194 OYSTERS, AND ALL ABOUT THEM. 



from time to time, the intensity of volcanic heat being 

 neither greater nor less than it is now. () 



The internal heat of the earth, the elevation and 

 depression of its crust, its belchings forth of vapours, ashes, 

 and lava, are its activities, in as strict a sense as are warmth 

 and the movements and products of respiration the activi- 

 ties of an animal. The phenomena of the seasons, of the 

 trade winds, of the Gulf-stream, are as much the results of 

 the reaction between these inner activities and outward 

 forces, as are the budding of the leaves in spring and their 

 falling in autumn the effects of the interaction between the 

 organization of a plant and the solar light and heat. (<:) 



It was, however, while certain mineral masses were in 

 progress of formation, and yet soft, that they were replen- 

 ished with the remains of animals which had lived in the 

 waters. Vestiges of their skeletons, coverings, and shelly 

 dwellings, are still discoverable. In the various strata, 

 excepting the earliest two or three, such remains occur of 

 organised creatures, in some instances vegetable, but prin- 

 cipally animal. Even in the lowest beds they may have 

 appeared ; but they would doubtless have been destroyed 

 by the heat communicated from below. Each system of 

 strata has species which belong to itself; a fact which 

 stands among the most remarkable discoveries of modern 

 times. 



It may be safely assumed that (my) readers have a 

 general conception of the nature of the objects to which 

 the word " species ' is applied ; but it has, perhaps, 

 occurred to few, even of those who are naturalists ex 



() " Principles of Geology," Sir Charles Lyell, vol. 2, p. 211. 



(c) " Geological Reform." See Professor T. H. Huxley's "Lay 



Sermons," &c., p. 237. 



