SCIENCE OF FLAT-FISH, OR SOLES AND TURBOT. in 



rangement, that our primitive ancestor was as clear as glass, and had 

 his eye inside his brain, as is still the case with the ascidian larva. As 

 soon as his descendants began to grow opaque, the eye was forced to 

 push itself outward, so as to reach the surface of the body ; and thus 

 at last, we may imagine, it came to occupy its present prominent posi- 

 tion on the full front of all vertebrate animals. 



To return to our sole, however, whom I have left too long waiting 

 in the sand to undergo his next transformation : as soon as he has 

 selected a side on which to lie, he begins to grow dark, and a pigment- 

 ary matter forms itself on the upper surface exposed to the light. 

 This is a very common effect of exposure, sufficiently familiar to ladies 

 and others, and therefore hardly calling for deliberate explanation. 

 But the particular form which the coloring takes in the true sole and 

 in various other kinds of flat-fish is very characteristic, and its origin 

 is one of the most interesting illustrations of, natural selection to be 

 found within the whole range of animated nature. In every case it 

 exactly resembles the coloration of the ground on which the particular 

 species habitually reposes. For example, the edible sole lies always 

 on sandy banks, and the spots upon its surface are so precisely similar 

 to the sand around it that in an aquarium, even when you actually 

 know from the label that there is a sole to be found in a particular 

 tank, you can hardly ever manage to spot him as long as he lies per- 

 fectly quiet on the uniform bottom. Turbot, on the other hand, which 

 prefers a more irregular pebbly bed, is darker brown in color, and has 

 the body covered on its upper side with little bony tubercles, which 

 closely simulate the uneven surface of the banks on which it basks. 

 The plaice, again, a lover of open, stony spots, where small shingle 

 of various sorts is collected together in variegated masses, has its top 

 side beautifully dappled with orange-red spots, which assimilate it in 

 hue to the party-colored ledges whereon it rests. In this last case the 

 brighter dabs of color undoubtedly represent the bits of carnelian and 

 other brilliant pebbles, whose tints of course are far more distinct 

 when seen in water by refracted light than when looked at dry in the 

 white and common daylight. "We all know how much prettier peb- 

 bles always seem when picked up wet on the sea-shore than under any 

 other circumstances. 



Some few flat-fish even possess the chameleon power of altering 

 their color, in accordance with the nature of the bottom on which they 

 are lying. The change is managed by pressing outward or inward 

 certain layers of pigment-cells, whose combination produces the de- 

 sired hues. 



The origin of this protective coloration must once more be set down 

 to that deus ex machind of modern biology, natural selection. In the 

 beginning, those flat-fish which happened to be more or less spotted 

 and speckled would be most likely to escape the notice of their ever- 

 watchful and rapacious foes ; while those which were uniformly 



