THUMBS AND BIG TOES 69 



which the two inner digits tend to be separated from, and 

 opposed to, the outer three. Certain arboreal Rodents 

 have developed very perfectly opposable thumbs and big 

 toes upon lines exactly similar to the Primates, and this 

 feature is seen very beautifully in an arboreal mouse 

 (Mus margarettce) discovered by Charles Hose in Borneo. 



Within the Primate phylum some very curious irregu- 

 larities are apparent in the distribution of the power of 

 opposing thumbs and big toes among the scattered living 

 types. It seems strange that no New-World monkey 

 possesses a perfectly opposable thumb, although all 

 possess an opposable big toe. Among the New- World 

 Primates the thumb is not perfectly opposable, and 

 is always permanently in line with the rest of the digits; 

 it tends to be small and unimportant, and may be entirely 

 undeveloped. 



At first sight it might seem that this arrangement was 

 correlated with the development of a prehensile tail, but 

 all the American Primates are not prehensile-tailed. 

 Nevertheless, it is not beyond possibility that some func- 

 tional factor may have been the common cause for both 

 developments ; a common factor may have led to the loss, 

 or non-development, of the opposable thumb and to the 

 perfection of the prehensile tail, but these two features 

 need not occur in combination. This I regard as the most 

 probable explanation, and in the actual method of climb- 

 ing characteristic of different groups the origin of the 

 different developments probably lies. 



The little Marmosets (Hapalidce), which have claws 

 instead of nails upon all the digits save the big toe, possess 

 a well-developed, but not opposable, thumb. In the 

 Cebidce the condition varies. The Howling Monkeys 

 (Mycetes] possess well-developed thumbs, prehensile tails, 

 and the usual opposable big toes. The Sakis (Pithecia) 

 have well-developed thumbs, but the tail is not prehensile ; 

 the Night Monkeys (Nyctipithecus] show the same 

 features. In the prehensile-tailed Spider Monkeys (Ateles) 



