170 



Inferior Oolite, all distinct from one another, all now totally 

 extinct, all very different from the present inhabitants of the 

 local coast line, and indicative of a warmer climate. Not in 

 this district, but in Eastern Yorkshire, we can further trace 

 the evolution of marine animals to the close of the Cretaceous 

 Period. Age after age, fauna succeeded fauna, until just 

 before the Glacial Period, the North Sea contained a suite of 

 animals very similar to those now living there. With the 

 advent of the Ice Age the pre-glacial animals were slowly 

 driven from the sea, and replaced by Arctic species, until 

 even these were driven out by the advent of the great 

 northern ice-sheet. 



After a longer or shorter period of refrigeration, the climate 

 gradually began to ameliorate, the ice-sheets retreated north- 

 wards, and ^ere followed by the marine species characteristic 

 of the Arctic province ; these in their turn were succeeded 

 by the temperate species with a few southern forms, until 

 the present status of the marine fauna was reached. This 

 succession of events was probably interrupted by the con- 

 version of the sea-floor into land, and minor fluctuations of 

 climate. 



The land-fauna can only be traced back to late Tertiary 

 times. What it may have been before then we have no local 

 evidence to show, but from data found in other parts of 

 Britain, it may in the future be possible to fill in the Tertiary 

 history of our animals. In Pliocene times the mammalia were 

 of African, northern, and temperate types, features probably 

 characteristic of all the other land animals of the district. 

 The approach of the Ice Age brought with it numerous Arctic 

 species, many of which survived that period on the driftless 

 area. In post-glacial times, the temperate and southern 

 animals belonging to our fauna once more entered the district, 

 unaccompanied, however, by any African or extreme southern 

 species. These gradually ousted the true Arctic species, 

 occupied all those habitats for which they are adapted, 

 and in so doing formed faunal associations very distinct from 

 one another. We also saw that many of our residents originated 

 in various parts of the world, the Arctic regions, Europe, 

 Asia, and America, long before the Ice Age, the Glacial 

 Period being merely an incident in their complex evolution. 



