G. GENEKAL NATURAL HISTORY AND ZOOLOGY. 343 



The mind, untrained in scientific logic (he remarked) in its 

 generalization respecting tlie animal kingdom, instinctively 

 associates its subjects into groups determined by the nature 

 of their -habitat ; and hence we have had the Vertebrates 

 separated into (1) those especially adapted for progression 

 on land (Quadrupeds) ; (2) those especially fitted for pro- 

 gression through the air (Birds) ; and (3) those adapted for 

 life in the water (Fishes) ; while the residue, not readily com- 

 binable with either of those classes, are either, as it were, 

 overlooked, or, as the Serpents, annexed as a kind of appen- 

 dix to the Quadrupeds, because they most resemble certain 

 of those animals the lizards. It was, therefore, a great ad- 

 vance when Linnoeus established a peculiar class (Mammalia) 

 for the viviparous quadrupeds and the whales, and thus for 

 the first time subordinated habitat and adaptation therefor 

 to structure. While at the present day the ancient ideas 

 have almost entirely disappeared from the system of nature 

 so far as regards the terrestrial vertebrates, they are still to 

 a great extent prevalent in the appreciation of the relations 

 of the aquatic ones ; for among the vertebrates collectively 

 known under the name of Fishes, there ai'e differences more 

 marked and radical than those between any of the higher 

 classes of the branch. Professor Gill therefore proposes not 

 only to distinguish several classes among the so-called 

 fishes, but to widely remove them, and recombine them as 

 follows : 



On the one hand is the lancelet {Branchiostoma)^ distin- 

 guished by the extension of the notochord (primitive basis 

 of the back-bone) to the anterior end of the vertebral column, 

 the attenuation of the spinal cord forward, and its simple 

 structure, the absence of auditory organs, the simple tubular 

 structure of the heart, and the development of the liver sim- 

 ply as a diverticulum of the intestine. This type is called 

 by Hackel the Suhphylmn Leptocardia or Acrania. 



On the other hand are all the other Vertebrates, which 

 agree in the termination of the notochord behind the pitui- 

 tary fossa, the enlargement of the spinal cord forward into a 

 brain, the development of auditory organs, the division of the 

 heart into (two to four) chambers, which in part (one or two) 

 specially receive the blood, and in part (one or two) specially 

 distribute it to the body again, and the differentiation of the 



