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 continents, 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 specieseof 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. ‘ 
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