; probably because the whole area of the early
formations now exposed to our researches was elevated at the end of the
Palaeozoic period, and remained so through the interval required for the
organic changes which resulted in the fauna and flora of the Secondary
period. The records of this interval are buried beneath the ocean which
covers three-fourths of the globe. Now it appears highly probable that a
long period of quiescence or stability in the physical conditions of a
district would be most favourable to the existence of organic life in
the greatest abundance, both as regards individuals and also as to
variety of species and generic group, just as we now find that the
places best adapted to the rapid growth and increase of individuals also
contain the greatest profusion of species and the greatest variety of
forms,--the tropics in comparison with the temperate and arctic regions.
On the other hand, it seems no less probable that a change in the
physical conditions of a district, even small in amount if rapid, or
even gradual if to a great amount, would be highly unfavourable to the
existence of individuals, might cause the extinction of many species,
and would probably be equally unfavourable to the creation of new ones.
In this too we may find an analogy with the present state of our earth,
for it has been shown to be the violent extremes and rapid changes of
physical conditions, rather than the actual mean state in the temperate
and frigid zones, which renders them less prolific than the tropical
regions, as exemplified by the great distance beyond the tropics to
which tropical forms penetrate when the climate is equable, and also by
the richness in species and forms of tropical mountain regions which
principally differ from the temperate zone in the uniformity of their
climate. However this may be, it seems a fair assumption that during a
period of geological repose the new species which we know to have been
created would have appeared; that the creations would then exceed in
number the extinctions, and therefore the number of species would
increase. In a period of geological activity, on the other hand, it
seems probable that the extinctions might exceed the creations, and the
number of species consequently diminish. That such effects did take
place in connexion with the causes to which we have imputed them, is
shown in the case of the Coal formation, the faults and contortions of
which show a period of great ac