The Origin of Species among Fiat-Fishes. 



(Continued from p. 177.) 



IT is stated by Romanes that specific distinctions are useless or non- 

 adaptive, while structures of generic, family, or higher taxonomic 

 value are usually adaptive. He explains this by a very curious 

 supposition : that the specific characters which have arisen by 

 spontaneous variation, and been allowed to perpetuate themselves 

 by heredity, are afterwards eliminated as so much surplusage in the 

 struggle for existence by natural selection. A greater or less time 

 would be required to effect this reduction, and thus we are to under- 

 stand how it is that useless structures, which do not impose much tax 

 upon the organism, occasionally persist even into genera, but rarely 

 into families or higher divisions. There is much ambiguity in con- 

 sidering this argument, from the fact that the definition of a genus is 

 so much a matter of individual opinion. We can nevertheless 

 inquire if there is really this difference of kind between useful and 

 useless characters, and if so, whether the useful or adaptive are 

 generally distinctive of the larger taxonomic divisions. We have 

 seen that, if we regard Zeugopterus as a genus, many of the generic 

 characters are not adaptive. According to the analysis of Jordan 

 and Goss the next higher division to which Zeugopterus belongs 

 is the sub-family Rhombinae (called by them Pleuronectinae), the 

 principal characteristics of which are, eyes on the left side, mouth 

 large and symmetrical, pelvic fins somewhat asymmetrical and ex- 

 tended at the base. The large mouth and symmetrical teeth are 

 certainly adaptive in relation to the predaceous habits of most of 

 these fish, which rise from the ground to seize moving prey, and 

 do not feed entirely on small animals living on the ground. But 

 this character is shared by the Hippoglossinae, and the extension of 

 the pelvic fins, the anterior extension of the dorsal, and the sinistral 

 position of the eyes are not obviously adaptive at all. 



We are thus driven to the whole family Pleuronectidae, which 

 are certainly distinguished by their adaptation to a horizontal position 

 on the sea-bottom. But before considering the family characters I 

 wish to direct attention to another group of species whose relations 

 are somewhat different to those of the three species of Zeugopterus. 

 As Jordan and Goss point out, the species allied to the plaice, 

 flounder, and dab form a distinct subdivision of the family, the 



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