SUCCESSION OF ANIMALS AND PLANTS. 151 



spines out of it, and every Fish in its own way; Turtles 

 their shields, Birds their wings, Quadrupeds their legs, and 

 Man, like all other Vertebrates, his whole skeleton; and 

 during each successive period in the history of our globe, 

 these structures are different in different species. What 

 similarity is there between these facts ? Do they not plainly 

 indicate the working of different agencies excluding one 

 another ? Truly the noble frame of Man does not owe its 

 origin to the same forces which combine to give a definite 

 shape to the crystal. And what is true of the carbonate 

 of lime is equally true of all inorganic substances; they 

 present the same characters, in all ages past, as those which 

 they exhibit now. 



Let us look upon the subject again in another light, and 

 we shall see that the same is also true of the influence of 

 all physical causes. Among these agents the most power- 

 ful certainly is electricity; the only one to winch, though 

 erroneously, the formation of animals has ever been directly 

 ascribed. The effects which it now produces, it has 

 always produced, and produced them in the same manner. 

 It has reduced metallic ores and various earthy minerals 

 and deposited them in crystalline form, in veins, during 

 all geological ages; it has transported these and other 

 substances from one point to another, in times past, as we 

 may do now in our laboratories under its influence. Eva- 

 poration upon the surface of the earth has always pro- 

 duced clouds in the atmosphere, which, after accumulating, 

 have been condensed in rain showers in past ages as now. 

 Rain-drop marks in the carboniferous and triassic rocks 

 have brought to us this testimony of the identity of the 

 operation of physical agents in past ages, and remind 

 us that what these agents do now they also did in 

 the same way in the oldest geological times, and have 



