176 MISCELLANEOUS OBJECTIONS TO THE [CHAP. VII. 



play, as well as in adapting the general shape of the body of these 

 fishes, and many other peculiarities, to their habits of life. We 

 should keep in mind, as I have before insisted, that the inherited 

 effects of the increased use of parts, and perhaps of their disuse, 

 will be strengthened by natural selection. For all spontaneous 

 variations in the right direction will thus be preserved ; as will 

 those individuals which inherit in the highest degree the effects 

 of the increased and beneficial use of any part. How much to 

 attribute in each particular case to the effects of use, and how 

 much to natural selection, it seems impossible to decide. 



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 

 reviewer 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 favour 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 (Cercopi- 

 thecus) 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 struc- 

 turally 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 whilst 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 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 



