418 PEOF. G. ELLIOT SMITH ON THE 



It is liavdlv necessary to insist on the Aalue of cerehral cliaraeters as an index of tlic 

 aflBnities of the animal, when it is recalled that the mammalian brain is perhaps the 

 only organ whicli can be trnly said to represent in itself tlie whole organism. For even 

 thou"h tlie l)rain is responsive to changes of habit in the animal, and innumerable other 

 factors which in various ways influence the jDroportions of its various parts and modify 

 its shape, yet, in its essential plan, it is perhaps the most conservative organ in the 

 whole body. Thus the same fundamental plan of tlie brain is found throughout the 

 Eutheria, and, with the exception of the striking changes in the constitution of the 

 cerebral commissures, it is common also to all the Metatlieria. In the foregoing notes 

 I have attempted to show that underlying the exceedingly variable and apparently 

 haphazard disposition of the sulci of the cerebral hemisphere — which, being tlie most 

 recently-moditied, are consequently the most changeable regions of the brain — there is a 

 fundamental group of homologous furrows, which are the common property of the 

 Avhole Meta- and Eutheria rather than the exclusive distinction of one Order. 



The fact of having thus discarded as obsolete and unjustifiable the belief that each 

 Order of mammals evolves for itself a number of furrows which are incapable of being 

 liomologized with those of other Orders, may seem to have broken down the barriers 

 Avhich separate the Lemurs from other mammals (for even those who have most 

 magnified the difierences between Apes and Lemurs have been compelled to adopt 

 from t!ie higher I'rimates their nomenclature for the Prosimian sulci). But this is 

 by no means so. Eor, having demonstrated the identity of the various elements 

 Avhich may unite in various manners to Ibrm the most heterogeneous series of patterns 

 in the difi'erent Orders, we are the l)etter able to appreciate the Ordinal value of 

 these varied groupings of the series of sulci, the morphological values of wiiich we 

 know, than we should if a new set of features of unknown importance were evolved in 

 each Order. 



It surely does not lessen the great systematic importance of the fact that the calcarine 

 and sujji'asylvian sulci behave in a very peculiar and identical manner in the Lemurs 

 and Apes, to know that the direct homologues of these furrows are disposed in a 

 strikingly different manner in the various other mammalian Orders. On the contrary, 

 the fact that the different patterns are composed of such stable elements enhances the 

 value of the plan of the sulci as an index of relationship. 



The peculiar constancy of the arrangement of the calcarine, retrocalcarino, and 

 paracalcarinc sulci is alone sufficient to indicate the close l)onds of affinity which unite 

 all the Lemurs, and even such difTerent members as Toi'si/i-s and C'h/roii/)/.s, and at the 

 same time separate them from all other mammals. 



The confluence of the calcarine and retrocalcarino sulci is peculiarly distinctive of the 

 Pi'imates, and in this respect the Lemurs conform to the Ordinal type. In many of the 

 Jsew- World Apes the paracalcarinc sulcus is much less perfectly developed than it is in 

 the Lemurs; and in this respect the latter closely resemble the highest Apes, in which 

 the paracalcarinc sulcus forms the ventral clement in that peculiar combination of two 

 sulci which is commonly distinguished as " parieto-occipital." In making this state- 

 ment I am not unmindful of the probability that this type of paracalcarinc sulcus may 

 be meiely its most primitive Ibrm. 



