46 FLAT-FISHKS. 



might the better be ascertained and referred to their probable 

 causes, but as the descent cannot, with any degree of certainty, 

 be traced, we are obliged to seek some explanation in the life- 

 history of the specie? here as compared with that elsewhere. 



As before observed, it seems to be a law of nature among 

 closely allied genera and species of fish, that denizens of a warm 

 southern range have fewer vertebrae and fin-supports than their 

 representatives in colder or more northern regions. In the order 

 to which these flat-fishes belong, the halibuts of high latitudes 

 have, as a rule, the fin-rays in greater number than more 

 southern species; and the same seems true of the flat-fishes them- 

 selves. To the naturalist species are only divergent varieties; 

 and wherever natural influences have conspired to produce 

 species, the same to a greater or less extent, may be, and pro- 

 bably are, operating to produce varieties within the species. The 

 localities from which the authorities cited drew their material, 

 especially in the case of P. amerlcnnus, namely Labrador, Cape 

 Breton, Grand Manan, Boston, New Bedford, &c., are fairly well 

 within the influence of the arctic currents, and the summer de- 

 gree of heat is below what obtains in the western bays of the 

 Gulf of St. Lawrence; while the difference in the winter temper- 

 ature of the two ranges must be small. The colder and more 

 uniform temperature and all it implies in the life history of the 

 creature, may tend to greater stability of type characters, while 

 an opposite set of conditions may disturb the equilibrium, and 

 promote variation. It will, however, be clear that this tendency 

 to variation may arise from either of two different sources, name- 

 ly, the inequality between the winter and summer temperature 

 of the water, or the greater heat of the latter aftecting the young 

 which pass the first few months of their lives in the warm shore 

 waters. The latter w-ould seem the more important. In the 

 growth of the creature from the larval to the adult stage, it 

 seems probable that the biological and climatic conditions of its 

 environment would react with greater effect on the system at 

 the immature and more plastic stage of life. The peculiar ad- 

 justment of the eyes in these fishes, at fi.rst normal as regards 

 position, to one side of the head is made very early in life, and 



