MAMMALIA. 463 
supremacy to development of the mental faculties. The 
brain develops in a typical vertebrate manner, and we may 
here merely note the following characteristics :— 
1. The cerebral hemispheres are large and encroach 
backwards over the thalamencephalon and the optic lobes. 
In the higher types their surface becomes much convoluted 
and they cover the cerebellum. 
2. The cerebral hemispheres are united across the 
middle line by the corpus callosum. 
3. The optic lobes become divided to form four, the 
corpora quadrigemina. 
4. One of the most striking characters of the mammalian 
brain is the great increase in proportionate size that has 
taken place in comparison with the brain of extinct forms. 
The brain of the Eocene mammals was far smaller in pro- 
portion to the total bulk than that of modern forms. This 
is probably due to the fact that since that epoch the race 
has not been so much to the strong as to the “cunning.” 
In the same way, if we compare the weight of a mammal’s brain 
with the total weight of the body, we find that there are three impor- 
tant laws. 
Firstly, in equally organised animals the relative weight of brain 
decreases with increase in size. Thus the smallest animals tend to have 
proportionately heavier brains. The relative brain-weight of a cat is 
given as z4,, whereas that of a tiger is ¢tg. On account of this law, 
we find that the relative brain-weight of man (#5) is exceeded by that 
‘of the lesser shrew (345) and the whiskered bat (75). 
Again, the relative brain-weight increases very rapidly in proportion 
to the organisation of the animal and in animals of equal size it varies 
with the organisation. 
Thus we may cite from Dubois the following equal-sized species:— 
Siamang (Simiide) . { 
Budlug (Cercopithecidze) Oey A, 
Civet- Cats. cocsinivce aceneaonnes Carnivora,....... eu 
Javan Pangolin,.... Edentata, ..........66655 4 
If the effect of the varying size of mammals be eliminated, a table 
showing degree of ‘‘cephalisation” can be formed, and this agrees 
generally with the recognised succession of the mammalian orders, the 
Metatheria, Edentata, Rodents and Insectivora taking the lowest 
places, followed by Ungulata, Cetacea, Carnivora and lower monkeys, 
and, lastly, anthropoid apes and man. 
Thirdly, taking extinct mammals into account, it would appear that 
in mammals of similar size and bodily organisation the relative brain- 
weight increases with the time, as we have seen that the greatest 
advance from Eocene times has been cerebral. 
