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often been interrupted, and as long blank intervals have intervened between successive formations, we ought not to expect to find, as I attempted to show in the last chapter, in any one or two formations all the intermediate varieties between the species which appeared at the commencement and close of these periods; but we ought to find after intervals, very long as measured by years, but only moderately long as measured geologically, closely allied forms, or, as they have been called by some authors, representative species; and these we assuredly do find. We find in short, such evidence of the slow and scarcely. sensible mutation of specific forms, as we have a just right to expect to find.
On the State of Development of Ancient compared with Living Forms.- We have seen in the fourth chapter that the degree of differentiation and specialisation of the parts of all organic beings, when come to maturity, is the best standard, as yet suggested, of their degree of perfection or highness. We have also seen that as the specialisation of parts and organs is an advantage to each being, so natural selection will constantly tend thus to render the organisation of each being more specialised and perfect, and in this sense higher ; not but that it may and will leave many creatures with simple and unimproved structures fitted for simple conditions of life, and in some cases will even degrade or simplify the organisation, yet leaving such degraded beings better fitted for their new walks of life. In another and more general manner we can see that on the theory of natural selection the more recent forms will tend to be higher than their progenitors; for each new species is formed by having had some advantage in the struggle for life over other and preceding forms. If under a nearly similar climate the eocene inhabitants of one quarter of the world were put into competition with the existing inhabitants of the same or some other quarter, the eocene fauna or flora would certainly be beaten and exterminated, as would a secondary fauna by an eocene, and a palæozoic fauna by a secondary fauna. So that by this fundamental test of victory in the battle for life, as well as by the standard of the specialisation of organs, modern forms ought on the theory of natural selection to stand higher than ancient forms. Is this the case ? A large majority of palæontologists would certainly answer in the affir
mative; but in my judgment I can, after having read the discussions on this subject by Lyell, and Hooker's views in regard to plants, concur only to a limited extent. Nevertheless it may be anticipated that the evidence will be rendered more decisive by future geological research.
The problem is in many ways excessively intricate. The geological record, at all times imperfect, does not extend far enough back, as I believe, to show with unmistakeable clearness that within the known history of the world organisation has largely advanced. Even at the present day, looking to members of the same class, naturalists are not unanimous which forms are highest : thus, some look at the selaceans or sharks from their approach in some important points of structure to reptiles as the highest fish; others look at the teleosteans as the highest. The ganoids stand intermediate between the selaceans and teleosteans; the latter at the present day are largely preponderant in number ; but formerly selaceans and ganoids alone existed ; and in this case, according to the standard of highness chosen, so will it be said that fishes have advanced or retrograded in organisation. To attempt to compare in the scale of highness members of distinct types seems hopeless : who will decide whether a cuttle-fish be higher than a bee—that insect which the great Von Baer believed to be “in fact more highly organised than a fish, although upon another type ” ? In the complex struggle for life it is quite credible that crustaceans, for instance, not very high in their own class, might beat the cephalopods or highest molluscs; and such crustaceans, though not highly developed, would stand very high in the scale of invertebrate animals if judged by the most decisive of all trials—the law of battle. Besides these inherent difficulties in deciding which forms are the most advanced in organisation, we ought not solely to compare the highest members of a class at any two distant periods—though undoubtedly this is one and perhaps the most important element in striking a balance—but we ought to compare all the members, high and low, at the two periods. At an ancient epoch the highest and lowest molluscs, namely, cephalopods and brachiopods, swarmed in numbers : at the present time both these orders have been greatly reduced, whereas other orders, intermediate in grade of organisation, have largely increased ; consequently some naturalists
have maintained that molluscs were formerly more highly developed than at present; but a stronger case can be made out on the other side, by considering the vast reduction at the present day of the lowest molluscs, more especially as the existing cephalopods, though so few in number, are more highly organised than their ancient representatives. We ought also to consider the relative proportional numbers of the high and low classes in the population of the world at the two periods : if, for instance, at the present day there be fifty thousand kinds of vertebrate animals, and if we knew that at some former period only ten thousand kinds had existed, we ought to look at this increase in number of the highest class, which implies a great displacement of lower forms, as a decided advance in the organisation of the world, whether the higher or the lower vertebrata had thus largely increased. We can thus see how hopelessly difficult it will apparently for ever be to compare with perfect fairness, under such extremely complex relations, the standard of organisation of the imperfectly-known faunas of successive periods of the earth's history.
