22 



a higher sphere of existence. In this destination alone may 

 we discern an adequate end and purpose in the great organic 

 scheme developed upon our planet. 



The progressive gradations in this scheme will be further 

 exemplified as I proceed to explain the principles and cha- 

 racters by which I have been guided in the formation of 

 the primary groups or divisions of the class Mammalia. 



Prior to the year 1836 it was held by comparative ana- 

 tomists that the brain in Mammalia differed from that in all 

 other vertebrate animals by the presence of the large mass 

 of transverse white fibres, called 'corpus callosum' by the 

 anthropotomist; which fibres, overarching the ventricles and 

 diverging as they penetrate the substance of either hemisphere 

 of the cerebrum, bring every convolution of the one into com- 

 munication with those of the other hemisphere, whence the 

 other name of this part the * great commissure.' In that year 

 I discovered that the brain of the kangaroo, the wombat, and 

 some other marsupial quadrupeds, wanted the ' great commis- 

 sure;' and that the cerebral hemispheres were connected 

 together, as in birds, only by the 'fornix' and ' anterior 

 commissure 1 .' Soon afterward, I had the opportunity of 

 determining that the same deficiency of structure prevailed 

 in the Ornithorhynchus and Echidna*. 



As many other modifications of structure, more or less 

 akin to those characterizing birds and reptiles, were found to 

 be associated with the above oviparous type of brain, together 

 with some remarkable peculiarities in the economy of repro- 

 duction 3 , I pointed out that the Mammalia might be divided 

 into 'placental' and ' implacentalV 



Impressed, however, with the fact that such binary divi- 

 sion, like that which might be based upon the leading differ- 

 ences of dentition, was too unequal to be natural, the larger 



1 See Philosophical Trans, for 1837, p. 87. 



2 Art. MONOTREMATA, Cyclopcedia of Anatomy and Physiology, Vo\. 111^.383. 



3 Art. MAESUPIALIA, torn. cit. p. 257. 



4 Art. MAMMALIA, torn. cit. p. 244. 



