294 
IMPEEFECTION OF THE 
Chap. IX. 
aware that two palaeontologists, whose opinions are 
worthy of much deference, namely Bronn and Wood¬ 
ward, have concluded that the average duration of each 
formation is twice or thrice as long as the average 
duration of specific forms. But insuperable difficulties, 
as it seems to me, prevent us coming to any just con¬ 
clusion on this head. When we see a species first ap¬ 
pearing in the middle of any formation, it would be rash 
in the extreme to infer that it had not elsewhere pre¬ 
viously existed. So again when we find a species disap¬ 
pearing before the uppermost layers have been deposited, 
it would be equally rash to suppose that it then became 
wholly extinct. We forget how small the area of Eu¬ 
rope is compared with the rest of the world; nor have 
the several stages of the same formation throughout 
Europe been correlated with perfect accuracy. 
With marine animals of all kinds, we may safely 
infer a large amount of migration during climatal 
and other changes ; and when we see a species first 
appearing in any formation, the probability is that it 
only then first immigrated into that area. It is well 
known, for instance, that several species appeared some¬ 
what earlier in the palaeozoic beds of North America 
than in those of Europe; time having apparently been 
required for their migration from the American to the 
European seas. In examining the latest deposits of 
various quarters of the world, it has everywhere been 
noted, that some few still existing species are common 
in the deposit, but have become extinct in the immedi¬ 
ately surrounding sea; or, conversely, that some are 
now abundant in the neighbouring sea, but are rare or 
absent in this particular deposit. It is an ^excellent 
lesson to reflect on the ascertained amount of migration 
of the inhabitants of Europe during the Glacial period, 
which forms only a part of one whole geological period; 
