Gradually the tundra animals spread toward the south with the cold climate which for the first time swept all over western Europe. The whole aspect of the country slowly changed with the approach of the reindeer, and the northern flora of the spruce, the fir, and the arctic willow clad the more sheltered river-valleys and hillsides, while the plateaus and fields were partly or wholly deforested. [Illustration: FIG. 96. Typical tundra fauna. "Gradually the tundra animals pressed toward the south with the cold climate which for the first time swept all over western Europe." The wolverene, _Gulo luscus borealis_; the barren-ground reindeer, _Rangifer tarandus_ (drawn from the living type); the arctic fox, _Canis lagopus_; the musk-ox, _Ovibos moschatus_; and the banded lemming, _Myodes torquatus_. One-twenty-fifth life size. The lemming (A) is also shown one-seventh life size. Drawn by Erwin S. Christman.] Thus the country became adapted chiefly to the tundra types of mammals; and in the middle Mousterian strata these herds, newly migrated from the far north and from the northeastern steppes bordering the Obi River, largely outnumber the steppe forms, which are limited to two or three species. Of these the principal types are the steppe horse, related to the Przewalski horse now living in the desert of Gobi, the steppe suslik (_Spermophilus rufescens_), and the steppe grouse, or moor-hen. The more characteristic forms of steppe life, such as the saiga antelope, the jerboa, and the kiang, were all later arrivals and did not appear until after the close of the Mousterian industry and the disappearance of the Neanderthal race. This was due to the fact that the climate surrounding the Neanderthal race in Mousterian times was cold and moist, with heavy rainfalls in summer and snow-storms in winter, a climate thoroughly suited to the arctic tundra mammals with their heavy covering of hair acting as a rain shed and the undercoating of wool protecting them in the most severe weather. The mammal life during the fourth glaciation, as it spread into the middle Rhine and Westphalian region, is fully recorded in the 'loess' deposits of Achenheim and in the famous grotto of Sirgenstein, on the upper Danube, lying northwest of Munich, where, together with traces of the most primitive Mousterian industry, are found remains of the mammoth, the bison, the reindeer, a species of wild horse, and the cave-bear. Following these mammals there is a record in the same deposit of the arrival of the Obi lemming, from northern Russia.