i8 95 . ORIGIN OF SPECIES AMONG FLAT-FISHES. 237 



interspinous regions, and almost absent on the blind side. It was 

 found that the ciliation, though varying considerably in different 

 individuals, was always much more strongly developed in the mature 

 males than , in the females. These specimens seem to be identical 

 with the variety pscudoflesus, described by Gottsche, but the sexual 

 difference has not been noticed before. Mobius mentions these 

 ciliated plaice, and observes that they form a transition to unusually 

 smooth specimens of the flounder. The flounder (P. flesus) is speci- 

 fically distinguished by the rough tuberculated scales which occur in 

 a single series at the bases of the longitudinal fins, along the lateral 

 line, and on the upper side of the head. Although I know from 

 observation that the degree of tuberculation varies in individual 

 flounders from the same locality, it is also a fact that the character is 

 most reduced in the Mediterranean, where its reduction distinguishes 

 a geographical variety called glabra, in which the head and lateral line 

 are nearly or quite destitute of tubercles. According to Moreau, this 

 variety extends into the Atlantic, but is rare to the north of the 

 Gironde. On the American side of the Atlantic no flat-fish is known 

 presenting the tuberculation characteristic of P. flesus, but in the 

 North Pacific there is a species, Platichthys stellalus, which has it much 

 more developed than in our P. flesus, all the scales being tubercular 

 and rough, but the largest ones, as in P. flesus, being placed along the 

 bases of the fins and the lateral line. 



It appears, therefore, that in the plaice there is a Baltic ciliated 

 variety, and in the flounder a southern smooth variety. Whether 

 there is any direct relation between northern habitat and spinulation 

 of the scales seems doubtful : at least it does not apply to other species. 

 We can see no indication of utility or selection in these cases, but 

 merely instances of variation, corresponding to geographical position. 

 The geographical isolation is not complete, and the characters within 

 the two several forms are not perfectly distinct, but connected by 

 intermediate forms. The differences between P. platessa and P. flesus 

 are, however, discontinuous, and the geographical ranges to a great 

 extent common. Here the isolation is physiological and the occurrence 

 of hybrids is not established. 



We have at present no evidence whatever that these diagnostic 

 differences in the character of the scales are adaptive, and the same 

 might be said of several other characters distinguishing the forms just 

 mentioned, such as the specific coloration and the number of fin-rays. 

 In fact, although certain differences are known in the habits and dis- 

 tribution of plaice, flounder, and dab, of which the most salient is that 

 the flounder lives in estuaries except when spawning, none of the 

 structural characters are known to correspond and be adapted to 

 these differences. Evidence, therefore, that generic and specific 

 characters are adaptive in the sub-family is wanting. On the other 

 hand, we must admit adaptation in many of the characters of the sub- 

 family, namely, in the characters of the mouth, which is asymmetrical, 



