152 AFFINITIES AND GEOGRAPHICAL 



the usual sense of naturalists, full of extremely curious minute 

 work. Their whole destiny seems to be of a high kind, for in 

 the stone record their line of forms stands parallel with others 

 in which the whole of the three lowest sub-kingdoms are 

 passed through. Polypiarian animals and encrinites appear 

 in the Silurian and many subsequent formations; at the 

 commencement of the carbonigenous era, the latter are so 

 abundant that we walk over large tracts of country, where 

 the rocks beneath our feet are almost wholly composed of 

 their remains. The Asteriadae appear in the upper Silurians, 

 and are but faintly seen until the Lias, when they become 

 conspicuous. In the Oolite, the Echinidae make their ap- 

 pearance. These are the last which we could expect to be 

 preserved in rocks, as the higher families possess no hard 

 parts ; otherwise, we might perhaps have seen the succession 

 of this class of fossils continued into the holothurise and 

 fistularidae. It cannot fail to be noticed how well the pro- 

 gression of forms agrees with the order of their appearance in 

 the geological ages. 



The ground is now cleared for the two grand series of in- 

 vertebrate animals, and first of the ARTICULATA. These are 

 generally describable as animals " composed of a succession of 

 rings, formed by the skin or outward integument, which from 

 its hardness constitutes a kind of external skeleton ;" one 

 class, however, the Annelides, have no hard investment. The 

 pedigree of the Articulata is very brief. The embryo in most 

 classes passes at once from the monad to the worm form, and 

 then the articulate character is assumed. It can therefore 

 scarcely be said that the radiate sub-kingdom comes before 

 the articulate, though the one is lower in organization than 

 the other. There is indeed reason to believe that the great 

 classes of the Articulata are distinct stirpes, the commence- 

 ment of each of which is little more than a step from the in- 

 organic form of matter. This may seem inconsistent with 

 the maxim Natura per saltum nihil agit; but maxims must 

 be obedient to facts, not facts to maxims, and we may deem 

 that a leap which in reality is none. 



