INTRODUCTION. 



A few words are needed to explain how it happens that in the following pages 

 we are describing the Pliocene floras of Limburg and the Prussian border, instead of 

 keeping to our own country, where so much remains to be done. And it will be well to 

 explain also why we treat the subject less from the point of view of the describer of 

 extinct plants than from that of the plant*geographer and the field*naturalist. 



For many years we have worked in the Pleistocene and Pliocene deposits of 

 Britain, in the hope of obtaining some approximate measure of geological time, some 

 idea of the succession of climatic changes, and some insight into the origins and migra* 

 tions of successive faunas and floras. We found, however, as others have found, that it 

 is necessary to use two parallel chronologies, the one founded on the changes amongst 

 the inhabitants of the lands and rivers, the other on the inhabitants of the sea. Not only 

 are the changes in the two sets of fossils not contemporaneous ; but even the climatic 

 conditions may be quite different for seas and lands closely adjoining. The bridge which 

 helps the migration of the land flora and fauna is the barrier which prevents the mingl* 

 ing of the faunas of two closely adjoining seas ; the strait which connects two seas is 

 the barrier which stops the spread of the fauna and flora of the land. 



The fossil fauna of the sea is fairly well known, and its Tertiary history is now 

 nearly continuous. But the landsanimals and plants are comparatively little studied, and 

 their geological record is full of gaps ; still it is on the land that we live, and it is the . 

 climate and past history of the landsareas which are of the greatest interest to us. 



The natural history of the land^areas is therefore our special study; but in 

 enquiries of this sort the field is so vast that before venturing to generalize it is neces* 

 sary first to specialize on some particular group. The group selected must be one which 

 responds readily to changes of climate and physical conditions, and is found in all 

 latitudes and at all altitudes. 



Mammals, or any warm-blooded animals, are unsafe guides ; for they have great 

 power of resisting and adapting themselves to climatic changes. Closely allied species 

 may inhabit arctic and tropical regions, and we cannot learn from any living mammal 

 under what conditions an extinct ally may have lived. The elephant and the rhinoceros 

 are now tropical; extinct elephants and rhinoceroses, clothed with thick fur, are found 

 frozen up in the soil of Siberia, and were evidently adapted to an arctic climate. 



The land and freshwater mollusca have proved of less use than was expected. The 

 coleoptera probably would be a very valuable group for this study, were their fossil 



