OP ARTS AND SCIENCES. 11 



been discovered among the true bony fishes, which date back as far as 

 the Jurassic Period. To whatever cause we may ascribe the peculiar 

 development of the Flounders, it seems to have been inactive during 

 the periods immediately preceding our own ; and, in the absence of 

 any plausible explanation of their appearance and development during 

 the present period, we must look to some e?:ceedingly subtle agency, 

 of which we have at present no conception. The causes usually as- 

 signed for the development of fishes with a binocular side are all 

 unsatisfactory ; and all are invalidated by the fact that similar condi- 

 tions constantly fail to produce like results. The Flounders are usually 

 said, for instance, to rest on one side, because the great width of the 

 body makes it the most natural position ; but there are many other 

 fishes of far greater width which always swim vertically, and never 

 show any tendency to assume the pleuronect mode of locomotion. In 

 fact, the great development of the dorsal and ventral fins gives to 

 Flounders special advantages over other fishes for maintaining a ver- 

 tical position. The young Flounder also shows a tendency thus to 

 rest on one side, at a time when the young fish is much like any other 

 fish, long before the habit could be of any special benefit or use. 



The absence of a swimming-bladder has also been assigned as a prin- 

 cipal cause of the peculiar mode of locomotion among Flounders. 

 Bat there is one of our Flounders in which a swimming-bladder is 

 already well developed in the young fish ; and this does not prevent 

 that particular species from adopting, as early as the others, the 

 Flounder mode of locomotion. 



The only other cause we can assign is that broad fishes, like the 

 Flounders, find it of course much easier to pursue their prey, if, 

 while swimming close to the bottom, they are protected from detec- 

 tion by a complicated system of pigment cells, for producing col- 

 ors or patterns within certain limits, so as to resemble sand, mud, or 

 gravel. This would gradually lead to the exclusive use of one side 

 (should the fish lie on either side), and would result in the atrophy of the 

 eye, unless the fish were able to transfer his eye to the other side, and 

 thus retain it ; when, as a secondary cause from this, the atrophy of 

 the pigment cells of one side would follow. If this, however, is the 

 natural explanation, why do not we find Flounders in almost all fami- 

 lies of fishes, — at least, among the broad forms of the group, — and 

 why were they not as common in earlier times as at the present day ? 

 We have also to face a very interesting point of heredity. It would 

 certainly seem far simpler for the Flounders to hand down, from gen- 

 eration to generation, the two eyes on one side of the body, and 



