188 MISCELLANEOUS OBJECTIONS TO THE [Chap. VII- 



be injurious, may be attributed to the habit, no doubt bene- 

 ficial to the individual and to the species, of endeavouring to 

 look upwards with both eyes, whilst resting on one side at the 

 bottom. We may also attribute to the inherited effects of use 

 the fact of the mouth in several kinds of fiat-fish being bent 

 towards 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 deve- 

 loped condition of the whole inferior half of the body, including the 

 lateral fins ; though Yarrell 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 

 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 can- 

 not 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 then- 

 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 anather 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 struc- 

 ture : " It is impossible to believe that in any number of ages the 

 first slight incipient tendency to grasp could preserve the lives of 



