EUTHERIA CORPUS CALLOSUM 



117 



and their mammalian 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. 7V) being small, 

 that seems to be not more than a slight difference of degree. 1 

 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- 

 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, but 



hip.com. 



anl.com 



jnecl 



-ve.nl.3 



section. ant.com, Anterior commissure ; 

 cbl, cerebellum ; c.mam, corpus mammil- 

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

 corpora quadrigemina ; gang.hab, ganglion 

 habenulare ; hip.com, hippocampal com- 

 missure ; med, medulla oblongata ; mid.com, 

 middle commissure ; olf, olfactory lobe ; 

 opt, optic chiasma ; tub.olf, tuberculum 

 olfactorium ; vent. 3, third ventricle. (From 

 Parker and Haswell's Zoology.) 



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 FIG. 57._Brain of Echidna aculeata ; sagittal 



the fact that this tooth is 

 differentiated contemporane- 

 ously with another series 

 formerly held to belong to 

 the so-called prelacteal denti- 

 tion. 2 There still remains, of 

 course, the actual fact that 

 the milk dentition is not for the most part functional, but its 

 significance breaks down 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 Eutherian. 



Huxley regarded the peculiarities in the reproductive organs 



1 Moreover, the " corpus callosum and the anterior commissure ... in ... Erin- 

 uceus and Dasypus are almost Monotreme-like." 



2 See Wilson and Hill, Quart. J. Micr. 3d. xxxix. 1899, p. 427. 



