558 Causes and Course of Organic Evolution 



representatives, a marked increase is noted in brain capacity 

 and complexity. But this is more marked in some genera, 

 tribes, or classes than in others, and reaches its climax in the 

 higher Primates. We will now endeavor to trace the cause or 

 causes which have operated to bring about this result. 



If we start with primitive pentadactyl marsupials like the 

 existing opossums, we are led by fairly easy transition types — 

 either existing or extinct — to the Phalangeridae and Phas- 

 colarctinidse, that in turn exhibit decided resemblances to some 

 of the ancient forms of Lemuridse. Now in all of these the 

 forelimbs are more and more used for a variety of purposes 

 other than progression, and not least for actions that aid, and 

 are directly centered round, the important sense organs of the 

 head. For this end the animals often rear themselves upward; 

 grasp food and convey it to the nostrils and mouth; tear it to 

 pieces; excavate nests for themselves, or grubs and other food 

 from rotting trees. In short, the anterior limbs are increas- 

 ingly used as arms and hands, that reach out in correlation with 

 the eyes and nose, to environal objects and convey therefrom 

 varied stimuli to the brain, through the different sense-centers 

 of the head. 



These therefore stand out in more and more marked con- 

 trast to the kangaroos and allies, in which the forelimbs, even 

 though raised in the air with the often semi-erect body, are 

 almost wholly organs of progression, accessory to the highly 

 evolved hindlimbs, that are almost purely cursorial. 



From the upper cretaceous rocks on to recent deposits a 

 succession of advancing and related types such as Indrodon, 

 Anaptomorphiis, Adapis, and others, all indicate advancing 

 affinities to the higher apes or Primates. But again the ancient 

 American genus Mixodedes shows characters that place it some- 

 what intermediate between the Rodentia and Lemuroidea, while 

 we have already suggested the origin of the Rodentia from 

 more ancient gnawing marsupials, thus indicating that even 

 the Rodentia, as we now understand it, may have had a some- 

 what composite origin. The evolution of the Carnivora also 

 from carnivorous marsupial ancestors has already been con- 

 sidered (p. 498). 



