562 ANNUAL KEPOKT SMITHSONIAN INSTITUTION, 1912. 



such actions as an arboreal mode of life demands (and secures, by 

 the survival only of those so fitted) and also a well-developed mus- 

 cular sensibility to enable such acts to be carried out with precision 

 and quickness. In the struggle 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, 

 Idnsesthetic, and motor functions, as well as to the purpose of pro- 

 viding 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 

 agUity. Moreover, these modifications do not interfere with the 

 primitive characters of limb and body. These small arboreal crea- 

 tures were thus free to develop their brains and maintain all the 

 plasticity of a generahzed structure, which eventually enabled them 

 to go far in the process of adaptation to almost any circumstances 

 that presented themselves. 



Amongst the members of this group, as in all the other mam- 

 malian phyla, the potency of the forces of natural selection was 

 immensely enhanced by the fact that the inquisitiveness of an ani- 

 mal which can learn by experience — i. e., is endowed with intelli- 

 gence — ^was leading these plastic insectivores into all kinds of situa- 

 tions which were favorable for the operation of selection. Various 

 members of the group became speciahzed in different ways. Of 

 such specialized strains the one of chief interest to us is that in 

 which the sense of vision became especially sharpened. 



THE ORIGIN OF PRIMATES. 



Toward 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 Tarsius,* quite emancipated the 

 creature from the dominating influence of olfactory impressions, the 

 sway of which was already shaken, but not quite overcome when its 

 tupaioid ancestor took to an arboreal life. This change was asso- 

 ciated with an enormous development of the visual cortex in the neo- 

 palUum, which not only increased in extent so as far to exceed that 



' " On the Morphology of the Brain in the Mammalia, with Special Reference to that of the Lemurs, 

 Recent and Extinct," Trana Linn. Soc. Lond., second series; Zoology, vol. 8, part 10, February, 1903. 



