164 Wort man — Studies of Eocene Mammalia in the 



character par excellence in which they have made the greatest 

 progress and in which their evolution has been most marked. 

 The very early development of prehensile hands and feet and 

 the hotter arrangement of the cerebral circulation have been 

 intimately associated w T ith this progress and are therefore 

 factors of prime taxonomic value and importance. In other 

 words, the Primates have adapted themselves more widely to 

 environment than any other mammalian group, in consequence 

 of which they present well-defined skeletal indices or equiva- 

 lents, so that whenever they are sufficiently preserved, it is 

 possible to recognize them with a degree of certainty not 

 usual!}' found in other groups of the Mammalia. 



Characters of the Cerebral Circulation. 



As regards the first set of these characters (the enlargement 

 of the brain), like all other specialized features which come to 

 distinguish a group or order in the final or advanced stages of 

 its evolution, they are of necessity less distinctly marked in 

 the earlier representatives than in the later ones ; so that, as 

 we approach the point of origin, greater and greater difficulties 

 are experienced in the application of such characters as a real 

 test of affinity. Thus it is that we find in the less specialized 

 members of the Primates, such as the lemurs and the oldest 

 true monkeys, the relative development of the cerebral hemi- 

 spheres to be little, if any, greater than that of many of the 

 Insectivora ; and were we compelled to depend upon this char- 

 acter alone, it would be in many instances practically impos- 

 sible to determine whether the animal in question were a 

 Primate or an Insectivore. 



Fully recognizing the importance of thus clearly distinguish- 

 ing between these small-brained Primates and other contiguous 

 groups, in the matter of certain cranial characters, I have been 

 led to make a careful and somewhat exhaustive study of the 

 manner in which the blood supply is furnished to the cerebral 

 hemispheres. Especial attention to this subject has been given, 

 on the assumption that it must have been not only intimately 

 associated with the progressive enlargement of the cerebral 

 hemispheres in the Primates, but was in a way not now clearly 

 understood, in some degree at least, responsible for it. If it is 

 true that certain fundamental differences of this character exist 

 between the several orders, the practical advantage to the pale- 

 ontologist will be great, since it is only on very rare occasions 

 that he has to deal with complete skeletons of the extinct 

 species. In order to bring out these characters more clearly, I 

 shall first consider the Insectivora, and I begin by quoting Hux- 

 ley's statement of the course of the entocarotid in the hedge- 



