Chap.VIL Theory of Natural Selection. 187 



swimbladder. Hence soon growing tired, they fall to the bottom 

 on one side. Whilst thus at rest they often twist, as Malm 

 observed, the lower eye upwards, to see above them ; and they 

 do this so vigorously that the eye is pressed hard against the 

 upper part of the orbit. The forehead between the eyes conse- 

 quently becomes, as could be plainly seen, temporarily contracted 

 in breadth. On one occasion Malm saw a young fish raise and 

 depress the lower eye through an angular distance of about seventy 

 degrees. 



We should remember that the skull at this early age is cartila- 

 ginous and flexible, so that it readily yields to muscular action. 

 It is also known with the higher animals, even after early youth, 

 that the skull yields and is altered in shape, if the skin or muscles 

 be permanently contracted through disease or some accident. With 

 long-eared rabbits, if one ear lops forwards and downwards, its 

 weight drags forward all the bones of the skull on the same side, of 

 which I have given a figure. Malm states that the newly-hatched 

 young of perches, salmon, and several other symmetrical fishes, 

 have the habit of occasionally resting on one side at the bottom ; 

 and he has observed that they often then strain their lower eyes 

 so as to look upwards ; and their skulls are thus rendered rather 

 crooked. These fishes, however, are soon able to hold themselves 

 in a vertical position, and no permanent effect is thus produced. 

 With the Pleuronectidse, on the other hand, the older they grow 

 the more habitually they rest on one side, owing to the increasing 

 flatness of their bodies, and a permanent effect is thus produced on 

 the form of the head, and on the position of the eyes. Judging 

 from analogy, the tendency to distortion would no' doubt be 

 increased through the principle of inheritance. Schiodte believes, 

 in opposition to some other naturalists, that the PleuronectidaB are 

 not quite symmetrical even in the embryo ; and if this be so, we 

 could understand how it is that certain species, whilst young, 

 habitually fall over and rest on the left side, and other species on 

 the right side. Malm adds, in confirmation of the above view, that 

 the adult Trachypterus arcticus, which is not a member of the 

 Pleuronectidse, rests on its left side at the bottom, and swims 

 diagonally through the water ; and in this fish, the two sides of the 

 head are said to be somewhat dissimilar. Our great authority on 

 Fishes, Dr. Giinther, concludes his abstract of Malm's paper, by 

 remarking that " the author gives a very simple explanation of the 

 abnormal condition of the Pleuronectoids." 



We thus see that the first stages of the transit of the eye from 

 one side of the head to the other, which Mr. Mivart considers would 



