EUTHERIA CORPUS CALLOSUM 



117 



and their maininalian relatives higher in the series ; but it is 

 not a character that should have been made use of by Huxley, 

 since he believed in the existence of a corresponding element in 

 the Dog. As to the corpus callosum (Fig. 50, p. 77) being small, 

 that seems to be not more than a slight difference of degree.^ 

 A number of other characters of secondary importance were added 

 by Huxley to the weight of evidence which led him to form a 

 group Metatheria for the Marsupials. Some of these, however, 

 are now known to be not evidence in that direction. For in- 



'.hezb 



hip.coTTV 



o-Ttt. com 



7ne<i- 



stance he observed that no 

 Marsupial had more than a 

 single successional tooth. It 

 seems at the present moment 

 to be fairly clear that Marsu- 

 pials have a milk dentition 

 like other Eutherians, l)ut 

 that only one of these teeth, 

 the fourth premolar, comes 

 to functional maturity. That 

 it is really one of a complete 

 milk series is evidenced by y\g. 57 

 the fact that this tooth is 

 differentiated contemporane- 

 ously with another series 

 formerly held to belong to 

 the so-called prelacteal denti- 

 tion."^ There still remains, of 

 course, the actual fact that 

 the milk dentition is not for the most part functional, l)ut its 

 significance breaks dow^n with these fresh discoveries. Of this 

 Professor Osborn has remarked : " The discovery of the complete 

 double series seems to have removed the last straw from the 

 theory of the marsupial ancestry of the Placentals." But Huxley 

 did not lay much stress upon this matter of the teeth, since he 

 observed that similar suppressions of the milk dentition were to 

 be found in many other mammals admittedly Eutheriau. 



Huxley regarded the peculiarities in the reproductive (»rga.ns 



tub.olf 

 eoLforn 



Brain of Ei-hidiiu aculeata ; sagittal 

 section. ant.com. Anterior eoniiuissure ; 

 cbl, cerebellum ; cm am, corpus mamniil- 

 lare ; col.forn, column of the fornix ; c.qu, 

 corpora quadrigemina ; gang. hah, ganglion 

 habenulare ; hvp.com, hipiiocampal com- 

 missure ; med, medulla oblongata : mld.coin, 

 middle commissure ; olf, olfactorj' lobe ; 

 opt, optic chiasma ; tuh.olf, tuberculum 

 olfactorium ; vent. 3, tliird ventricle. (From 

 Parker and Haswell's Zoology.) 



- Moreover, the " corpus callo.sum and the anterior coniniissurc ... in 

 aceus and Dasi/pus are almost Monotrenie-like." 



• See Wilson and Hill, Quart. .7. Mier. Sci. xxxix. 1899, p. 427. 



Erin- 



