THEOBT OF NATURAL SELEGTION. 323 



of changing their color in accordance with the surround- 

 ing surface, or the presence of bony tubercles on the upper 

 side of the turbot, are due to the action of the light. 

 Here natural selection has probably come into 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 per- 

 haps of their disuse, will be strengthened by natural selec- 

 tion. For all spontaneous variations in the right direc- 

 tion 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 appar- 

 ently owes its origin exclusively to use or habit. The 

 extremity of the tail in some American monkej's 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 number 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 neces- 

 sity 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 (Mas messorius) which do not pos- 

 sess a structurally prehensive tail; but ho 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 analo- 

 gous 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 



