THE VEGETABLE AND ANIMAL KINGDOMS. 113 



accounts for. We have seen that, overlooking an era with 

 doubtful vestiges of life, there was first one in which only 

 sea plants and invertebrate marine animals flourished ; after- 

 wards one presenting the meaner (cartilaginous) fishes ; and 

 that higher (osseous) fishes, reptiles, birds, and mammals, 

 came at long intervals throughout the subsequent ages. 

 Even when we pass into details, we find the succession to be 

 in so many instances conformable to the gradation of the 

 special groups of animals, that no doubt can remain that such 

 is the case in all. Thus, for example, there is, between the 

 Silurians and the oolite, a clear advance from humble to more 

 highly organized cephalopoda and echinodermata. In the 

 same time, the trilobite is exchanged for the superior but con- 

 nected form, limulus, and the brachiopoda sink beneath the 

 new and superior class of bivalves, the lamellibranchiata. 

 We see, in an order of fishes of the carbonigenous era, an 

 approximation to the reptile class. By-and-by, come ichthy- 

 osauri, half-fish half-crocodile; afterwards, a succession of 

 forms ending in true crocodiles. Some difficulties have, 

 indeed, been brought forward ; but they are, on a just view 

 of the sciences concerned, of no real importance, and I only 

 deem them worthy of notice in a subordinate place. ( 55 ) 



Leaving for a future section the particulars of the animal 

 scale, which will there lend us further illustration, it may 

 now be observed that, while the external features of the 

 various creatures are so different, there has been traced, 

 throughout large groups of them, a fundamental unity of 

 organization, as implying, with respect to these groups, that 

 all were constructed upon one plan, though in a series of im- 

 provements and variations giving rise to the special forms, 

 and bearing reference to the conditions in which each animal 

 lives. Starting from the primitive germ, which, as we have 

 seen, is the representative of a particular order of full-grown 

 animals, we find all others to be merely advances from that 

 type, with the extension of endowments and modification of 

 forms which are required in each particular case ; each form, 

 also, retaining a strong affinity to that which precedes it, 



