The first difficulty which meets us in this question is that of distinguishing clearly between the different geological periods. No hard-and-fast line separates the Quaternary from the Pliocene, the Pliocene from the Miocene, or the Miocene from the Eocene. They pass from one into the other by insensible gradations, and the names given to them merely imply that such considerable changes have taken place in the fauna as to enable us to distinguish one period from another. And even this only applies when we take the periods as a whole, and see what have been the predominant types, for single types often survive through successive periods. The course of evolution seems to be that types and species, like individuals, have their periods of birth, growth, maturity, decay, and death. Thus fish of the ganoid type appear sparingly in the Silurian, culminate in the Devonian, and gradually die out in the later formations. So also Saurian reptiles appear in the Carboniferous, culminate in the Lias, and die out with the Secondary, or so nearly so that the crocodilia are their sole remaining representatives. And this applies when we attempt to take our first step backwards in tracing the origin of man, and follow him from the Quaternary into the Pliocene. When did the Pliocene end and the Quaternary begin? Within which of the two did the first great glacial period fall? Does pre-glacial mean Pliocene, or is it included in the Quaternary? and to which do the oldest human remains belong, such as the skeletons of Spy? The difficulty of answering these questions is increased because, as we go back in time, the human remains which guide us in the Quaternary age necessarily become scarcer. Mankind must have been fewer in number, and their relics to a great extent removed by denudation. Thus the evidence from caves, which affords by far the most information as to Quaternary man, entirely fails us as to the Pliocene and earlier periods. This may be readily accounted for when we consider the great amount of the earth's surface which has been removed by denudation. In fact we have seen that nearly 2000 feet of a mountain range must have disappeared from denudation in the Weald of Kent, since the streams from it rolled down the gravels with human implements, scattered over the North Downs as described by Professor Prestwich. What chance would Tertiary caves have of surviving such an extensive denudation? Moreover, if any of the present caves existed before the glacial period, their original contents must have been swept out, perhaps more than once, before they became filled by the present deposits. There is evidence in many caves that this was the case, from small patches of the older deposit being found adhering to the roof, as at Brixham and Maccaguone in Sicily, in which latter case flakes of chipped stone and pieces of carbon were found by Dr. Falconer in these patches of a hard breccia.