820 THE ORIGIN OF SPECIES 



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 pro- 

 portion 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 many 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 changmg 

 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 

 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 U* natural selection, it seems impos- 

 sible to decide. 



