OBJECTIONS TO THE THEORY 821 



I may give another instance of a structure whicli ap- 

 parently owes its origin exclusively to use or habit. The 

 extremity of the tail in some American monkeys has 

 been converted into a wonderfully perfect prehensile 

 organ, and serves as a fifth hand. A reviewer who 

 agrees with Mr. Mivart in every detail remarks on this 

 structure: "It is impossible to believe that in any num- 

 ber of ages the first slight incipient tendency to grasp 

 could preserve the lives of the individuals possessing it, 

 or favor their chance of having and of rearing offspring." 

 But there is no necessity for any such belief. Habit, and 

 this almost implies that some benefit great or small is 

 thus derived, would in all probability suffice for the 

 work. Brehm saw the young of an African monkey 

 (Cercopithecus) clinging to the under surface of their 

 mother by their hands, and at the same time they hooked 

 their little tails round that of their mother. Professor 

 Henslow kept in confinement some harvest mice (Mus 

 messorius) which do not possess a structurally prehensile 

 tail; but he frequently observed that they curled their 

 tails round the branches of a bush placed in the cage, 

 and thus aided themselves in climbing. I have received 

 an analogous account from Dr. Giinther, who has seen a 

 mouse thus suspend itself. If the harvest mouse had 

 been more strictly arboreal, it would perhaps have had 

 its tail rendered structurally prehensile, as is the case with 

 some members of the same order. Why Cercopithecus, 

 considering its habits while young, has not become thus 

 provided, it would be difficult to say. It is, however, 

 possible that the long tail of this monkey may be of 

 more service to it as a balancing organ in making its 

 il prodigious leaps than as a prehensile organ. 



