CONCLUSION. 5 6i 



looked upon as parts of one and the same great cycle. We note, 

 as we advance from Pliocene times, how the climatic conditions 

 of the colder epochs of the Glacial Period increase in severity 

 until they culminate with the appearance of that great northern 

 mer de glace which overwhelmed all Northern Europe, and 

 reached as far south as the 50th parallel of latitude in Saxony. 

 Thereafter the glacial epochs decline in importance until in the 

 Postglacial period they cease to return. The genial climate of 

 Interglacial ages probably also attained a maximum towards the 

 middle of the Pleistocene Period, and afterwards became less 

 genial at successive stages, the temperate and equable condi- 

 tions of early Postglacial times being probably the latest 

 manifestation of the Interglacial phase. The Ice Age, there- 

 fore, cannot be considered as a prolonged and uninterrupted 

 period of glaciation, separating, as by an impassable barrier, one 

 geological horizon from another. Ever and anon the character- 

 istic glacial conditions vanished, and a mild and genial climate 

 succeeded, thus giving rise, again and again, to great migrations 

 and numerous extinctions and modifications of species, and to 

 the present peculiar distribution of fauna and flora. It was 

 probably by similar means that species were dispersed and 

 modified at much more distant epochs in the world's history ; 

 and if it be true that the greater changes of climate of which 

 this volume treats were brought about by astronomical and 

 physical causes, then we cannot doubt that the same causes 

 must have been in operation at different and widely separated 

 periods in the past. Already many indications that such was 

 the case have been noted, and more particularly, as might have 

 been expected, in the Tertiary formations. Within the past few 

 years the late Signor Gastaldi's view that the great erratics of 

 the Miocene of the Moncalieri-Valenza hills, near Turin, are of 

 glacial origin, has been extended to various other regions of 

 Europe j 1 and that such observations will continue to multiply as 



1 See Jules Martins : Bull. Soc. Giol. France, 2" Ser. t. xix. pp. 153, 450 ; 

 3° Ser. t. i. p. 390 ; t. ii. p. 269 ; Observations sur divers produits d'origine gla- 

 ciaire en Bourgogne, Paris, 1873. The phenomena described by M. Martins have 

 been otherwise explained by M. Delafond {Bull. Soc. Giol. France, 3 e Ser. t. iv. 



2o 



