xxxii I^■TE0DUCT10X. 



of scales and fiii-ra^s, but the latter appears to have constantly S gill-rakcrs on the 

 lower part of the anterior arch, Avhilst the former has 7. 



Cenfrojjomus vin'dis and C. undec'nnaUs agree in the number of scales and gill-rakers, 

 but the former appears to have a ray less in the second dorsal fin. A similar difference 

 is found between Pldliipiuis maculafus and P. dormitator, the former having a ray 

 more in the anal tin. 



The members of each pair of species in all cases differ from each other in at least one 

 more or less tangible " substantive " character, such as the relative depth of the body, 

 length of the head, size of the mouth, length of the fin-rays, or size of the teeth. 



A^ hen there is agreement in all the characters which can be numerically expressed 

 and the distinction appears to rest on a single feature, as in the case of Dormitator 

 latlfrons, separated from I), maculatiis by the larger head, or Gcrrcs Uiiecttus, differing 

 from G. 2)lumieri in the less elevated dorsal fin, the two forms can scarcely be regarded 

 as more than sub-specifically distinct. 



In other cases the Atlantic and Pacific forms differ in two tangible characters, 

 e. g. Gerres olitliodomus and G. iieruvianus, the latter of which is distinguished by the 

 larger scales in the thoracic region and the somewhat smaller mouth. From such 

 pairs of species we pass to others, the members of which may be separated by three, 

 four, or more tangible characters. 



From the data given above we may conclude that a greater or less amount of change 

 may take place in a species when isolated for a considerable period, and that some of 

 the changes which take place may be non-adaptive. 



The fact that in so many cases species on opposite coasts may be paired is more in 

 harmony with the view that there has been a gradual modification during isolation 

 than with the supposition that a " mutant " has arisen which has replaced the 

 parent form. 



C. T. E. 



F,:hruarij lOOS. 



