14 
NATURAL SELECTION 
I 
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 condi- 
tions 6f 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 rich- 
ness 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 connection 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 
