Gobioidei, Discocephali, and Teniosomi 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 (Gobiide) 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 Gobiide, 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 
II—30 
