must have presented the strangest appearance of all the Dinosaurs yet discovered.
The affinities of birds and reptiles have been so clearly demonstrated of late years, as to cause Professor Huxley and many other comparative anatomists to bridge over the wide gap which was formerly considered to divide the two classes, and to bracket them together in one class, to which the name Sauropsidæ has been given.35
There are, indeed, not a few remarkable forms, as to the class position of which, whether they should be assigned to birds or reptiles, opinion was for a long time, and is in a few instances still, divided. It is, for example, only of late years that the fossil form Archæopteryx36 (Fig. 4, p. 19) from the Solenhofen slates, has been definitely relegated to the former, but arguments against this disposal of it have been based upon the beak or jaws being furnished with true teeth, and the feather of the tail attached to a series of vertebræ, instead of a single flattened one as in birds. It appears to have been entirely plumed, and to have had a moderate power of flight.
Fig. 6. – Sivatherium (restored), from the Upper Miocenedeposits of the Siwalik Hills. (After Figuier.)
On the other hand, the Ornithopterus is only provisionally classed with reptiles, while the connection between the two classes is drawn still closer by the copious discovery of the birds from the Cretaceous formations of America, for which we are indebted to Professor Marsh.
The Lepidosiren, also, is placed mid-way between reptiles and fishes. Professor Owen and other eminent physiologists consider it a fish; Professor Bischoff and others, an amphibian reptile. It has a two-fold apparatus for respiration, partly aquatic, consisting of gills, and partly aerial, of true lungs.
So far, then, as abnormality of type is concerned, we have here instances quite as remarkable as those presented by most of the strange monsters with the creation of which mythological fancy has been credited.
Fig. 7. – Skeleton of Megatherium. (After Figuier.)
Among mammals I shall only refer to the Megatherium, which appears to have been created to burrow in the earth and to feed upon the roots of trees and shrubs, for which purpose every organ of its heavy frame was adapted. This Hercules among animals was as large as an elephant or rhinoceros of the largest species, and might well, as it has existed until a late date, have originated the myths, current among the Indians of South America, of a gigantic tunnelling or burrowing creature, incapable of supporting the light of day.37
CHAPTER II.
EXTINCTION OF SPECIES
In reviewing the past succession of different forms of ancient life upon the globe, we are reminded of a series of dissolving views, in which each species evolves itself by an imperceptible gradation from some pre-existing one, arrives at its maximum of individuality, and then slowly fades away, while another type, either higher or lower, evolved in turn from it, emerges from obscurity, and succeeds it on the field of view.
Specific individuality has in all cases a natural term, dependent on physical causes, but that term is in many cases abruptly anticipated by a combination of unfavourable conditions.
Alteration of climate, isolation by geological changes, such as the submergence of continents and islands, and the competition of other species, are among the causes which have at all times operated towards its destruction; while, since the evolution of man, his agency, so far as we can judge by what we know of his later history, has been especially active in the same direction.
The limited distribution of many species, even when not enforced by insular conditions, is remarkable, and, of course, highly favourable to their destruction. A multiplicity of examples are familiar to naturalists, and possibly not a few may have attracted the attention of the ordinary observer.
For instance, it is probably generally known, that in our own island, the red grouse (which, by the way, is a species peculiar to Great Britain) is confined to certain moorlands, the ruffs and reeves to fen districts, and the nightingale,38 chough, and other species to a few counties; while Ireland is devoid of almost all the species of reptiles common to Great Britain. In the former cases, the need of or predilection for certain foods probably determines the favourite locality, and there are few countries which would not furnish similar examples. In the latter, the explanation depends on biological conditions dating prior to the separation of Ireland from the main continent. Among birds, it might fairly be presumed that the power of flight would produce unlimited territorial expansion, but in many instances the reverse is found to be the case: a remarkable example being afforded by the island of Tasmania, a portion of which is called the unsettled waste lands, or Western Country. This district, which comprises about one-third of the island upon the western side, and is mainly composed of mountain chains of granites, quartzite, and mica schists, is entirely devoid of the numerous species of garrulous and gay-plumaged birds, such as the Mynah mocking-bird, white cockatoo, wattle bird, and Rosella parrot, though these abundantly enliven the eastern districts, which are fertilized by rich soils due to the presence of ranges of basalt, greenstone, and other trappean rocks.
