368 NATURAL SCIENCE. Nov., 



extreme rarity of fish remains, the comparative scarcity of truly 

 aquatic mollusca, and the infrequent occurrence of perennial aquatic 

 plants. The amphibious mollusca are common enough, especially 

 such forms as can survive in the mud beneath a dried-up crust. 

 With these amphibious forms are associated several continental 

 species now rare or entirely extinct in Britain. We find, for instance, 

 that one of the most abundant shells is usually the Succinea ohlonga, 

 now so scarce in Britain. We have also Hydvohia marginata, Corhicula 

 fluminalis, Unio littoralis, and several species of Helix, all these forms 

 having now disappeared from our islands. It might be thought at 

 first sight that the assemblage pointed to a warmer cHmate ; but a 

 careful analysis of the list does not support this view, and the only 

 character which the species possess in common is that all of them now 

 live in sunnier and drier regions than ours, though not necessarily in 

 warmer ones. The Pleistocene mammals found in Britain also point 

 in the same direction, though not so decidedly, for many of them 

 belong to extinct species whose former habitat is unknown to us. We 

 notice, however, two or three species, such as the Saiga antelope and 

 certain of the small rodents, which belong distinctly to the desert 

 fauna of Central Asia. When the corresponding strata are traced 

 eastward into Central Europe the evidence becomes much stronger, 

 for Professor Nehringhas discovered in Germany a mammalian fauna 

 corresponding closely with that now inhabiting the Central Asian 

 steppes. 



If the loess of Central Europe be examined, it is found rarely to 

 contain aquatic mollusca, but to yield in myriads such species as love 

 sand-dunes and dust. The loess, as Baron von Richthofen has shown, 

 is a desert-deposit such as now drifts before the winds in the dry 

 regions of Central Asia. Thick dust-deposits like the loess do not 

 extend so far to the west as Britain, where the climate must always 

 have been comparatively moist owing to the proximity of the ocean ; 

 but in England there are inland deposits of blown sand, where sand 

 does not now drift owing to the growth of vegetation, and these also 

 probably point to drought. In the composition of our surface-soils we 

 have also, I believe, evidence of the former wafting to and fro of fine 

 material, which could not be obtained from the weathering of the 

 underlying rock. The soil on our chalk Downs, for instance, is always 

 full of small rounded grains of quartz, which cannot be derived from 

 the underlying strata, for the upper and middle Chalk in this country 

 do not yield anything but carbonate of lime, flint, and a little fine 

 clay. The loess period seems to have affected Britain, though not so 

 strongly as it did Central Europe. 



The erosion of the valleys, undoubtedly exceptionally rapid 

 during certain parts of the Pleistocene Period, and the formation of 

 enormous sheets of valley gravel, remain, therefore, the sole evidence 

 of the former existence of a Pluvial Period. Let us re-examine this 

 evidence from a new point of view, and see what would be the necessary 



