162 On (he Differences between Types in the 



above them, have followed at a later period ; that still later the birds, 

 which, according to their anatomy, rank above reptiles, have next 

 made their appearance ; and that mammalia, which stand highest, 

 have been introduced last, and even among these the lower families 

 seem to have been more numerous, before the higher ones prevailed 

 over them. Man, at last, has been created, only after all other types 

 had acquired their full development. These facts which, in such 

 generality are fully exemplified in every country in the order of suc- 

 cession of the different fossil characteristics of the various geological 

 deposits, shew plainly that a gradation really exists in this succession, 

 and constitutes one of the most prominent characters of the develop- 

 ment of the animal kingdom as a whole. 



If we investigate, however, this gradation, and the order of succes- 

 sion of animals more closely, we cannot but be struck with the differ- 

 ent relations which exist between the fossils and the living animals. 

 Many extinct types have been pointed out as characteristic of differ- 

 ent geological periods, which combine, as it were, peculiarities which 

 at present are found separately in different families of animals. 



I may mention as such, the Ichthysaur, with their fish-like ver- 

 tebrae, their dolphin or porpoise-like general form, and several spe- 

 cial characters reminding us of their close relation to the Crocodilian 

 reptiles ; thus combining characters of different classes in the most 

 extraordinary manner. 



Again, the Pterodactyli, in which reptilian characters are com- 

 bined with peculiarities reminding us both of birds and bats. 



Again, the large carnivorous fishes of the coal period, combining 

 peculiarities of the Saurians, with true fish characters ; and so on. 



These relations are of an entirely different kind from those which 

 I have pointed out between some of the older fossils and the early 

 stage of growth of the living representatives of the same families. 



For instance, the fossil fishes with a heterocercal tail, found be- 

 low the new red sandstone, down to the lowest deposits, reminds us 

 of the peculiar termination of the vertebral column in all fish em- 

 bryos of species living in the present period, to whatever family they 

 may belong, indicating a similarity of structure in the oldest repre- 

 sentative of this class, with the earliest condition of the germs of 

 those animals in our days. 



Let us now examine whether we can properly understand the 

 bearings of these relations, and the meaning of such differences. 



In the first place, I have mentioned the gradual progress, which 

 is observed in the succession of the different classes of Vertebrata. 

 This progress is exemplified by a series of types which differ from 

 each other, but which shew, when arranged in a series, a gradation 

 which agrees in general with the structural gradation, which we may 

 establish upon anatomical evidence. For instance, the salamanders, 

 with their various forms, rank below the tailess Batrachians. 



And where we have a succession of those animals in the tertiary 



