THE PLACOIB BEAIN. 141 



camorphosis as those of the toad uTid frog, or of the amphibia 

 generally. And some of their number, — the common dog- 

 fish, for instance, — are ovo viviparous, bringing forth their 

 young, like the common viper and the viviparous lizard, alive 

 and fully formed. 



"But such features," says the author of the "Vestiges," 

 referring chiefly to certain provisions connected with the re- 

 productory system in the placoids, " are partly partaken of by 

 families in inferior sub-kingdoms, showing that they cannot 

 truly be regarded as marks of grade in their own class." 

 Nay, single features do here and there occur in the inferior 

 sub-kingdoms, which very nearly resemble single features in the 

 placoid character and organization, — which even very nearly 

 resemble single features in the human character and organi- 

 zation j but is there any of the inferior sub-kingdoms in which 

 there occurs such a collocation of features 1 or does such a 

 collocation occur in any class of animals, — setting the pla- 

 coids wholly out of view, — which is not a high class ? Nay, 

 further, does there occur in any of the inferior sub-kingdoms, 

 — existing even as a single feature, — that most prominent, 

 leading characteristic of this series of fishes, — a large brain ? 



But is not the " cartilaginous structure" of the placoids 

 analogous to the embryonic state of vertebrated animals in 

 general ? Do not the other placoid peculiarities to which 

 the author of the " Yestiges" refers, — such as the heterocer- 

 cal or one-sided tail, the position of the mouth on the under 

 side of the head, and the rudimental state of the maxillaries 

 and intermaxillaries, — bear further analogies with the em- 

 bryonic state of the higher animals 1 And is not " embyro- 

 nic progress the grand key to the theory of development ?" 

 Let us examine this matter. " These are the characters," 

 says this ingenious writer, " which, above all, I am chiefly 

 concerned in looking to ; for they are features of embryonic 

 progress, and embryonic progress is the grand key to the 



