PRESIDENTIAL ADDRESS. 583 
its instruments in, and taken firm possession of, the cerebral hemisphere, long 
before the advent in this dominant part of the brain of any adequate represen- 
tation of the other senses, but also, and chiefly, because to a small land-grubbing 
animal the guidance of smell impressions, whether in the search for food or as 
a means of recognition of friends or enemies, was much more serviceable than 
all the other senses. Thus the small creature’s mental life was lived essentially 
in an atmosphere of odours, and every object in the outside world was judged 
primarily and predominantly by its smell; the senses of touch, vision, and 
hearing were merely auxiliary to the compelling influence of smell. 
Once such a creature left the solid earth and took to an aquatic or an arboreal 
life all this was changed, for away from the ground the guidance of the olfactory 
sense lost much of its usefulness; and, in the case of aquatic mammals, the 
whole smell apparatus atrophied, and in some cases vanished. We need not 
stop to consider the aquatic mammal, because a life in the water calls for such 
marked specialisation of structure that such creatures disappear from the race 
for mammalian sunremacy. But the case is very different with arboreal mammals. 
Life amidst the branches limits the usefulness of olfactory organs. but it is 
favourable to the high development of vision, touch, and hearing. Moreover, 
it demands an agility and quickness of movement that necessitates an efficient 
motor cortex to control and co-ordinate such actions as an arboreal mode of life 
demands (and secures, by the survival only of those so fitted), and also a well- 
developed muscular sensibility to enable such acts to be carried out with pre- 
cision and quickness. In the strugale for existence, therefore, all arboreal 
mammals, such as the Tree-Shrews, suffer a marked diminution of their olfactory 
apparatus, and develop a considerable neopallium, in which relatively large 
areas are given up to visual, tactile, acoustic, kinesthetic and motor functions, 
as well as to the purpose of providing a mechanism for mutually blending in 
consciousness the effects of the impressions pouring in through the avenues of 
these senses. 
Thus a more equable balance of the representation of the senses is brought 
about in the large brain of the arboreal animal; and its mode of life encourages 
and makes indispensable the acquisition of agility. Moreover. these modifications 
do not interfere with the primitive characters of limb and body. These small. 
arboreal creatures were thus free to develop their brains and maintain all the 
plasticity of a generalised structure, which eventually enabled them to go far in 
the vrocess of adaptation to almost any circumstances that presented themselves. 
Towards the close of the Cretaceous period some small arboreal Shrew-like 
creature took another step in advance, which was fraught with the most far- 
reaching consequences; for it marked the birth of the Primates and the definite 
branching off from the other mammals of the line of man’s ancestry. 
A noteworthy further reduction in the size of the olfactory parts of the brain, 
such as is seen in that of Jarsius,?" quite emancipated the creature from the 
dominating influence of olfactory impressions, the sway of which was alreadv 
shaken, but not quite overcome, when its Tupaioid ancestor took to an arboreal 
life. This change was associated with an enormous development of the visual 
cortex in the neonallium, which not only increased in extent so as far to exceed 
that of Txmaia, but also became more highly specialised in structure. Thus, 
in the primitive Primate, vision entirely usurped the controlling place once 
accunied by smell; but the sienificance of this chance is not to be measured 
merely as the substitution of one sense for another. The visual area of cortex 
is nart of the neonallium, and when its importance thus became enhanced the 
whole of the neopallinm felt the influence of the changed conditions. The sense 
of touch also shared in the effects, for tactile impressions and the related kin- 
msthetic sensibility, the importance of which to an agile tree-living animal is 
obvious, assist vision in the conscious appreciation of the nature and the various 
properties of the things seen and in learning to perform agile actions which are 
guided bv vision. 
An arboreal life also added to the importance of the sense of hearing; and 
the cortical representation of this sense exhibits a noteworthy increase in the 
22 €On the Morphology of the Brain in the Mammalia, with Special Reference 
to that of the Lemurs, Recent and Extinct,’ 7’rans. Linn. Soc. Jiond., second 
series; Zoology, vol. viii., part 10, February 1903. 
