188 LAY SERMONS, ESSAYS, AND REVIEWS. [x. 



kind dawns upon the mind ; and under this aspect the smallness 

 of the total change becomes as astonishing as was its greatness 

 under the other. 



There are two hundred known orders of plants ; of these not 

 one is certainly known to exist exclusively in the fossil state. 

 The whole lapse of geological time has as yet yielded not a single 

 new ordinal type of vegetable structure. 1 



The positive change in passing from the recent to the ancient 

 animal world is greater, but still singularly small. No fossil 

 animal is so distinct from those now living as to require to be 

 arranged even in a separate class from those which contain 

 existing forms. It is only when we come to the orders, which 

 may be roughly estimated at about a hundred and thirty, that 

 we meet with fossil animals so distinct from those now living as 

 to require orders for themselves ; and these do not amount, on 

 the most liberal estimate, to more than about 10 per cent, of 

 the whole. 



There is no certainly known extinct order of Protozoa ; there 

 is but one among the Ccelenterata that of the rugose corals ; 

 there is none among the Mollusca ; there are three, the Cystidea, 

 Blastoidea, and Edrioasterida, among the Echinoderms ; and two, 

 the Trilobita and Eurypterida, among the Crustacea; making 

 altogether five for the great sub-kingdom of Annulosa. Among 

 Vertebrates there is no ordinally distinct fossil fish : there is 

 only one extinct order of Amphibia the Labyrinthodonts ; but 

 there are at least four distinct orders of Reptilia, viz. the 

 Ichthyosauria, Plesiosauria, Pterosauria, Dinosauria, and perhaps 

 another or two. There is no known extinct order of Birds, and 

 no certainly known extinct order of Mammals, the ordinal 

 distinctness of the &quot; Toxodontia &quot; being doubtful. 



The objection that broad statements of this kind, after all, 

 rest largely on negative evidence is obvious, but it has less force 

 than may at first be supposed ; for, as might be expected from 

 the circumstances of the case, we possess more abundant positive 

 evidence regarding Fishes and marine Mollusks than respecting 



1 See Hooker s &quot; Introductory Essay to tlie Flora of Tasmania,&quot; p. xxiii. 



