THE INTRODUCTION OF NEW SPECIES. 19 



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 con- 

 nexion 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 of 

 great activity and violent convulsions, and it is in 

 the formation immediately succeeding this that the 

 poverty of forms of life is most apparent. We 

 have then only to suppose a long period of somewhat 

 similar action during the vast unknown interval at 

 the termination of the Palaeozoic period, and then 

 a decreasing violence or rapidity through the Second- 

 ary period, to allow for the gradual repopulation of 

 the earth with varied forms, and the whole of the 

 facts are explained.* We thus have a clue to the 

 increase of the forms of life during certain periods, 

 and their decrease during others, without recourse 



* Professor Ramsay has since shown that a glacial epoch 

 probably occurred at the time of the Permian formation, 

 which will more satisfactorily account for the comparative 

 poverty of species. 



G 2 



