GRADATION AMONG ANIMALS. 97 



marshes and the fern-forests of the Carboniferous 

 period, Reptiles and Insects found their place ; 

 and only when the earth was more extensive, 

 when marshes had become dry land, when islands 

 had united to form cpntinents, when mountain- 

 chains had been thrown up to make the inequali- 

 ties of the surface, were the larger quadrupeds 

 introduced, to whose mode of existence all these 

 circumstances are important accessories. 



But while all the types and most of the classes 

 were introduced upon the earth simultaneously 

 at the beginning, these types and classes have 

 nevertheless been represented in every great geo- 

 logical period by different sets or species of ani- 

 mals. In this sense, then, there has been a gra- 

 dation in time among animals, and every succes- 

 sive epoch of the world's physical history has had 

 its characteristic population. We have found 

 that there is a correspondence between the grada- 

 tion of structural complication among adult ani- 

 mals as known to us to-day, which we may call 

 the Series of Rank, and the gradation of embry- 

 ological changes in the same animals,- which we 

 may call the Series of Growth ; and there is 

 also a correspondence between these two series 

 and the order of succession in time, that estab- 

 lishes a certain gradation in the introduction of 

 animals upon earth, and which we may call the 

 Series of Time. 



