Evuiot SmitH, Brain of Archeocet. 47 
ance, and becomes much less peculiar, even though it may not 
be wholly explained. 
In most Eocene mammals the cerebral hemispheres were 
exceedingly diminutive in comparison with those of their mod- 
ern descendants and successors. Moreover, the bulk of the 
primitive mammalian hemisphere was composed of those parts 
(hippocampus and lobus pyriformis), which are pre-eminently 
olfactory ; in other words, the neopallium (z.e., that part of the 
pallium which is neither hippocampus nor pyriform lobe) is 
especially insignificant. It is a well-known fact that the sense 
of smell loses much of its importance in mammals of aquatic 
habits (e.g., Ornithorhynchus, the Sirenia, the Pinnipedia, and 
especially the Cetacea), and in these animals the olfactory parts 
of the brain dwindle to very small proportions. In the 
Odontoceti the olfactory bulb and its peduncle actually disap- 
pear. The Archeoceti, 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 hemi- 
spheres. 
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 Archzoceti 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 pres- 
ent day. In all mammals which lead a life ‘‘in the open’’ it 
has become a condition of their survival that the neopallium 
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 
