Translator s Preface. ix 



epoch, which, although intermittent, was of con- 

 siderable duration. The last great geological 

 event which our globe experienced was in fact 

 this Ice Age, and the pure naturalist has not 

 hitherto attributed in my opinion sufficient im- 

 portance to the direct modifying effects of this 

 prolonged period of cold. It is scarcely possible 

 that such a vast climatic change as that which 

 came on at the close of the Pliocene Period 

 should have left no permanent effect upon our 

 present fauna and flora, all the species of which 

 have survived from the glacial age. The great 

 principle of Natural Selection leads us to see 

 how pre -glacial forms may have become adapted 

 to the new climatic conditions (which came on 

 gradually) by the " survival of the fittest " or 

 " indirect equilibration. 1 ' The influence of the 

 last Glacial Epoch as a factor in determining the 

 present geographical distribution of animals and 

 plants has already been amply treated of by many 

 writers since the broad paths were traced out by 

 Darwin, Lyell, and Wallace. The last named 

 author has indeed quite recently discussed this 

 branch of the subject most exhaustively in his 

 work on " Island Life" above mentioned. The 

 reference of a particular group of phenomena 

 the seasonal dimorphism of butterflies to the 

 direct action of the Glacial Period and the subse- 

 quent influence of the ameliorating climate, was 

 however the first step taken in this neglected 



