Sept. 1898] SPECIES, SEX, AND THE INDIVIDUAL 185 



The conception of correlation means that, owing to something in the 

 constitution of the animal, two or more features can only vary to- 

 gether in certain directions, for example it might be found that a 

 dog could not have less hair without having smaller teeth, and the 

 advantage of larger teeth might in such case be alleged as sufficient 

 explanation of long or thick hair. 



It is of course not necessary to the selection argument that all 

 characters should be useful at the present day. They might have 

 been evolved in a former period when they were useful under other 

 conditions, and be still inherited after they have ceased to be useful. 



However, the more we study the forms and structure of animals 

 the more difficult we find it is to believe that all peculiarities by 

 which the various species are classified are to be explained directly 

 or indirectly by adaptation. We may study the question in refer- 

 ence to the characters distinguishing subdivisions of any degree, but 

 it has been most discussed in reference to the peculiarities of species, 

 and these can only be studied in actual examples. 



The case with which I am most familiar is that of 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 investigated. The 

 dab is principally characterised by the presence of well-developed 

 ctenoid or spinulated scales all over both sides of the body, and by a 

 semicircular 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 is a series of these enlarged 

 scales, which form thorny tubercles ; and there are others along the 

 lateral line. Other peculiarities are the smooth ridge behind the 

 eyes, and the small number of the fin-rays. 



In other respects these fish are much alike. There are differ- 

 ences in their habits and life histories. The flounder lives in 

 estuaries and rivers, only descending to the sea in the spawning 

 season. The plaice and dab are almost invariably found together. 

 The plaice feeds mostly on molluscs, the dab chiefly on crustaceans, 

 worms and echinoderms. The young plaice congregate near shore, 

 while young dabs are found at various depths. The plaice begins 

 to spawn earlier in the year than the dab. 



Xow it is quite impossible, at any rate up to the present time, to 

 find the slightest indication that the specific characters of these three 

 species are useful in relation to these slight differences of life-history. 



