FAUNA OF THE POST-PLIOCENE. 345 
northern hemisphere by the gradual supervention of the Glacial 
period. Previous to this the climate must have been temper- 
ate or warm-temperate ; but as the cold gradually came on, 
two results were produced as regards the living beings of the - 
area thus affected. In the first place, all those Mammals 
which, like the Mammoth, the Woolly Rhinoceros, the Lion, 
the Hyzena, and the Hippopotamus, require, at any rate, mode- 
rately warm conditions, would be forced to migrate southwards 
to regions not affected by the new state of things. In the 
second place, Mammals previously inhabiting higher latitudes, 
such as the Reindeer, the Musk-ox, and the Leniming, would 
be enabled by the increasing cold to migrate southwards, and 
to invade provinces previously occupied by the Elephant and 
the Rhinoceros. A precisely similar, but more slowly-executed 
process, must have taken place in the sea, the northern Mollus- 
ca moving southwards as the arctic conditions of the Glacial 
period became established, whilst the forms proper to temperate 
seas receded. As regards the readily locomotive Mammals, 
also, it is probable that this process was carried on repeatedly 
in a partial manner, the southern and northern forms alternately 
fluctuating backwards and forwards over the same area, in ac- 
cordance with the fluctuations of temperature which have been 
shown by Mr James Geikie to have characterised the Glacial 
period as a whole. We can thus readily account for the inter- 
mixture which is sometimes found of northern and southern types 
of Mammalia in the same deposits, or in deposits apparently 
synchronous, and within a single district. Lastly, at the final 
close of the arctic cold of the Glacial period, and the re-estab- 
lishment of temperate conditions over the northern hemisphere, 
a reversal of the original process took place—the northern 
Mammals retiring within their ancient limits, and the southern 
forms pressing northwards and reoccupying their original 
domains. 
The J/nvertebrate animals of the Post-Pliocene deposits re- 
quire no further mention—all the known forms, except a few 
of the shells in the lowest beds of the formation, being iden- 
tical with species now in existence upon the globe. The only 
point of importance in this connection has been previously 
noticed—namely, that in the true Glacial deposits themselves 
a considerable number of the shells belong to northern or 
Arctic types. 
As regards the Vertebrate animals of the period, no extinct 
forms of Fishes, Amphibians, or Reptiles are known to occur, 
but we meet with both extinct Birds and extinct Mammals. 
The remains of the former are of great interest, as indicating 
