THE DEPTHS OF THE SEA INTRODUCTION (3) C. WYVILLE THOMSON

Our acceptance of the doctrines of specific centres and of representation, or, at all events, the form in which we may be inclined to accept these, 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, and to the genius of Professor Ernst Haeckel, Dr. Fritz Müller, and others of their enthusiastic disciples and commentators. I do not think that I am speaking too strongly whenI 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 is, no doubt, very great difficulty 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 other law 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, capable, within a limited period, under favourable circumstances, of converting one species into what, according to our present ideas, we should be forced to recognize 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 individuaIs comprising a species have a definite range of variation strictly limited by the circumstances under which the group of individuaIs is placed. Except in man, and in domesticated animaIs in which it is artificially increased, this individual variation is usually so slight as to be unappreciable except to a practised eye; but any extreme variation which passes the natural limit in any direction clashes in some way with surroundjng 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 individuaIs of the species be gradually altered, with the gradual change of circumstances the limit of variation is contracted in one direction and relaxed in another; it becomes more dangerous to diverge towards one side and more desirable to diverge towards the other, and the position of the lines limiting 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 of others. This deflection, carried on for ages in the same direction, must eventually carry the divergence of the varying race far beyond any limit within which we are in the habit of admitting identity of species. But the process must be 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 faunae, each as rich and varied as the fauna of the present day, themselves teem with fossils fully representing alI the existing classes of animaIs, except perhaps the highest. If it be 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 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 of so-called specific centres, it does not practically affect the question of their existence, or of the laws regulating the distribution of species from their 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 casehas yet been observed of one species passing through a series of inappreciable modifications into another. Every species appears to have an area of maximum development, and this has been called the metropolis of the species; and practically we must employ the same methods in investigating the laws of its distribution as if we still regarded it as having been specially created in its metropolis. It is the same in dealing with the law of representation. Accepting an evolution doctrine, we should certainly regard closely allied or 'representative' species as having descended comparatively recently from a common ancestry, and as having diverged from one another under somewhat different conditions of life. It is possible that as our knowledge increases we may be able to trace the pedigree of our modern species, and some attempts have already been made to sketch out the main branches of the universal genealogical tree (Ernst Haeckel); but practically we must continue to accord a specific rank to forms which exhibit characters to which we have been in the habit of assigning specific value. Every species has three maxima of development, - in depth, in geographic space, in time. ln depth, we find a species at first represented by few individuaIs, which become more and more numerous until they reach a certain point, after which they again gradually diminish, and at length altogether disappear. So also in the geographic and geologic distribution of animaIs. Sometimes the genus to which the species belongs ceases with its disappearance, but not unfrequently a succession of similar species are kept up, representative as it were of each other. When there is such a representation, the minimum of one species usually commences before that of which it is representative has attained its correspondent minimum. Forms of representative species are similar, often only to be distinguished by critical examination. [...]