80 THE BRITISH MAMMALS PAST AND PRESENT. 



Ireland, Cumberland, and Scotland were represented by their 

 mountain summits in the form of islands. Once again the sea-bed 

 rose to sink again, and during this period of subsidence the waters 

 finally cut their way through St. George's Channel and afterwards 

 through the Straits of Dover, so as to leave the geography of the 

 British Isles much as it is to-day. 



From this rapid survey of our country's physical history since 

 the first British mammal yet found dropped his teeth on the Rhaetic 

 mud, it is clear that, even considering the changes we have recorded 

 which are by no means all the conditions of existence must have 

 altered frequently and greatly. Throughout, the aquatic animals 

 must have had by far the safer time of it, though, everi they must at 

 times have been shut in or perished from change of climate, or want 

 of food, or the increase of enemies. But the land animals must 

 either have been cut off at each submergence, or retired to a con- 

 tinent, when they could find one, there to breed and leave their 

 descendants to venture on to the new land surface when vegetation 

 had rendered it habitable. 



That numbers of species must have passed away in this area 

 and left no traces behind them seems to be almost inevitable. As 

 the rocks were all deposited in water, the land animals, whose 

 remains have been preserved, must either have met their deaths by 

 drowning, or been washed into the water when dead, or swallowed 

 by some aquatic carnivore who failed to digest their teeth and other 

 hard parts. In any case they are not likely to be met with far from 

 the shore, and hence it is that so few have been found in marine 

 deposits. In the Secondary rocks they appear only in the estuarine 

 beds of the Rhaetics and Stonesfield Slate ; in the beds above the chalk 

 they are more numerous because our Tertiary period was mainly 

 one of lakes, lagoons, estuaries, and shallow seas ; and in the caves 

 of the Pleistocene they owe their preservation, not to contempo- 

 raneous deposition, but mainly to the formation of stalagmite due to 

 the percolation of water through the roof of the natural cavity they 

 had chosen as their home for the dying. 



Considering the preponderance of the chances against preserva- 

 tion the wonder is that our fossil animals should be so numerous. 

 Evidently the faunas of the past were as rich as those of the present, 

 though the classes were in different proportion. And the abundance 

 of animal life agrees with the known luxuriance ot vegetable life, for 

 it is dependent on it, just as the vegetable life is dependent on the 

 land surface as modified by denudation under the influences of 

 climate. As examples of this interdependence of Nature we have 

 the first appearance of the grasses in Cretaceous clays followed by 

 the incoming of the grass-feeding ungulates of the Eocene, then 

 the arrival of the larger carnivores as soon as there was prey suit- 

 able for them, and finally the destruction of nearly all at the hands 

 of omnivorous man, whom, for obvious reasons, we have not in- 

 cluded in our list, though he was certainly here before our final 

 separation from the continent, but perhaps in a form which many 

 of his descendants would not care to recognise. 



Some brief notes on our past faunas would seem to be necessary. 

 The earliest British mammals as yet known are two species of 



