470 EVOLUTIONARY MORPHOLOGY 



the evolution of man must be based, it remains to consider what 

 causes were probably operative during the history of human 

 descent. It may be said at once that just as the rise of the 

 mammals was due to the development of the brain and forma- 

 tion of a neopallium, so a continuation and perfection of that 

 process led to the rise of the Primates and man, and that this 

 development was largely associated with the sense of sight. 



It has been seen that the history of the Primates can be 

 traced from Insectivore-like ancestors, through Tarsioid, 

 monkey, and ape stages, and that their evolution was accom- 

 plished under arboreal conditions of life. Now, the Insecti- 

 vora, Tarsioidea, monkeys and apes have living representatives 

 at the present day, some of which have changed but little from 

 their Eocene ancestors. Without in the least suggesting that 

 these living forms are on the main line of descent (which 

 indeed it has been shown carefully that they are not), they 

 may be taken and studied for their brains and organs of sight, 

 as showing grades of structure approximately representative 

 of the stages through which it is known that the Primates 

 passed. 



Of the Insectivora, Macroscelides (the jumping shrew) may 

 be taken as a primitive mammal, in which the neopallium is 

 developed, but the archipallium related to the sense of smell 

 is still very large. In particular it is important to notice that 

 the region of the neopallium (parietal region) related to the 

 sense of sight is small. 



Tupaia (the tree shrew) is related to Macroscelides, and the 

 difference which it shows in its brain is related to the habit of 

 living in trees. Life in trees is conducive to the better develop- 

 ment of the sense of sight, for jumping from one branch to 

 another, and inefficient perception of spatial relations would 

 lead to disaster. Accordingly, it is not surprising to find that 

 the visual area of the neopallium of Tupaia is better developed 

 than in Macroscelides, and that in the nature of its retina and 

 other features connected with the eyes, Tupaia approaches the 

 Lemurs. 



The stage represented by Tarsius, which is also arboreal, is 

 of great importance, for here for the first time the sense of 



