23 



group never presenting the same degree of correspondence of 

 organic structure as the smaller moiety, I continued to pursue 

 investigations, with the view of gaining an insight into the 

 more natural and equivalent primary groups of the Mammalia; 

 having my attention more especially directed to the cerebral 

 organ in this quest. 



In 1842, I was able to demonstrate, in the ' Hunterian 

 Course of Lectures' delivered at the Royal College of Sur- 

 geons, the leading modifications of the mammalian brain, and 

 their peculiar value in classification by reason of their asso- 

 ciation with concurrent modifications of other systems of 

 organs. 



Xevertheless there were genera of Mammals, e. g. the 

 sloths, anteaters, armadillos, roussettes, giraffes, rhinoceroses, 

 &c. to which the cerebral test had to be applied. Fortunately 

 the rare species of these genera successively arrived at the 

 Zoological Gardens in London, and afforded me the means 

 of applying that test ; so that, at length, having dissected 

 the brain in one species at least, of almost every genus or 

 natural family of the Mammalian class, I felt myself in a 

 position to submit to the judgment of my fellow-labourers in 

 zoology, at the Linnamn Society, in 1857, the generalised 

 results of such dissections, comprising a fourfold primary divi- 

 sion of the MAMMALIA, based upon the four leading modi- 

 fications of cerebral structure in that class. 



In some mammals the cerebral hemispheres are but feebly 

 and partially connected together by the ' fornix ' and ' ante- 

 rior commissure:' in the rest of the class the part called 

 ' corpus callosum' is added, which completes the connecting 

 or ' commissural ' apparatus. 



With the absence of this great superadded commissure 1 is 

 associated a remarkable modification of the mode of develop- 

 ment of the offspring, which involves many other modifica- 

 tions; amongst which are the presence of the bones called 

 ' marsupial,' and the non-development of the deciduous body 



1 On the Structure of the Brain in Marsupial Animals, Philos. Trans. 1837. 



