APPARITION OF SPECIES 107 



and the mammals of the early Tertiary. A remark 

 able contrast is afforded by the later Tertiary and 

 modern time, in which, with the exception of man 

 himself, and perhaps a very few other species, no new 

 forms of life have been introduced, 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. Evolu 

 tionists 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 alternating withj 

 Bothers of greater quiescence. It is further to be ob 

 served here that in a limited way and with reference 

 to certain forms of life we can see a reason for these 

 intermittent creations. 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 submergence alternated with 

 others of elevation ; and it is manifest that each 

 period of continental submergence gave scope 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 in the 

 Arctic regions periods when plants such as now live 



