Pictet has estimated the number of species of mammals inhabiting Europe in the palanthropic period at ninety-eight,[12] of which only fifty-seven now live there, the remainder being either wholly or locally extinct--that is, they are either not now existing in any part of the world, or are found only beyond the limits of Central, Western, and Southern Europe. The extinct species also include the largest and noblest of all. It has been remarked that the assemblage of palanthropic species in Europe and Western Asia is so great and varied that with our present experience we can scarcely imagine them to have existed contemporaneously in the same region. For example, the association of species of elephant and rhinoceros, the musk-sheep, the reindeer, the Cape hyena, and the hippopotamus seems to be incongruous. [12] Zittel, in a recent paper (1893), gives 110 species of mammals in the pleistocene and early modern. Of these about twenty of the largest and most important are extinct. Various theories have been proposed to remove the difficulty. Modern analogies will allow us to believe in such astounding facts if we take into account the probability of a warm climate, especially in summer, along with a wooded state of the country providing much shelter, and wide continental plains affording facilities for seasonal migrations. There were no doubt also climatal changes in the course of the age, which may have tended to the remarkable mixture of animal types in its deposits. In connection with this there is now every reason to believe that while, in its earlier part, the palanthropic age was distinguished by a warm climate, in its later portion a colder and more inclement atmosphere crept over the northern hemisphere. As an illustration of this, it is known that in the earlier part of the period a noble species of elephant named _Elephas antiquus_, and a rhinoceros (_R. Merkii_), abounded in Europe; but as the age advanced these species disappeared, and were replaced by the mammoth (_E. primigenius_) and the woolly rhinoceros (_R. tichorhinus_), animals clothed like the musk-ox in dense wool and hair, and evidently intended for a rigorous climate. With and succeeding these last species, the reindeer becomes characteristic and abundant. It is, as we shall see, a point of much importance in what may be called the prehistoric history of man, that he was introduced in a period of genial temperature as well as of wide continental extension, and survived to find his physical environment gradually becoming less favourable, and the age ending in that great cataclysm which swept so many species of animals and tribes of men out of existence, and reduced the dry land of our continents to its present comparatively limited area.