220 



EXPLANATIONS. 



to account for the development of one long line, but of 



many comparati vely short ones. And, as I have also re- 

 marked, there is a rapidity of generation amongst the 

 lower animals which may well suggest something like 

 that "rush of life," which, if we were to judge from 

 British strata alone, would seem to ha ye taken place in 

 the early seas. But, fortunately, none of these specu- 

 lative answers to the objection are required ; for the ques- 

 tion first arises, Does the lowest band of the English 

 Lower Silurians indicate, beyond all question, the point 

 of time at which animal life commenced upon our planet ? 

 Are we quite sure that cephalopoda were among the first 

 of all earth's living creatures ? Far from it. It has 

 only been ascertained that certain comparatively small 

 cephalopods are found as far down as any other animals 

 of inferior organization at certain spots in Wales and 

 Cumberland. When we remember that, in modern seas, 

 certain kinds of such animals haunt special places suit- 

 able for their subsistence — that we may have Crustacea 

 and mollusks exclusively at one place, and radiata (as 

 corals and zoophytes) at some other, not perhaps far dis- 

 tant, but different with respect to depth or some other 

 circumstance — we can conceive that cephalopods may 

 occur in the first fossil bands in the places which have 

 been examined in England, and yet remains of inferior 

 animals may be found by themselves on the same or a 

 lower level in some as yet unexplored place not far off; 

 so that a time-interval may there appear to allow for a 

 progressive development. Such seems but a reasonably 

 cautious surmise, when we are told by a high authority 

 that there are " detached Silurian districts in England, 

 presenting particular changes and modifications, arising 

 from difference of depth, and the variety of currents, and 

 chemical combinations in the seas in which they were 

 formed;" and that, "in consequence of this variety of 

 physical condition, there is a correbponding diversity in 

 the traces of organic life in each situation."* What, 

 however, places the matter beyond doubt is, that in 

 North America, where the early stratified rocks are even 

 more amply developed than with us, the highest inver- 

 tebrated forms do not appear at the first. In the earliest 

 ascertained fossiliferous strata, the Potsdam Sandstone, 

 * Professor Phillips, BrHish Association, 1845. Athenaeum's 

 Report 