Another equally striking instance is given by my late father, Mr. J. Gould, in his work on the humming-birds. Of two species, inhabiting respectively the adjacent mountains of Pichincha and Chimborazo at certain elevations, each is strictly confined to its own mountain; and, if my memory serves me correctly, he mentions similar instances of species peculiar to different peaks of the Andes.
Limitation by insular isolation is intelligible, especially in the case of mammals and reptiles, and of birds possessing but small power of flight; and we are, therefore, not surprised to find Mr. Gosse indicating, among other examples, that even the smallest of the Antilles has each a fauna of its own, while the humming-birds, some of the parrots, cuckoos, and pigeons, and many of the smaller birds are peculiar to Jamaica. He states still further, that in the latter instance many of the animals are not distributed over the whole island, but confined to a single small district.
Continental limitation is effected by mountain barriers. Thus, according to Mr. Wallace, almost all the mammalia, birds, and insects on one side of the Andes and Rocky Mountains are distinct in species from those on the other; while a similar difference, but smaller in degree, exists with reference to regions adjacent to the Alps and Pyrenees.
Climate, broad rivers, seas, oceans, forests, and even large desert wastes, like the Sahara or the great desert of Gobi, also act more or less effectively as girdles which confine species within certain limits.
Dependence on each other or on supplies of appropriate food also form minor yet practical factors in the sum of limitation; and a curious example of the first is given by Dr. Van Lennep with reference to the small migratory birds that are unable to perform the flight of three hundred and fifty miles across the Mediterranean. He states that these are carried across on the backs of cranes.39
In the autumn many flocks of cranes may be seen coming from the North, with the first cold blast from that quarter, flying low, and uttering a peculiar cry, as if of alarm, as they circle over the cultivated plains. Little birds of every species may be seen flying up to them, while the twittering cries of those already comfortably settled upon their backs may be distinctly heard. On their return in the spring they fly high, apparently considering that their little passengers can easily find their way down to the earth.
The question of food-supply is involved in the more extended subject of geological structure, as controlling
“It is now generally admitted by biologists who have made a study of the Vertebrata that birds have come down to us through the Dinosaurs, and the close affinity of the latter with recent struthious birds will hardly be questioned. The case amounts almost to a demonstration if we compare with Dinosaurs their contemporaries, the Mesozoic birds. The classes of birds and reptiles as now living are separated by a gulf so profound that a few years since it was cited by the opponents of evolution as the most important break in the animal series, and one which that doctrine could not bridge over. Since then, as Huxley has clearly shown, this gap has been virtually filled by the discoveries of bird-like reptiles and reptilian birds. Compsognathus and Archæopteryx of the old world, and Icthyornis and Hesperornis of the new, are the stepping-stones by which the evolutionist of to-day leads the doubting brother across the shallow remnant of the gulf, once thought impassable.” —
Professor Carl Vogt regards the Archæopteryx “as neither reptile nor bird, but as constituting an intermediate type. He points out that there is complete homology between the scales or spines of reptiles and the feathers of birds. The feather of the bird is only a reptile’s scale further developed, and the reptile’s scale is a feather which has remained in the embryonic condition. He considers the reptilian homologies to preponderate.”
“It enters Europe early in April, spreads over France, Britain, Denmark, and the south of Sweden, which it reaches by the beginning of May. It does not enter Brittany, the Channel Islands, or the western part of England, never visiting Wales, except the extreme south of Glamorganshire, and rarely extending farther north than Yorkshire.” – A. R. Wallace,
