1865.] and Homological Interpretations. 133 



Marsupial brains are homologous with the ' anterior commissure,' why 

 should the name of f corpus callosum ' he refused to them ? " 



" These fibres [as Mr. Flower repeats after me] are part of the great 

 system of transverse fibres bringing the two hemispheres into connexion 

 with each other," &c. * 



But however germane such speculations may be to Philosophical Ana- 

 tomy, they are altogether out of place in plain zoological definitions. In 

 these, to be of use or to be understood, we must adopt Linneean sharply- 

 defined terms, such as 'bract,' 'spath,' 'sepal,' 'petal,' 'paw,' 'foot,' 

 ' fin,' ' hand.' If the zoologist believes that he has found characters in the 

 brain leading to an improved classification of a group, he must enunciate 

 those characters in terms by which they will be understood, agreeably with 

 the current and accepted anatomical definitions of the part. It may be long 

 ere either my homological notices or my successors' lead Anthropotomists 

 to dissociate the ' psalterial fibres ' or ' transverse commissure of the hippo- 

 campi ' from the rest of their complex idea of a ' fornix,' or compel them to 

 change the definition of that part of the brain. 



No amount of subtle suggestions of signification of delicate radiations of 

 fibres or laminae will make the ' hippocampal commissure ' of the Wombat 

 equivalent to the ' corpus callosum ' of the Beaver, in the eye of the natu- 

 ralist : if the essential element of his idea of a ' corpus callosum ' be a mass 

 of transverse fibres crossing the hemispheric fissure, and he does not find 

 them there on divaricating the hemispheres, he will not see them elsewhere 

 at anybody's bidding. 



If a group of mammals want such commissural fibres, and another group 

 possess them, the classifier will avail himself of a well-defined term expressing 

 such difference, without prejudice to his reception of any homological de- 

 termination of the parts, or their rudiments, in anatomical works of the ap- 

 plier of the term. 



Finally, I submit the following contrast. Mr. Flower represents the sum 

 of " the literature of the subject " of his paper (p. 71 ) as " a statement by 

 Professor Owen (Phil. Trans. 1837) of the absence in the Marsupials of 

 the ' corpus callosum/ " and he opposes to that statement " the result of 

 his present investigation" (ib.). MM. F. Cuvier and Laurillard, in their 

 description of the marsupial brain, in the posthumous edition of the ' Le- 

 cons d' Anatomic Comparee,' sum up my contribution to the literature of the 

 subject as follows : L'observation de M. Owen sur cette disposition du 

 ccrveau des marsupiaux a etc repoussee a tort comme errone'e. II ne nie 

 pas 1'existence du ' corps calleux ' dans les marsupiaux, comme on 1'a sup- 

 pose ; il declare formellement qu'on pent voir, si on le veut, dans ce qui 

 reste de la commissure, le rudiment d'un ' corps calleux ' ; mais il releve 

 avec raison 1' absence dans les marsupiaux d'un ' corps calleux ' comparable 

 a celui des autres mammiferes." Vol. iii. p. 101, 8vo, 1845. 

 * Proc. Roy. Soc. vol. xiv. p. 73. 



