12 POST-TERTIARY ENTOMOSTRACA. 



Professor Edward Forbes, in his paper " On the Connection between the Distribution 

 of the Existing Fauna and Flora of the British Isles and the Geological Changes which 

 have affected their area, especially daring the Epoch of the Northern Drift,"-^ while 

 referring in his catalogue of " species of marine animals found fossil in beds of the 

 Glacial Epoch " to localities so broadly defined as " Scotland," " drift beds of Scotland 

 and Ireland," and " Clyde beds," nevertheless, by the whole course of his arguments 

 plainly indicates that these beds themselves must differ from each other in their general 

 characteristics, and require to be studied separately rather than be roughly gathered into 

 one group. 



Among the " chief conclusions " (some of which subsequent researches have neces- 

 sarily modified) in which he sums up the results of the facts and arguments stated in the 

 essay the following occur, which bear upon the existence, among the Post-tertiary fossili- 

 ferous deposits under discussion, of many varieties of beds deposited neither at one time 

 nor under one set of circumstances. 



" The greater part of the terrestrial animals and flowering plants now inhabiting the 

 British Islands are members of specific centres beyond their area, and have migrated to it 

 over continuous land before, during, or after the Glacial Epoch" (p. 399). 



" The termination of the Glacial Epoch in Europe was marked by a recession of an 

 Arctic fauna and flora northwards, and of a fauna and flora of the Mediterranean south- 

 wards, and in the interspace thus produced there appeared on land the general Germanic 

 fauna and flora, and in the sea that fauna termed Celtic " (p. 401). 



" The causes which thus preceded the appearance of a new assemblage of organized 

 beings were the destruction of many species of animals, and probably also of plants, either 

 forms of extremely local distribution or such as were not capable of enduring many 

 changes of conditions, — species, in short, with very limited capacity for horizontal or 

 vertical diffusion" (p. 401). 



" All the changes before, during, and after the Glacial Epoch appear to have been 

 gradual and not sudden, so that no marked line of demarcation can be drawn between 

 the creatures inhabiting the same element and the same locality during two proximate 

 periods" (p. 401). 



The recession of one fauna and flora and the advance of another, the changes in the 

 local distribution of species caused by the elevation and subsidence of the land, and the 

 gradual passage from one set of conditions to another, must be indicated in the varying 

 composition of the different deposits. 



The first important attempt to classify the various beds belonging to the " Scotch 

 glacial drift," as well as explain their origin and determine their sequence, was made by 

 Prof. A. Geikie, whose treatise " On the Phenomena of the Glacial Drift of Scotland " was 

 pubhshed in 1863.^ 



1 'Memoirs of the Geological Survey of Great Britain,' vol. i. London, 18-16. 



2 ' Transactions of Geological Society of Glasgow,' vol. i, part ii. 



