THEORY OF NATURAL SELECTION. 209 



endeavoring to look upward with both eyes, while resting on 

 one side at the bottom. We may also attribute to the inher- 

 ited effects of use the fact of the mouth in several kinds of 

 flat-fish being bent toward the lower surface, with the jaw- 

 bones stronger and more effective on this, the eyeless side of 

 the head, than on the other, for the sake, as Dr. Traquair 

 supposes, of feeding with ease on the ground. Disuse, on 

 the other hand, will account for the less developed condition 

 of the whole inferior half of the body, including the lateral 

 tins ; though Yarrel thinks that the reduced size of these fins 

 is advantageous to the fish, as " there is so much less room 

 for their action than with the larger fins above." Perhaps 

 the lesser number of teeth in the proportion of four to seven 

 in the upper halves of the two jaws of the plaice, to twenty- 

 five to thirty in the lower halves, may likewise be accounted 

 for by disuse. From the colorless state of the ventral surface 

 of most fishes and of man}' other animals, we may reasonably 

 suppose that the absence of color 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 color 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 variations in the 

 right direction will thus be preserved ; as will those individ- 

 uals 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 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 pf ages the first slight incipient tendency 



