THE SPINOUS PROCESSES 109 



Sloths, it cannot be said that their arboreal activities, 

 although distinctive enough, tend to lead far in the 

 struggle of evolution. 



If this be the true history of the arboreal Bradypodidce, 

 it would seem to be one not easily applied to the origin 

 of the arboreal Primate stock. No lumbering gait, or 

 mere clinging to branches, seems to have led them to the 

 tree-tops; and, indeed, as we have seen in a previous 

 section, an early acquired mammalian activity appears 

 the most probable factor in bringing about the enterprise. 

 Taken as a whole, the Primates show a distinctly retro- 

 verted and anteverted series of dorso-lumbar spinous 

 processes, the two sets being separated by an anticlinal 

 vertebra marking a centre of movement, which is very 

 obvious in the arboreal activities of most monkeys. 



The same conditions are present in most members of 

 the Insectivora. Among the Menotyphlidce, the Oriental 

 Tree Shrews (Tupaiadce), which are deserving of especial 

 notice, have twelve ribs, the spines of the vertebrae 

 anterior to the tenth dorsal slope backwards, the tenth 

 or eleventh is upright, and the nine posterior spines slope 

 forwards. These are active arboreal creatures jumping 

 from branch to branch, and having a very definite centre 

 of movement at the hind end of the thoracic region 

 (see Fig. 37). 



There are many reasons for supposing that, in some 

 such form as a primitive tree-haunting Insectivore, a 

 picture of an earlier stage of the Primate phylum is to 

 be seen most perfectly among living Mammals. Some 

 exceedingly primitive form, of which a very much elabo- 

 rated modern evolution may be seen in the existing 

 Tupaiadce, probably pioneered the Primate stock in the 

 conquest of the branches; this pioneer form was, in all 

 probability, a small active animal, perhaps with the 

 commencing possession of a centre of movement situated 

 at the hinder end of the thoracic vertebrae. 



Great interest centres round the Lemurs in the study 



