220 PROGRESSIVE DEVELOPMENT. 



position of the nebular theory in respect to its philosophical 

 credibility, is more fully represented in a previous portion of 

 this work. 



The theory of progressive succession in the organic king- 

 doms, as advocated by the " Vestiges," is disputed mainly on 

 the following grounds : First, that fishes of a high organization 

 occur (as it is said) in the oldest of the fossiliferous rocks ; 

 secondly, that in several instances the passage from a lower to 

 a higher system of rocks, is accompanied by an abrupt and 

 entire transition in the organic kingdoms, exhibiting none of 

 the links of progressive gradation which the theory of the 

 " Vestiges" supposes to exist ; and thirdly, that in some in- 

 stances several widely different and previously unknown 

 species seem to have been introduced at about the same epoch, 

 with apparently no links of connection between them. 



To the allegation that fishes of a high organization occur in 

 the oldest of the fossiliferous rocks, the author of the " Ves- 

 tiges," in his sequel to that work, replies by quotations from 

 geologists, showing a discrepancy in their statements upon this 

 point, which, however, he shows may be explained by the fact, 

 that since the statements of some of them were put forth, " the 

 lower fossiliferous rocks have been divided into several dis- 

 tinct formations, in the lowest of which it is fully admitted 

 there are no vertebrata. He, moreover, argues that the 

 cephalopoda and gasteropoda, mollusks of a high organization, 

 whose remains are found in the oldest series of fossiliferous 

 rocks, might, as transmuted species, have come in soon after 

 the commencement of the formation of those rocks, as owing 

 to a " rapidity of generation " and " rush of life," which is 

 sometimes characteristic of certain of the lower orders of 

 animals. 



In answer to the argument which negatives the idea of 



