CHARACTERS OF THE CLASSES OF VERTEBRATE ANIMALS. 19 



in the " fretful porcupine," spines in the hedgehog, scales ^in the 

 manis, bony scutes in the armadillo ; whilst a few species at the two 

 extremes of the series are naked, for example, the whale and the 

 man. 



Thus far the review of the general anatomical characteristics of 

 the Mammalian class seems not to indicate a very marked superiority : 

 in the energetic contraction of the muscular fibre, in the rapidity of 

 the actions of the heart and lungs, Mammals are surpassed by Birds ; 

 but the functions which attain their highest development in the 

 Mammalian class are of far nobler character than those which are 

 more immediately connected with the maintenance of animal life. 

 The progressive expansion of the brain is greatest, and the final pre- 

 dominance of reason over instinct is achieved, in the present class : 

 sensation is its characteristic rather than muscular energy or irrita- 

 bility ; the instincts become more varied, they are also less mechanical 

 and more educable. In Mammalia we first find the cerebral hemi- 

 spheres acquiring an additional extent of the grey and vascular 

 surface by convolutions, which increase in number and depth as the 

 species approximate Man : a fornix first, and then a corpus callosum, 

 are introduced into the Mammalian brain, to bring into mutual com- 

 munication the various parts of the hemispheres. A new cranial 

 bone, the squamosal, developed from the proximal piece of the man- 

 dibular arch, now, for the first time, takes a share in the forma- 

 tion of the walls of the cranial cavity. The organs of the senses 

 attain their most complex structures in the present class. The ear 

 has a perfect spiral cochlea, and the distinct appendage called outer 

 ear. No bony plates are ever developed in the sclerotic coat of the eye. 

 The turbinated bones and pituitary membrane of tlie nose present in 

 most Mammalia a great, and in some a prodigious extent of surface. 

 The tongue is large, soft, and papillose, and is supplied with a gusta- 

 tory as well as a motor and respiratory nerve. 



We see the locomotive extremities progressively endowed with more 

 varied and complicated powers. At first retaining, in the Cetacea for 

 example, their primitive embryonic form of simple flattened fins ; 

 they next, with ampler proportions, acquire the full development of 

 the normal joints and segments, and liave their extremities enveloped 

 in dense hoofs : next we find tlie digits liberated, and armed with 

 claws confined to the upper surface, leaving the under surface of 

 the toes free for the exercise of touch : tlien we have certain digits 

 endowed with special offices, and, by a particular position, enabled to 

 oppose the others, so as to seize, retain, and grasp : lastly, in Man, 

 the offices of support and locomotion are assigned to a single pair ot 

 members, so organised as to sustain the body erect ; leaving the 



c 2 



