Chap. VII.] THEORY OP NATURAL SELECTION. 291 



gradual transition from Hippoglossus pinguis, which 

 does not in any considerable degree alter the shape in 

 which it leaves the ovum, to the soles, which are entire- 

 ly thrown to one side." 



Mr. Mivart has taken up this case, and remarks that 

 a sudden spontaneous transformation in the position of 

 the eyes is hardly conceivable, in which I quite agree 

 with him. He then adds: " if the transit was gradual, 

 then how such transit of one eye a minute fraction of 

 the journey towards the other side of the head could 

 benefit the individual is, indeed, far from clear. It seems, 

 even, that such an incipient transformation must rather 

 have been injurious." But he might have found an 

 Answer to this objection in the excellent observations 

 published in 1867 by Malm. The Pleuronectidas, whilst 

 irery young and still symmetrical, with their eyes stand- 

 ing on opposite sides of the head, cannot long retain a 

 vertical position, owing to the excessive depth of their 

 bodies, the small size of their lateral fins, and to their 

 being destitute of a swimbladder. Hence soon grow- 

 ing tired, fhey 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 cartilaginous and flexible, so that it readily yields to 

 muscular action. It is also known with the higher ani- 

 nals, even after early youth, that the skull yields and 



