30 



ARBOREAL MAN 



being likened by all comparative anatomists to that in 

 the typical pre-mammalian Vertebrates. In the rest of 

 the mammalian orders the appearance of the clavicle, 



if we take the systematic posi- 

 tion of the animal as our guide, 

 can only be described as hap- 

 hazard. Among the Metatheria 

 (Marsupials), only Perameles (the 

 Bandicoot) fails to possess a 

 clavicle. Among Eutheria (higher 

 Mammals), a complete clavicle 

 is present in all Insect ivora, 

 except the aberrent aquatic 

 Potamogale ; it is present in some 

 Edentata, in all Cheiroptera, and 

 all Primates. It is entirely 

 absent in Cetacea and Sirenia 

 and Ungulata (see Fig. 10); in 

 most Carnivora it appears onl}^ 

 as a rudiment, though in some 

 members of this order it attains 

 a fair degree of development; 

 in the Rodent ia it is sometimes 

 well developed and sometimes 

 entirely absent . The only under- 

 lying principle which seems to 

 explain the rather random de- 

 velopment of this bone in differ- 

 ent IMammals appears to be 

 found in the functional demands 

 made upon the movements J of 

 the fore-limb. So long as^ no 

 more demand is made than the 

 simple backward and forward movement at the shoulder- 

 joint, such as is seen in the walking and trotting of pure 

 quadrupeds, this strut is either not develo^Ded, or attains 

 no greater perfection than that of a mere isolated inter- 



FiG. 10. — The Shoulder 

 Girdle of an Ungu- 

 late, TO SHOW THE 



Absence of the Clav- 

 icle. 



The fore-limb has no strut 

 to keej) it poised at the 

 side of the thorax. 



