TRANSLATOR'S PREFACE xv 



In the same way we might argue that the structural cou- 

 ditions coimected wdth the upright position in man are not 

 themselves inherited, but that what is inherited is the 

 power in the body of retaining the changes caused by the 

 assumption of the upright position in the child. 



According to Mr. Wallace's argument, the distortion of 

 the skull in flat fishes and the peculiar asymmetry of the 

 eyes are not inherited at all, but are due in every generation 

 entirely to the efforts of each individual young fish actinc^ 

 on a peculiarly sensitive skull. But in the young turbot 

 and brill the metamorphosis is very nearly or quite com- 

 pleted long before the little fish have abandoned their pelagic 

 mode of life and retired to the sea-bottom. In these species 

 the young have a large air-bladder, although in the adults 

 that organ is completely wanting. In consequence of this 

 the young fish swim at the surface of the water until they 

 have reached a considerable size, long after the right eye has 

 migrated to the left side. These young fishes swim at the 

 surface in a horizontal position, as do adult flat fishes when 

 they swim up a short distance from the sea -bottom. Mr. 

 Wallace evidently means by the " efforts of the young fish," 

 the efforts which it makes to use its lower eye after it has 

 assumed the habit of lying on its side on the bottom. Now 

 these young turbot and • brill do not lie on the bottom, and 

 do not therefore need to make efforts to bring their right eye 

 up to the edge of the body, yet the eye migrates all the same. 

 It may be urged that they assume the horizontal position, 

 and therefore have to twist their right eye round : but wliy 

 do they assume the horizontal position ? The reason cannot 

 lie in the great depth of the body, as Mr. Wallace suggests 

 with reo-ard to flat fishes in general, for in the John Dorey 



