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 accordance 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
intermixture 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-establishment
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 _Invertebrate_ animals of the Post-Pliocene deposits require
no further mention--all the known forms, except a few of the shells
in the lowest beds of the formation, being identical 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.
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