It is clear that so complete and sudden a change in the higher forms of life, does not represent the normal state of things. Species and genera have not, at all times, become so rapidly extinct. The time occupied by the "Recent period," that is the {150}time _since_ these changes took place is, geologically, minute. The time of the whole of the Post-Pliocene period, as measured by the amount of physical and _general_ organic change known to have taken place, is exceedingly small when compared with the duration of the Pliocene period, and still smaller, probably, as compared with the Miocene. Yet during these two periods we meet with no such break in the continuity of the forms of life, no such radical change in the _character_ of the fauna (though the number of specific and generic changes may be as great) as we find in passing from the Post-Pliocene to recent times. For example, in Central Europe numerous hyænas, rhinoceroses, and antelopes, with the great _Machairodus_, continued from Miocene all through Pliocene into Post-Pliocene times; while hippopotami and elephants continued to live through a good part of the Pliocene and Post-Pliocene periods,--and then all suddenly became extinct or left the country. In North America there has been more movement of the fauna in all the periods; but we have similar great felines, horses, mastodons, and elephants, in the Pliocene and Post-Pliocene periods, while _Rhinoceros_ is common to the Miocene and Pliocene, and camels range continuously from Miocene, through Pliocene, to Post-Pliocene times;--when all alike became extinct. Even in South America the evidence is, as far as it goes, all the same way. We find _Machairodus_, _Equus_, _Mastodon_, _Megatherium_, _Scelidotherium_, _Megalonyx_, and numerous gigantic armadillos, alike in the caves and in the stratified tertiary deposits of the Pampas;--yet all have since passed away.