Gobioidei, Discocephali, and Tsniosomi 465 



former time more developed, parallel in their nature to such 

 survivals in costume as the two buttons on the back of a man's 

 coat, once useful for the attachment of a sword-belt. But in 

 this fish we have no case of survival, but one of unusual develop- 

 ment ; the family (Gobiidcz) to which it belongs presents no 

 similar case, although its members have somewhat similar 

 habits, and the conviction grows upon us, as we consider the 

 subject, that the long jaws serve some useful purpose in the 

 economy of the creature. In view of the half -terrestrial life 

 led by this fish, I am inclined to suspect that the expansion of 

 the upper jaw may serve for the retention of a small quantity of 

 water, which, slowly trickling downward into the mouth and gills, 

 keeps the latter moist when, from an unusually low tide or a 

 dry season, the waters of its native creek fail, perhaps for several 

 hours, to reach the holes in which the fishes dwell. It may be 

 objected to this view that, were such an appendage necessary or 

 even useful, other species of Gobiida, whose habits are similar, 

 would show traces of a similar adaptation. This, however, by 

 no means follows. Nature has many ways of working out the 

 same end ; and it must be remembered that every real species, 

 when thoroughly known, differs somewhat in habits from its 

 congeners, or at least from its family friends. To take an 

 illustration from the mammalia. The chimpanzee and the 

 spider-monkey are both quadrumanous and both arboreal, yet 

 the end which is attained in the former by its more perfect 

 hands is reached in the latter by its prehensile tail. 



"Why may not the extremely long channel formed by the 

 jaw of this rather abnormal member of the goby family be 

 another mode of provision for the requirements of respiration? " 



Of the Asiatic genera, Periophthalmus and Boleophthalmus 

 are especially notable. In these mud-skippers the eyes are 

 raised on a short stalk, the fins are strong, and the animal has 

 the power of skipping along over the wet sands and mud, even 

 skimming with great speed over the surface of the water. It 

 chases its insect prey among rocks, leaves, and weeds, and out 

 of the water is as agile as a lizard. Several species of these 

 mud-skippers are known on the coasts of Asia and Polynesia, 

 Periophthalmus barbarus and Boleophthalmus chinensis being 

 the best known. Awaous crassilabris is the common oopu, or 



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