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RELATION OF PAST FORMS TO PRESENT.
ture, so all the faunas and floras of the past are related to one another, and to existing species. Out of about 120 orders of animals, as animals are classified by the naturalist, you will not, at the outside estimate, find above ten or a dozen extinct, and the proportion of extinct plants is still smaller.1 The whole fall into one grand natural system. "As Buckland long ago remarked, All fossils can be classed either in still existing groups, or between them. That the extinct forms of life help to fill up the wide intervals between existing genera, families, and orders, cannot be disputed. For if we confine our attention either to the living or to the extinct alone, the series is far less perfect than if we combine both into one general system. With respect to the Vertebrata, whole pages could be filled with striking illustrations from our great palæontologist Owen, showing how extinct animals fall in between existing groups. Cuvier ranked the Ruminants and Pachyderms as the two most distinct orders of mammals; but Owen has discovered so many fossil links, that he has had to alter the whole classification of these two orders, and has placed certain Pachyderms in the same sub-order with Ruminants; for example, he dissolves, by fine gradations, the apparently wide difference between the pig and the camel. In regard to the Invertebrata, Barrande (and a higher authority could not be named) asserts that he is every day taught that Palæozoic animals, though belonging to the same orders, families, or genera with those living at the present day, were not at this early epoch limited in such distinct groups as they now are.”2 A fourth fact is, that there exists a wonderful relation
1 Huxley, Causes of the Phenomena of Organic Nature.
ship in the same continent between the dead and the living. The fauna now occupying each separate area of the earth's surface is very closely allied to the fauna which existed on that area during recent geological times. Mr Clift many years ago showed that the fossil mammals from the Australian caves were closely allied to the living kangaroo and other marsupials of that continent. The wombat is a characteristic Australian form of mammal, and no fossil wombat has been detected out of Australia and Tasmania. In South America a similar relationship is manifest; for while sloths and armadilloes exist in that continent, and nowhere else, large teeth and great pieces of tesselated armour are found, exactly like those of the living sloth and armadillo, except in size. In Europe, Asia, and Africa, the large mammals are at present rhinoceroses, hippopotamuses, elephants, lions, tigers, oxen, horses, &c.; and if you examine the newest tertiary deposits, which contain the animals and plants which immediately preceded these, you do not find gigantic specimens of ant-eaters and kangaroos, but you find rhinoceroses, elephants, lions, tigers, &c., closely allied to those now living, though still of different species. What do these facts mean? It cannot be an immutable law that pouched animals should be produced only in Australia; for we know that Europe, in ancient times, was peopled by numerous marsupials. It cannot be said that the old forms having become extinct, because of unfitness to some new external condition, the existing marsupials were specially created to fit the modified environment; for sundry animals found elsewhere are so much more completely in harmony with these new Australian conditions, that, when taken to Australia, they rapidly supplant the marsupials.
It is maintained by many naturalists that the difficulties and anomalies of natural history are lessened by these parallel facts in palæontology, and that by connecting the two the key may be found to both. With a larger number of forms to compare, they find less difficulty in classifying them; and, giving up all attempts at linear order or circular arrangement, the conception finally arrived at is, that an ordinary genealogical tree represents, on a small scale, a system of grouping analogous to that which exists among organisms in general— much as would be the case if they were descended from a common ancestor. The relationship in structure and form, the existence of rudimentary parts, the apparent gradation of species into species in some cases, the close relationship of species in adjacent areas, the absence from some habitats of the species best suited to them— with several other anomalies—are said to be just what they would be on the supposition of a genealogical connexion, with the first parents placed far back in time. The analogous palæontological facts-of closely allied species occurring in closely following strata, of no species living again, the community of structure of fossil forms among themselves and with existing creatures, the close alliance of living and recently extinct species on the same continent--are also what they would be on the supposition of genealogical connexion. The difficulties of arrangement resemble those which would be encountered in the endeavour to classify the branches and twigs of a tree: the highest form in a sub-kingdom is the extremity of a great branch, from which there is no access to another great branch except by going back to some place of bifurcation low down in the tree.
It is not pretended that every twig can be traced to
its branch, and every branch to the trunk, down through the long succession of rocks; for there are but fragments, in many cases, where branches might be looked for. But our knowledge of the geological record is far from complete; the record itself is imperfect, and we have evidence of a pre-geologic era of unknown duration. Sedimentary strata, earlier than any we know, have been melted up, and the fossils they probably contained destroyed; two-thirds of the earth's surface are covered by water, and cannot be examined; a great part of the exposed land is inaccessible to the geologist, or at least untravelled by him; and the greater part of the remainder has been very imperfectly explored. The crust of the earth, with its embedded fossils, must not be looked at as a well-filled museum, but as a poor collection made at hazard and at rare intervals. Defective as are the layers of stone, the record of contemporaneous life which they contain is necessarily still more defective and fragmentary: jelly-fishes, and other creatures without hard parts, could not be expected to leave any traces; the bones of many creatures would be crushed by their devourers; and even the bones of large animals might be dissolved through the trickling of water containing a superfluity of carbonic acid. Still, as the experienced anatomist, starting with two or three bones, or perhaps only one, can accurately build up the entire skeleton, so (though in a lesser degree) it has become possible for science to reconstruct the life-tree of the globe.
THE THEORY OF THE EVOLUTION OF LIVING THINGS.
THE theory of Evolution, which we have seen to be true in Astronomy, Geology, and other sciences, and which is true of the individual organism developed from its parent, was sure to be applied sooner or later to the more general facts of Biology. Of the manner in which it has been so applied I shall now endeavour to give a faithful outline, without adding a word of criticism; for it is the Theory of the Evolution of Living Things, as put before us by its acknowledged exponents, which we have to make acquaintance with in the present chapter.
Until recent years the great majority of naturalists believed that each species of animal and plant had been separately created; that the species had always been as distinct from one another as they are now, and would always remain immutable. But during the first sixty years of the present century no fewer than thirty authors published views involving, in one form or another, the theory of the introduction of new species in accordance with some regular natural law.1 At the beginning of this period we find Lamarck, and at the close of it Mr Charles Darwin; while, conspicuous in the intermediate space, stands the anonymous author of the Vestiges of Creation. It will not be necessary to consider the forms-always inadequate and sometimes grotesque
1 Darwin's Origin of Species: Historical Sketch.