8 SEXUAL DIMORPHISM 



But when we examine and compare species we find that 

 they are distinguished by characters, many of which are not 

 obviously adaptive, and it is open to question in many cases 

 whether any of the specific peculiarities are adaptations. 

 Darwin's remark on this subject is as follows : " In looking 

 at many small points of difference between species which, as 

 far as our ignorance permits us to judge, seem quite unim- 

 portant, we must not forget that climate, food, etc., have no 

 doubt produced some effect. It is also necessary to bear in 

 mind that, owing to the law of correlation, when one part 

 varies, and the variations are accumulated through natural 

 selection, other modifications, often of the most unexpected 

 nature, will ensue." Darwin himself, then, was obliged to 

 admit that many specific characters were not adaptive, so far 

 as he could discover. 



As an example specially familiar to myself I will refer to 

 the flat fishes. 



The Plaice, Flounder, and Dab, are three species of the same 

 genus, whose habits and life-histories are fairly well known, 

 and whose structural peculiarities have been minutely investi- 

 gated. The Dab is principally characterised by the presence 

 of well-developed ctenoid or spinulated scales all over both 

 sides of the body, and by the semi-circular curve of the 

 lateral line above the pectoral fin. 



In the Plaice we see conspicuous red spots ; the scales are 

 for the most part smooth, cycloid, and reduced, the lateral 

 line is straight, and the bony ridge behind the eyes is 

 elevated into five tubercles. 



In the Flounder we find another condition of the scales, 

 some are smooth and reduced in size as in the Plaice, while 

 others are enlarged and developed beyond the condition seen 

 in the Dab. Along the bases of the marginal fins there are 

 series of these enlarged scales, which form thorny tubercles, 

 and there are others along the lateral line. Other peculi- 



