THEORY OF NATURAL SELECTION 243 



twenty-five to thirty in the lower halves, may likewise be 

 accounted for by disuse. From the colourless state of the 

 ventral surface of most fishes and of many other animals, we 

 may reasonably suppose that the absence of colour in flat- 

 fish on the side, whether it be the right or left, which is 

 undermost, is due to the exclusion of light. But it cannot 

 be supposed that the peculiar speckled appearance of the 

 upper side of the sole, so like the sandy bed of the sea, or 

 the power in some species, as recently shown by Pouchet, of 

 changing their colour in accordance with the surrounding 

 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 perhaps of their disuse, will be 

 strengthened by natural selection. For all spontaneous varia- 

 tions 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 appar- 

 endy owes its origin exclusively to use or habit. The ex- 

 tremity of the tail in some American monkeys has been con- 

 verted 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 im- 

 possible 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 

 (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 



