they will in the struggle for life have to beat all the older forms with which they come into close competition. We may therefore conclude that if under a nearly similar climate the eocene inhabitants of the world could be put into competition with our existing inhabitants, the former would be beaten and exterminated, as would a secondary fauna by an eocene, and a paleozoic 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 paleontologists would certainly answer in the affirmative; but in my judgment I cannot, after having read the discussions on this subject by Lyell, Bronn, and Hooker, look at this conclusion as fully proved, though highly probable.

It is no valid objection to this conclusion or to the general belief that species in the course of time change, that certain Brachiopods have been but slightly modified from an extremely remote geological period, although no explanation can be given of this fact. It is not an insuperable difficulty that Foraminifera have not progressed in organisation, as insisted on by Dr. Carpenter, since that most ancient of all epochs the Laurentian formation of Canada; for some organisms would have to remain fitted for simple conditions of life, and what better for this end than these lowly organised Protozoa? It is no great difficulty that fresh-water shells, as Professor Phillips has remarked, have remained almost unaltered from the time when they first appeared to the present day; but in this case we can see that these shells will have been subjected to less severe competition than the molluscs which inhabit the far more extensive area of the sea with its innumerable inhabitants. Such

objections as the above would be fatal to any view which included advance in organisation as a necessary contingent. They would be fatal to my view if Foraminifera, for instance, could be proved to have first come into existence during the Laurentian epoch, or Brachiopods during the lower Silurian formations; for if this were proved, there would not have been time sufficient for the development of these organisms up to the standard which they had then reached. When once advanced up to any given point, there is no necessity on the theory of natural selection for their further continued progress; though they will, during each successive age, have to be slightly modified, so as to hold their places in relation to their changing conditions of life. All such objections hinge on the question whether we have any sufficient knowledge of the antiquity of the world and of the periods when the various forms of life first appeared; and this may be boldly disputed.

The problem whether organisation on the whole has advanced 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 to be ranked as 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 cuttlefish 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, not very high in their own class, might beat cephalopods, the 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 been 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 any 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 must believe, that if all the animals and plants of Great Britain were set free in New Zealand, in the course of time a multitude of British forms would become thoroughly naturalized there, and would exterminate many of the natives. On the other hand, from the progress of this displacement in New Zealand, and from hardly a single inhabitant of the southern hemisphere having become wild in any part of Europe, we may well 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 with the embryological development of recent forms. I must follow Pictet and Huxley in thinking that the truth of this doctrine is 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 admirably 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 rich in fossils are discovered far beneath the lowest Silurian stratum -a discovery of which the chance is 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