92 GRADATION AMONG ANIMALS. 



will, in the most cursory manner, give a sketch 

 of the great geological periods, as generally ac- 

 cepted now by geologists. 



The first of these periods has been called the 

 Azoic or lifeless period, because it is the only one 

 containing stratified deposits in which there are 

 no remains of organic life, and it is therefore 

 supposed that at that early stage of the world's 

 history the necessary conditions for the mainte- 

 nance of animals and plants were not yet estab- 

 lished. After this, every great geological period 

 that follows has been found to be characterized 

 by a special set of animals and plants, differing 

 from all that follow and all that precede it, till 

 we arrive at our own period, when Alan, with the 

 animals and plants that accompany him on earth, 

 was introduced. 



There is, then, an order of succession in time 

 among animals ; and if there has been any tran- 

 sition between types and classes, any growth of 

 higher out of lower forms, it is here that we 

 should look for the evidence of it. According to 

 this view, we should expect to find in the first 

 Leriod in which organic remains are found at all 

 only the lowest type, and of that type only the 

 lowest class, and, indeed, if we push the theory 

 to its logical consequences, only the lowest form s 

 of the lowest class. What are now the facts? 

 Tb is continent affords admirable opportunities foi 



