63<5 ^ PllOF. R. A. DAllT ON TUE 



contour of the Zeuglodont brain. Of course, the enlarged cere- 

 bellum has played a role in the lateral expansion of the cranial 

 cavity, but the extraordinary dimensions, even for an Eocene 

 animal, of this cerebellum can only be explained by the tribute 

 coming from trigeminal nerves whose Gasserian ganglia rival the 

 cerebral cortex in size. Hence the inlluonce of the cerebellum is 

 fundamentally trigeminal in origin. 



It is true then that, although the trigeminus has played the 

 chief part iia this expansion, the internal factors include also an 

 olfactory and a visual factor and may well have been assisted in 

 Zeuglodonts, as Andrews (1908) suggested, by the "pressure 

 on the anterior end, more or less in the direction of the long 

 axis," during motion through the water, "and during very rapid 

 movement this pressure must be considerable." 



Now a reduction in value of the sense of smell and possibly of 

 sight and a compensatory dependence upon the sense of touch in 

 the muzzle — i, e., a certain degree of trigeminal specialisation — 

 are equally to be anticipated in the ancestors of modern Cetacea. 

 Might it not be that the Z^uglodontida), in this respect as in their 

 osteological conformity, are to be regarded as ancestral to Cetacea? 

 This might be admitted if it wei-e not for the demonstration of 

 the already-marked specialisation of Prozevglodon airox and the 

 gradual deterioration by specialisation within the phyletic 

 series as already discussed. In this light the passage of Cetacean 

 ancestors through a "trigeminal" stage in evolution can only be 

 cited as convergence, or may be explained by the hypothesis that 

 the Zeuglodontidse and the true ancestors of Cetacea may have 

 had a common ancestry in the earliest Eocene. 



Despite the positive views concerning the ancestral relation- 

 ship of Zeuglodontidse to Cetacea stated at the outset, it must 

 not be assumed that palaeontologists are agreed concerning this 

 interpi^etation. 



As eai'ly as 1877 Marsh said: "That the connection (between 

 Zeuglodon and Cetacea) was a direct one, however, is hardly 

 probable, since tlie diminutive brain, large number of simple 

 teeth, and i-educed limbs in the whales all indicate them to be an 

 old type Avhich doubtless branched off from the more primitive 

 stock leading to the Carnivores." 



Weber (1886, p. 243) also referred to Zetiglodon as "einen verun- 

 gliickten Yersuch Cetaceen herauszubilden," and although there 

 have been wavering opinions by many since, which even Weber 

 himself has shared, many, such as Miiller, Fraas, and Stromer, 

 have enunciated the same view. 



Fraas, in 1905, summed up the evidence to that date available 

 and showed that the Zeuglodontidse, arising from some possible 

 Creodont stock, have passed through some stage corresponding 

 to his Eocetus (Mesocetus) and, by differentiation, have produced 

 various forms, including the gigantic ones which generally have 

 marked the acme of evolutionary progress in many groups of 

 animals, after which the race has disappeared. He has felt it 



