142 THE PLACOID BRAIN. 



theory of development." Bold assertion, certainly ; but, then, 

 assertion is not argument ! The statement is not a reason 

 for the faith that is in the author of the "Vestiges," but 

 simply an avowal of it ; it is simply a confession, not a de- 

 fence, of the Lamarckian creed ; and, instead of being ad- 

 mitted as embodying a first principle, it must be put strin- 

 gently to the question, in order to determine whether it con- 

 tain a principle at all. 



In the first place, let us remark, that the cartilaginous 

 structure of the placoids bears no very striking analogy to the 

 cartilaginous structure of the higher vertebrata in the em- 

 bryonic state. In the case of the Delphinidce, with their soft 

 skeletons, the analogy is greatly more close. Bone consists 

 of animal matter, chiefly gelatinous, hardened by a difiusion 

 of inorganic earth. In the bones of young and foetal mam- 

 malia, inhabitants of the land, the gelatinous prevails ; in the 

 old and middle-aged there is a preponderance of the earth. 

 Now, in the bones of the dolphin there is comparatively 

 little earth. The analogies of its internal skeleton bear, not 

 on the skeletons of its brethren the mature full-grown mam- 

 mals of the land, but on the skeletons of their immature or 

 foetal offspring. But in the case of the true placoids that ana- 

 logy is faint indeed. Their skeletons contain true bone : the 

 vertebral joints of the sharks and rays possess each, as has 

 been shown, an osseous nucleus, which retains, when sub- 

 jected to the heat of a common fire, the complete form of 

 the joint ; and their cranial framework has its surface always 

 covered over with hard osseous points. But, though their 

 skeletons possess thus their modicum of bone, unlike those of 

 embryonic birds or mammals, they contain, in what is pro- 

 perly their cartilage, no gelatine. The analogy signally fails 

 in the very point in which it has been deemed specially to 

 exist. The cartilage of the Chondropterygii is a substance 

 so essentially different from that of young or embryonic birds 



