18 ON THE LAW WHICH HAS REGULATED 
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 con- 
ditions 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 in- 
crease of individuals also contain the greatest pro- 
fusion 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 pro- 
bably 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 phy- 
sical conditions, rather than the actual mean state 
in the temperate and frigid zones, which renders 
them less prolific than the tropical regions, as exem- 
plified 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 
