Sept. 1898] SPHCIES, 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 molluses, 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. 
Now 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. 