We shall appreciate under one important point of view this difficulty the more clearly, by looking to the case of certain existing faunas and floras. From the extraordinary manner in which European productions have recently spread over New Zealand, and have seized on places which must have been previously occupied, we may believe, if all the animals and plants of Great Britain were set free in New Zealand, that in the course of time multitude of British forms would become thoroughly naturalised there, and would exterminate many of the natives. On the other hand, from what we see now occurring in New Zealand, and from hardly a single inhabitant of the southern hemisphere having become wild in any part of Europe, we may doubt, if all the productions of New Zealand were set free in Great Britain, whether any considerable number would be enabled to seize on places now occupied by our native plants and animals. Under this point of view, the productions of Great Britain may be said to be higher than those of New Zealand. Yet the most skilful naturalist from an examination of the species of the two countries could not have foreseen this result.
Agassiz insists that ancient animals resemble to a certain extent the embryos of recent animals of the same
classes ; or that the geological succession of extinct forms is in some degree parallel to the embryological development of recent forms. I must follow Pictet and Huxley in thinking that the truth of this doctrine is very far from proved. Yet I fully expect to see it hereafter confirmed, at least in regard to subordinate groups, which have branched off from each other within comparatively recent times. For this doctrine of Agassiz accords well with the theory of natural selection. In a future chapter I shall attempt to show that the adult differs from its embryo, owing to variations supervening at a not early age, and being inherited at a corresponding age. This process, whilst it leaves the embryo almost unaltered, continually adds, in the course of successive generations, more and more difference to the adult.
Thus the embryo comes to be left as a sort of picture, preserved by nature, of the ancient and less modified condition of each animal. This view may be true, and yet it may never be capable of full proof. Seeing, for instance, that the oldest known mammals, reptiles, and fish strictly belong to their own proper classes, though some of these old forms are in a slight degree less distinct from each other than are the typical members of the same groups at the present day, it would be vain to look for animals having the common embryological character of the Vertebrata, until beds far beneath the lowest Silurian strata are discovered—a discovery of which the chance is very small.
On the Succession of the same Types within the same areas, during the later tertiary periods.—Mr. Clift many years ago showed that the fossil mammals from the Australian caves were closely allied to the living marsupials of that continent. In South America, a similar relationship is manifest, even to an uneducated eye, in the gigantic pieces of armour like those of the armadillo, found in several parts of La Plata ; and Professor Owen has shown in the most striking manner that most of the fossil mammals, buried there in such numbers, are related to South American types. This relationship is even more clearly seen in the wonderful collection of fossil bones made by MM. Lund and Clausen in the caves of Brazil. I was so much impressed with these facts that I strongly insisted, in 1839 and 1845, on this “ law of the succession of types,"—on
wonderful relationship in the same continent between the dead and the living." Professor Owen has subsequently extended the same generalisation to the mammals of the Old World. We see the same law in this author's restorations of the extinct and gigantic birds of New Zealand. We see it also in the birds of the caves of Brazil. Mr. Woodward has shown that the same law holds good with sea-shells, but from the wide distribution of most genera of molluscs, it is not well displayed by them. Other cases could be added, as the relation between the extinct and living land-shells of Madeira ; and between the extinct and living brackish-water shells of the AraloCaspian Sea.
Now what does this remarkable law of the succession of the same types within the same areas mean? He would be a bold man, who after comparing the present climate of Australia and of parts of South America under the same latitude, would attempt to account, on the one hand, by dissimilar physical conditions for the dissimilarity of the inhabitants of these two continents, and, on the other hand, by similarity of conditions, for the uniformity of the same types in each during the later tertiary periods. Nor can it be pretended that it is an immutable law that marsupials should have been chiefly or solely produced in Australia ; or that Edentata and other American types should have been solely produced in South America. For we know that Europe in ancient times was peopled by numerous marsupials ; and I have shown in the publications above alluded to, that in America the law of distribution of terrestrial mammals was formerly different from what it now is. North America formerly partook strongly of the present character of the southern half of the continent; and the southern half was formerly more closely allied, than it is at present, to the northern half. In a similar manner we know from Falconer and Cautley's discoveries, that northern India was formerly more closely related in its mammals to Africa than it is at the present time. Analogous facts could be given in relation to the distribution of marine animals.
On the theory of descent with modification, the great law of the long enduring, but not immutable, succession of the same types within the same areas, is at once explained ; for the inhabitants of each quarter of the world will obviously tend to leave in that quarter, during