192 THE ORIGIN OF SPECIES 



I may give another instance of a structure which apparently 

 owes its origin exclusively to use or habit. The extremity of the 

 tail in some American monkeys has been converted into a wonder- 

 fully perfect prehensile organ, and serves as a fifth hand. A re- 

 viewer, who agrees with Mr. Mivart in every detail, remarks on 

 this structure: "It is impossible to believe that in any number of 

 ages the first slight incipient tendency to grasp could preserve the 

 lives of the individuals possessing it, or favor their chance of hav- 

 ing 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 them- 

 selves 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 mak- 

 ing its prodigious leaps, than as a prehensile organ. 



The mammary glands are common to the whole class of mam- 

 mals, and are indispensable for their existence; they must, there- 

 fore, have been developed at an extremely remote period, and we 

 can know nothing positively about their manner of development. 

 Mr. Mivart asks: "Is it conceivable that the young of any animal 

 was ever saved from destruction by accidentally sucking a drop 

 of scarcely nutritious fluid from an accidentally hypertrophied 

 cutaneous gland of its mother? And even if one was so, what 

 chance was there of the perpetuation of such a variation?" But 

 the case is not here put fairly. It is admitted by most evolutionists 

 that mammals are descended from a marsupial form; and if so, 

 the mammary glands will have been at first developed within the 

 marsupial sack. In the case of the fish (Hippocampus) the eggs 

 are hatched, and the young are reared for a time, within a sack 

 of this nature; and an American naturalist, Mr. Lockwood, be- 



