The Geographical Distribution of Fishes 239 



like fashion, young fishes from the tropics drift northward in the 

 Kuro Shiwo to the coasts of Japan, but never finding a per- 

 manent breeding-place and never joining the ranks of the Japa- 

 nese fishes. But to this there have been, and will be, occasional 

 exceptions. Now and then one among thousands finds per- 

 manent lodgement, and by such means a species from another 

 region will be added to the fauna. The rest disappear and 

 leave no trace. A knowledge of these currents and their in- 

 fluence is eventual to any detailed study of the dispersion of 

 fishes. 



The occurrence of the young of many shore fishes of the 

 Hawaiian Islands as drifting plankton at a considerable distance 

 from the shores has been lately discovered by Dr. Gilbert. 

 Each island is, in a sense, a "sphere of influence," affecting 

 the fauna of neighboring regions. 



Species Changed through Natural Selection. In the third class, 

 that of species changed in the process of adaptation, most 

 insular forms belong. As a matter of fact, at some time or 

 another almost every species must be in this category, for isola- 

 tion is a source of the most potent elements in the initiation 

 and intensification of the minor differences which separate re- 

 lated species. It is not the preservation of the most useful 

 features, but of those which actually existed in the ancestral 

 individuals, which distinguish such species. Natural selection 

 must include not only the process of the survival of the fittest, 

 but also the results of the survival of the existing. This means 

 the preservation through heredity of the traits not of the species 

 alone, but those of the actual individuals set apart to be the 

 first in the line of descent in a new environment. In hosts of 

 cases the persistence of characters rests not on any special use- 

 fulness or fitness, but on the fact that individuals possessing 

 these characters have, at one time or another, invaded a cer- 

 tain area and populated it. The principle of utility explains 

 survivals among competing structures. It rarely accounts for 

 qualities associated with geographical distribution. 



Extinction of Species. The extinction of species may be 

 noted here in connection with their extension of range. Prof. 

 Herbert Osborn has recognized five different types of elimina- 

 tion. 



