60 REPORT OF COMMISSIONERS OF INLAND FISHERIES. 



water, and that this should agree exactly with the conditions in the 

 Turbot tribe, on the one hand, and the Flounder tribe on the 

 other. There are some of the Calif ornian flat-fishes which, as we 

 might perhaps expect, may perhaps be considered to form transition 

 groups;' some species of those waters are right-sided and some are 

 left-sided, and in some species it seems to be true that the fishes 

 are indifferently right or left, or at least reversed individuals are 

 common. These interesting facts suggest to us that natural selection 

 may favor sinistral fishes in the tropics and dextral fishes in cold 

 water. It is difficult to imagine this, though right- and left -handed- 

 ness may be dependent on conditions in the environment of which 

 we know nothing. 



Another remarkable condition is the relation ^\hich seems to 

 exist between the number of vertebrae possessed by the various 

 species and their grographical distribution. It is true of other groups 

 of fishes that in some cases their northern representatives have an 

 increased number of vertebrae, but this relation is shown in a strking 

 way in the Flounder famil5\ The following are the averages of the 

 numbers of vertebrae found in the species of the three tribes of the 

 Flounder family: 



Flounder Tribe (Pleuronectince) . 



Average number of vertebrae in 13 species, 42. 

 AH these species live in northern waters. 



Halibut Tribe (HippogJossince). 



Average number of vertebrae in 15 species, 41. 



Of these 15 species, 4 are cold water and 9 occur along the Californian 

 and Middle Atlantic and Gulf Coasts. 



Turbot Trihe{PseUinre). 



Average number of vertebrse in 20 species, 34.6. 

 These are tropical or sub-tropical species. 



The causes of this remarkable relation are not fully understood. 

 It has been suggested that the reduction in the number of vertebrae 

 of the warm-water species is a result of the higher specialization 

 demanded of those fishes living in the tropics where the struggle for 



