The results obtained by the exploration of the caves described in this chapter are to be taken merely as the first-fruits of a new line of inquiry, which is likely to throw light on many points relating to art, history, and the range of the animals, and not as being perfect or final. On the continent, no historic caves of importance have as yet been explored. CHAPTER IV. CAVES USED IN THE AGES OF IRON AND OF BRONZE. The Difference between Historic and Prehistoric Time.--The Prehistoric Fauna.--The Archæological Classification.--Caves of the Iron Age.--Caves of the Bronze Age in Britain.--The Caves of Césareda in Portugal probably occupied by Cannibals.--The Cave of Reggio in Apulia. _The Difference between Historic and Prehistoric Time._ It will be necessary before we examine the group of caves used by man in prehistoric times, to point out the important difference in the measurement of time within and beyond the borders of history. When we speak, for example, of the date of the Norman Conquest, we imply that we can ascertain by historical records, not merely that it succeeded the invasion of Britain by the English or Danes, and happened before our own time, but that the interval which separates it from those events can be accurately measured by the unit of years. If, however, we attempt to ascertain the date of any event which happened outside the historical limit, we shall find that it is a question solely of relation. When we speak, for example, of the neolithic age, we merely mean a certain stage of human progress which succeeded the palæolithic, and preceded the bronze age, but we have no proof of the length of the interval dividing it from the one or the other. The historic “when?” implies “how long ago?” the prehistoric “when?” merely implies a definition before and after certain events, without any idea of the measurement of the intervals.