The Progress of Species. 157 



173. Nay, the progress of species does not even 

 mean, in any uniform sense, that the earliest species 

 under a type were necessarily the lowest, ProgreM not 

 to be followed always by a higher, a Descent of 

 Echinoderms, or the highest type of s P ecies - 

 radiates, were represented by species called cystids 

 and crinids, long before the inferior type of polyps 

 existed. The highest group of cryptogams, the 

 ground pines, were a prevailing form of terrestrial 

 vegetation long before there were mosses. There 

 were huge crocodilians in the world, long before 

 there were limbless snakes. 



174. To do justice to the good taste of Mr. Dar- 

 win, as also of Messrs. Haeckel and Vogt, in ad- 

 mitting that, after all, at least the principal species 

 of animals must have sprung from different origins, 

 we must remember that the systems of animal 

 structure classified as the zoophyte, the radiate, the 

 molluscan, the articulate and the vertebrate, are 

 irreducible to one another: they are not on the 

 same plan. " Can it be said," argues Flourens, 

 "that there is only one form of nervous system? 

 Is the nervous system of the zoophyte the same as 

 that of the mollusk, that of the mollusk the same 

 as that of the articulate animal, etc.? If not, how 

 can they be of one type?" And, carrying back the 

 same analysis to the embryos of these classes, Mul- 

 ler says: " The human embryo never resembles a 

 radiate, an insect, a mollusk, a worm. The plan of 

 formation in these animals is altogether different 



