THE INTRODUCTION OF NEW SPECIES, 19 
differ from the temperate zone in the uniformity of 
their climate. However this may be, it scems 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 Paleozoic 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. 
co 2 
