IN MODERN SCIENCE. 



125 



duced, while many old forms have perished. 

 This is somewhat unfortunate, since, in such 

 a period of stagnation as that in which we 

 live, we can scarcely hope to witness either 

 the creation or the evolution of a new species. 

 Evolutionists themselves — those, at least, who 

 are willing to allow their theory to be at all 

 modified by facts — now perceive this ; and 

 hence we have the doctrine, advanced by 

 Mivart, Le Conte, and others, of "critical 

 periods," or periods of rapid evolution alter- 

 nating with others of greater quiescence. It 

 is further to be observed here that in a limited 

 way and with reference to certain forms of 

 life we can see a reason for these intermittent . 

 creatipns. The greater part of the marine 

 fossils known to us are from rocks now raised 

 up in our continents, and they lived at periods 

 when the continents were submerged. Now, 

 in geological time these periods of submer- 

 gence alternated with others of elevation ; and 

 it is manifest that each period of continental 

 submergence gave scop'e for the introduction 

 of numbers of new marine species, while each 

 continental elevation, on the other hand, gave 

 opportunity for the increase of land-life, fur- 

 ther, periods when a warm climate prevailed- 

 ii« 



