October 11, 1901.] 



SCIENCE. 



559 



animals are reducible to three very simple 

 propositions. 



Each species of animal is found in every 

 part of the earth having conditions suitable 

 for its maintenance, unless : 



(a) Its individuals have been unable to 

 reach this region through barriers of some 

 sort; or, 



(&) Having reached it, the species is 

 unable to maintain itself, through lack of 

 capacity for adaptation, through severity 

 of competition with other forms, or through 

 destructive conditions of environment ; or 

 else, 



(e) Having entered and maintained it- 

 self, it has become so altered in the pro- 

 cess of adaptation as to become a species 

 distinct from the original type. 



SPECIES ABSENT THROUGH BARRIERS. 



The absence from the Japanese fauna of 

 most European or American species comes 

 under the first head. The pike has never 

 reached the Japanese lakes, though the 

 shade of the lotus leaf in the many clear 

 ponds would suit its habits exactly. The 

 grunt* and porgiesf of our West Indian 

 waters and the crenilabri of the Mediter- 

 ranean have failed to cross the ocean and 

 therefore have no descendants in Japan. 



SPECIES ABSENT THROUGH FAILURE TO MAIN- 

 TAIN FOOTHOLD. 



Of species under (6), those who have 

 crossed the seas and not found lodgment, 

 we have, in the nature of things, no record. 

 Of the existence of multitudes of estrays 

 we have abundant evidence. In the Gulf 

 Stream ofi" Cape Cod are every year taken 

 many young fishes belonging to species at 

 home in the Bahamas and which find no 

 permanent place in the ITew England fauna. 

 In like fashion, young fishes from the 

 tropics drift northward in the Kuro Shiwo 

 to the coasts of Japan, but never finding a 



* Ssemulon. 

 t Calamus. 



permanent breeding-place and never join- 

 ing the ranks of the Japanese fishes. But 

 to this there have been, and will be, occa- 

 sional exceptions. ISTow and then one 

 among thousands finds permanent lodg- 

 ment, and by such means a species from an- 

 other 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. 



SPECIES CHANGED THROUGH NATURAL SE- 

 LECTION. 



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 isolation is a source 

 of the most potent elements in the initia- 

 tion and intensification of the minor differ- 

 ences which separate related species. It 

 is not the preservation of the most useful 

 features, but of those which actually existed 

 in the ancestral individuals, which distin- 

 guish such species. I have elsewhere noted 

 that 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 indi- 

 viduals 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 usefulness or fitness, but 

 on the fact that individuals possessing these 

 characters have, at one time or another, 

 invaded a certain area and populated it. 

 The principle of utility explains survivals 

 among competing structures. It rarely ac- 

 counts for qualities associated with geo- 

 graphical distribution. 



BARRIERS CHECKING MOVEMENT OF FISHES. 



The limits of the distribution of indi- 

 vidual species or genera must be found in 



