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 



