14 NATURAL SELECTION j 



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 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 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 



