It is easy to refer a given cave to the age of the reindeer or of the mammoth because it contains the remains of those animals, but the division has been rendered worthless for chronological purposes, by the fact that both these animals inhabited the region north of the Alps and Pyrenees at the same time, and are to be found together in nearly every bone-cave explored in that area. The difference between the contents of one palæolithic cave and another, is probably largely due to the fact that man could more easily catch some animals than others, as well as to the preference for one kind of food before another. And the abundance of the reindeer, which is supposed to characterise the reindeer period, may reasonably be accounted for by the fact, that it would be more easily captured by a savage hunter, than the mammoth, woolly rhinoceros, cave-bear, lion, or hyæna. The classification will apply, as I have shown in my essay on the pleistocene mammalia,[234] neither to the caves of this country, of Belgium, nor of France, and my views are shared by M. de Mortillet,[235] after a careful and independent examination of the whole evidence. The division of the caves also into ages, according to the various types of implements found in them, proposed by M. de Mortillet, seems to be equally unsatisfactory; for there is no greater difference in the implements of any two of the palæolithic caves, than is to be observed between those of two different tribes of Eskimos, while the general resemblance is most striking. The principle of classification by the relative rudeness, assumes that the progress of man has been gradual, and that the ruder implements are therefore the older. The difference, however, may have been due to different tribes, or families, having co-existed without intercourse with each other, as is now generally the case with savage communities; or to the supply of flint, chert, and other materials for cutting instruments, being greater in one region than in another. _Relation of Cave-dwellers to Eskimos._ [Illustration: FIG. 122.--Eskimos Spear-head, bone (1/2).] [Illustration: FIG. 123.--Eskimos Arrow-straightener of Walrus Tooth (1/1). (Brit. Mus.)]