1903.] 



The Brain of the Archceoceti. 



327 



brain dwindle to very small proportions. In the Odontoceti the 

 olfactory bulb and its peduncle actually disappear. The Archseoceti, 

 therefore, are subject to two factors, which will account in some 

 measure for their small cerebrum. For, in addition to the smallness of 

 the brain to which most Eocene mammals are subject, there is their 

 aquatic mode of life. This causes a reduction in size of just those 

 portions of the pallium which form the greater part of the Eocene 

 hemispheres. 



In the modern Cetacea the neopallium attains to the greatest 

 absolute size which it ever reaches in any mammal. This fact cannot, 

 however, be considered fatal to the belief in the close affinity of the 

 Archaeoceti and the Cetacea, because the extraordinary dissimilarity 

 between the brains in the two sub-orders is such as we know to have 

 been produced by the operation of well-recognised causes in the 

 long lapse of time which separates the dawn of the Tertiary period 

 from the present clay. In all mammals which lead a life " in the 

 open" it has become a condition of their survival that the neo- 

 pallium must increase in size in each successive generation : failing 

 this, the creature must either adopt a " retired and safe mode of life " 

 or become extinct. Numerous examples might be quoted in support 

 of this hypothesis. But the case of the Sirenia shows us how little 

 we really know of the factors which determine the size of the brain. 

 These creatures began the struggle for existence in Eocene times with 

 relatively large brains, in spite of their aquatic mode of life ; and 

 they have been succeeded by generations of descendants whose latest 

 progeny at the present day have a brain-equipment only slightly 

 superior to their earlier Tertiary ancestors (vide 'Catalogue,' op. cit, 

 p. 344, et seq.). Even if we admit that the modern Manatees and 

 Dugongs- lead an eminently safe and retired life, which is in marked 

 contrast to the venturesome and " open " life of the whales and 

 porpoises, much still remains to be satisfactorily explained. 



Perhaps the most striking feature of the brain of Zeugloclon is the 

 extreme disproportion between the size of the enormous cerebellum 

 and the diminutive cerebrum. In this respect the fossil brain presents 

 a most marked contrast to that of all recent mammals, and especially 

 to that of the Cetacea. This relatively great size of the cerebellum 

 is not peculiar to the Archaeoceti, but is common to other extinct 

 mammals of large size. In my memoir on the brain in the Edentata* 

 the difficulty presented itself of adequately explaining a similar 

 phenomenon in Glyptodon ; and it must be borne in mind, in even 

 attempting to do this, (1) that the obtrusive greatness of the cere- 

 bellum presents itself only in large mammals and not in lowlier 

 vertebrates, and (2) that the size of the cerebellum is not proportionate 



* "The Brain in the Edentata," 'Linnean Society's Trans.,' 2nd series, Zoology, 

 vol. 7, part 7, 1899, p. 381. 



