138 MAMMALIAN DESCENT. [Lect. Y.. 



mammal, and have superficial cartilages strapped to them.. 

 These superficial cartilages are largest in the Chimseroid 

 fishes ; and the deej) cartilages lessen in bulk as we 

 ascend into the various culminations of the fishy, rep- 

 tilian, and avian types. Here, in the Hedgehog, we 

 are much nearer the cartilaginous fishes in this respect 

 than when amongst Eeptiles and Birds ; in this, as in 

 some other things, even the lower kinds of the Eutheria, 

 or placental Mammalia, show a relationship to the 

 lowest sorts of fishes known. But there are no fishes 

 now living, low enough, or generalised enough, to he 

 at all good living representatives of the stock out of 

 which the highest and Ijest sort of living Vertebrata 

 must have arisen. We must, however, try and be con- 

 tent with such tilings as we have, and by comparing 

 the early stages of the mammal with even the perma- 

 nent condition of some of the cartilaginous fishes, we 

 do get some light upon this dark and difticult subject. 

 If you will ])ut together what is familar to you in the 

 parts of the human face, and recollect some of the many 

 things I have from year to year reiterated al)out the 

 face of Sharks and Skates, you will have a very good 

 idea of the marvellous transformation the mammalian 

 embryo undergoes during development. Meckel's carti- 

 lages (the deep or inner lower jaws) are immense in the 

 embryo of the Hedgehog ; they meet and unite in front, 

 at the chin, and there form a single bar or basi- 

 mandil)ul;ii' rod. Just l)chind this single, terminal, part 



