THE ARGUMENT FROM EMBRYOLOGY 87 



change in position of the eye being accompanied by 

 very considerable twisting and distortion of the skull. 

 (Fig. 12.) 



Inasmuch as flat fish in all other respects agree 

 with more ordinarily-constituted fish, and as their 

 special peculiarities — i.e., the lateral compression of 

 the body, the difference of colouring on the two 

 sides, and the singular position of the eyes — may all 

 be readily and naturally explained by their habits, 

 which again would be clearly advantageous to the fish 

 in aiding them to escape from enemies, it becomes 

 in the highest degree probable that flat fish are 

 descended from normally-formed fish, which first 

 acquired the habit of lying on one side for the sake 

 of protection, and then underwent structural changes 

 in consequence of this habit. 



If this be correct, then the developmental history 

 of the flat fish becomes intelligible by assuming that 

 each individual has an inherited tendency to repeat 

 in its own development the history of the species ; 

 every flat fish during its own growth passing through 

 the same series of changes by which we have sup- 

 posed the whole race of flat fish to have acquired 

 their special peculiarities. 



The case with regard to the sole is really a 

 very strong one : for the only alternative view is 

 that flat fish are not descended from normally- 

 shaped fish, but have sprung into existence in- 

 dependently ; and not only is this view absolutely 

 contradicted by what we know of other animals, but 

 it would render the development of the flat fish an 

 incomprehensible mystery. The one view gives a 



