i895- ORIGIN OF SPECIES AMONG FLAT-FISHES. 235 



latter we had three distinct species not geographically separated : here 

 we have three species whose geographical ranges are, according to 

 present knowledge, exclusive. We have no evidence that the ranges 

 are not continuous, and perhaps if specimens were collected at succes- 

 sive stations along the arctic coasts of Europe, Asia, and America, it 

 would be found that the three species of Limanda were connected con- 

 tinuously by intermediate forms. In that case we should have a species 

 with a very wide range exhibiting geographically distinct races. We 

 have no evidence that these forms would not interbreed if brought 

 together. They are geographically isolated, not physiologically. We 

 have, therefore, another illustration of the principle that isolation 

 results in divergence. Even if the forms pass gradually into one another, 

 it is certain that the American individuals are prevented by distance 

 of habitat from breeding with European or with those of the Pacific. 



Can we suppose in this case that there are differences in the 

 environments which make the specific peculiarities advantageous to 

 each species ? Are rugose prominences on the head required in the life 

 of the dab on the east coast of America and not in Europe ? or is the 

 angle between the snout and head useful to the European dab and 

 not to the dab of Alaska ? We have at present no facts which support 

 such an assumption. There may be some differences in the conditions 

 of life which produce these differences of structure as a result, the 

 differences of structure themselves playing no part whatever in the 

 struggle for existence. This also we do not know. What we do 

 know is, that forms so closely similar as to be almost one widely- 

 ranging species, present these constant peculiarities in places distant 

 enough from one another to prevent interbreeding. On the selection 

 theory, the variations in the different habitats are indefinite and 

 similar, the selection is different ; on the other view, which I prefer, 

 the selection, so far as these structures are concerned, is absent, the 

 variations are definite and different in the isolated groups. 



Closely allied to Limanda is a species in the Pacific, called by 

 Jordan and Goss Lepidopsetta bilineata. Only one species is placed in 

 the genus, the establishment of which seems superfluous. This 

 form is distinguished from Limanda only by the presence of an 

 accessory branch of the lateral line, which starts from the anterior 

 part of the lateral line, extending in all members of the family above 

 the eyes, and runs backward along the base of the dorsal fin. This 

 species is very common on the Pacific coast of America from 

 California to Alaska, and its scales become rougher in the north. 

 The evidence that this species and Limanda aspera are distinct seems 

 sufficient, and as they are not geographically isolated, we have a case 

 of divergence in the same area. We have no evidence that the 

 accessory branch of the lateral line is required by L. bilineata and not by 

 L. aspera ; it is probably a definite but non-adaptive variation. It is a 

 very interesting fact that this variation is known as a constant character 

 only in the Pacific, and that there it occurs in a large number of species. 



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