time. I will give a brief sketch of the general result to which Forbes was led by his labours, and I shall have to point out that, although we are now inclined to look somewhat differently on certain very fundamental points, and, although recent investigations with better appliances and more extended experience have invalidated many of his conclusions, to Forbes is due the credit of having been the first to treat these questions in a broad philosophical sense, and to point out that the only means of acquiring a true knowledge of the rationale of the distiibution of our present fauna is to make ourselves acquainted with its history, to connect the present with the past. This is the direction which must be taken by future inquiry:— Forbes as a pioneer in this line of research was scarcely in a position to appreciate the full value of his work. Every year adds enormously to our stock of data, and every new fact indicates more and more clearly the brilliant results which are to be obtained by following his methods, and by emulating his enthusiasm and his indefatigable industry. Forbes believed implicitly, along with nearly all the leading naturalists of his time, in the immutability of species. He says:—“Every true species presents in its individuals certain features, specific characters, which distinguish it from every other species: as if the Creator had set an exclusive mark or seal on each type.” He likewise believed in specific centres of distribution. He held that all the individuals composing a species had descended from a single progenitor, or from two, according as the sexes might be united or distinct, and that, consequently, the idea of a species involved the idea of the relationship in all the individuals of common descent; and rhe converse, that there could by no possibility be community of descent except in living beings which possessed the same specific characters. He supposed that the original individual or pair was created at a particular spot where the conditions were suitable for its existence and propagation, and that the species extended and migrated from that spot on all sides, over an area of greater or less extent, until it met with some natural barrier in the shape of unsuitable conditions. No specific form could have more than a single cencre of distribution. If its area appeared to be broken up, a patch not in connection with the original centre of distribution occurring in some distant locality, it was accounted for by the formation, through some geological change, after the first spread of the species, of a barrier which cut off part of its area, or by some accidental transport to a place where the conditions were sufficiently similar to those of its original habitat to enable it to become naturalised. No species once exterminated was ever re-created, so that in those few cases in which we find a species abundant at one period over an area, absent over the same area for a time, and recurring at a later period, it must be accounted for by a change in the conditions of the area which forced the emigration of the species, and a subsequent further change which permitted its return. Forbes defined and advocated what he called the law of “representation.” He found that in all parts of the world, however far removed, and however completely separated by natural barriers, where the conditions of life are similar, species, and groups of species, occur, which, although not identical, resemble one another very closely; and he found that this similarity existed likewise between groups of fossil remains and between groups of fossils and groups of recent forms. Admitting the constancy of specific characters, these resemblances could not be accounted for by community of descent, and he thus arrived at the generalisation that in localities placed under similar circumstances, similar, though specifically distinct, specific forms were created. These he regarded as mutually representative species. Our acceptance of the doctrines of “specific centres” and of “representation” or at all events the form in which we may be inclined to accrpt them, depends greatly upon the acceptance or rejection of the fundamental dogma of the immutability of species, and on this point there has been a very great change of opinion within the last ten or twelve years—a change certainly due to the remarkable ability and candour with which the question has been discussed by Mr. Darwin and Mr. Wallace. I do not think that I am speaking too strongly when I say that there is now scarcely a single competent general naturalist who is not prepared to accept some form of the doctrine of evolution. There are no doubt very great difficulties in the minds of many of us in conceiving that, commencing from the simplest living being, the present state of things in the organic world has been produced solely by the combined action of “atavism,”the tendency of offspring to resemble their parents closely, and “variation,”the tendency of offspring to differ individually from their parents within very narrow limits; and many are inclined to believe that some Jaw, as yet undiscovered, other than the “survival of the fittest”must regulate the existing marvellous system of extreme and yet harmonious modification. Still, it must be admitted that variation is a vera causa, probably capable, within a limited period, under favourable circumstances, of convening one species into what, according to our present ideas, we should be forced to recognise as a different species; and such being the case, it is perhaps conceivable that during the lapse of a period of time—still infinitely shorter than eternity—variation may have produced the entire result. The individuals composing a species have a definite range of variation strictly limited by the circumstances under which the group of individuals is placed. Except in man and in domesticated animals, in which it is artificially increased, this individual variation is usually so slight as to be inappreciable except to a practiced eye; and any extreme variation which passes the nasural limit in any direction clashes in some way with surrounding circumstances, and is dangerous to the life of the individual. The normal or graphic line, or “line of safety,” of the species, lies midway between the extremes of variation. If at any period in the history of a species, the conditions of life of a group of individuals of the species are gradually altered; with the gradual change of circumstances the limit of variation is contracted in one direction and relaxed in anorher, it becomes more dangerous to diverge towards one side, and more desirable to diverge towards the other, and the position of the lines limiring variation is altered. The normal line, the line along which the specific characters are most strongly marked, is consequently slightly deflected, some characters being more strongly expressed at the expense ol others. This deflection, carried on for ages in the same direction, must eventually carry the divergence of the varying race far beyond any limits within which we are in the habit of admitting identity of species. But the process must be, so to speak, infinitely slow. It is difficult to form any idea of ten, fifty, or a hundred millions of years; or of the relation which such periods bear to changes taking place in the organic world. We must remember, however, that the rocks of the Silurian system, overlaid by ten miles thickness of sediment, entombing a hundred successive faunce, each as rich and varied as that of the present day, are themselves teeming with fossils fully representing all the* existing classes of animals except the very highest. If it is possible to imagine that this marvellous manifestation of eternal power and wisdom involved in living nature can have been worked out through the law of "descent with modification “alone, we shall certainly require from the physicists the very longest row of cyphers which they can afford. Now, although the admission of a doctrine of evolution must affect greatly our conception of the origin and rationale?>? so-called specific centres, it does not practically affect the question of their existence, or of the laws regulating the distribution of species from these centres by migration, by transport, by ocean currents, by elevations or depressions of the land, or by any other causes at work under existing circumstances. So far as practical naturalists are concerned, species are permanent within their narrow limits of variation, and it would introduce an element of infinite confusion and error if we were to regard them in any other light. The origin of species by "'descent with modification “is as yet only a hypothesis. During the whole period of recorded human observation, not one single instance of the change of one species into another has been detected, and, singular to say, in successive geological formations, although new species are constantly appearing, and there is abundant evidence of progressive change, no single case has as yet been observed of one species passing through a series of inappreciable modifications into another.

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