Contributions to the theory of natural selection. A series of essays ... By Alfred Russel Wallace ...

18 ON THE LAW WHICH IIAS REG ULATED
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