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 Archæopteryx[36] (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. [Illustration: FIG. 6.--SIVATHERIUM (RESTORED), FROM THE UPPER MIOCENE DEPOSITS 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. [Illustration: 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.