ANIMALS AND THEIR ENVIRONMENTS. 313 



whose heads from disease have become fixed either sideways 

 or backways, one of the eyes has changed its position and 

 the bones of the skull have become modified." So also, if 

 one ear of a lop-eared rabbit tends to fall downwards and 

 forwards, its mere weight is found to affect the development 

 and growth of all the bones of the skull, and to cause a for- 

 ward protrusion of the head on that particular side. Me- 

 chanical causes, and the mere action of weight or strain, may 

 thus produce changes of surprising extent in structures of 

 greater firmness than the soft skulls of fishes. The evidence 

 in support of the evolutionist's theory of flat-fish modification, 

 however, is also strengthened by certain cases of distortion 

 which follow upon the habit evinced by the young of cer- 

 tain well-known and symmetrical fishes of resting on one 

 side. Young trout, salmon, and perch have been found to 

 acquire unsymmetrical skulls from this habit ; and they have 

 also been seen to strain their lower eyes in the endeavour to 

 look upward, after the fashion of the young flat-fish. One 

 authority, indeed, declares that it is possible that the young 

 of the most modified flat-fishes are in reality unsymmetrically 



r 



FIG. 61. Trachypterus, or vaagmar, one of the "deal-fishes." 



developed even within the egg. This condition, if actually 

 present, must necessarily be viewed as the inherited result 

 of the typical development of the unsymmetrical state in 

 ancestral forms ; and its occurrence would render easy of 

 explanation the cause of the young fish losing its balance so 

 soon after its escape from the egg. In some fishes, which 

 are widely removed in their systematic position from the 

 flat-fishes, there is a want of symmetry which compels them 

 to rest on one side. Such are the curious " deal-fishes " 

 {Trachyptcnts arcturus), or vaagmars (Fig. 61), which derive 



