No. 597] PRIMARILY UN ADAPTIVE VARIANTS 569 



If this hypothesis which seems to fit into the known facts 

 with surprising neatness, be accepted, the writer believes 

 that it will explain a force in direct opposition to the 

 commingling of blood along a line of geographical de- 

 marcation of adjacent races, tending to drive those races 

 apart in structure and making possible the derivation 

 from them of adjacent species without geographical 

 isolation. In fact we find in the incompatibility of 

 closely related forms a powerful centrifugal force for 

 differentiating species and forcing them apart 4 — and it 

 also explains complementary coexisting forms, as it 

 would tend to make coexisting forms complementary and 

 would not act to change or eliminate them if they already 

 were so. In the case of the foreign intermediate we have 

 a geographically isolated form less influenced by the cen- 

 trifugal forces, therefore varying less, retaining inter- 

 mediate or primitive characters. This centrifugal force 

 should always be considered as balanced against blood- 

 relationship, doubtless the chief cause of resemblance in 

 species. In the outcrop we have a case where the centrif- 

 ugal force is inoperative and the centripetal tendency 

 brings about a parallelism different in fundamental na- 

 ture from the more familiar environmentally induced 

 parallels. 



There seems to be an analogy between the outcrop and 

 homologous rectigradations in Paleoevolution. 



Having given this rather concentrated outline of my 

 hypothesis and the class of facts it is designed to explain 

 it will not be out of place to mention other widely scat- 

 tered examples of the important classes of variant? al- 

 luded to. I will begin with the foreign intermediate. 



Trichiurus is a long band-like silvery fish with a fila- 

 mentous tail found along the shores of warm seas, where 

 also occur numerous representatives of the Scombroid 

 or mackerel-like fishes. The two are utterly unlike, yet 

 a clear line of relationship is found through intermediate 

 forms (for instance Lepidopus) from the ocean depths. 



* See, however, Gulick, J. T., "Evolution, Racial and Habitual," 1905, 



