ON PRIMARILY UNADAPTIVE VARIANTS 1 



JOHN TREADWELL NICHOLS 



American Mi-skum of Natural History 



This paper deals with vertebrate variants (forms or 

 species of animals more or less related but differing from 

 one another) which, although geographical, are not 

 direct or obvious responses to the environment. 



Several types of variants are defined. Representa- 

 tive forms occupying adjacent regions are designated as 

 adjacent races or species: Forms intermediate in struc- 

 ture between adjacent forms and occupying territory 

 remote from them as foreign intermediates: Related 

 forms occupying the same territory and contrasted in 

 superficial characters as complements: Forms separated 

 geographically and showing greater resemblance (not in- 

 duced by environmental adaptation) than their degree 

 of relationship would presuppose, as outcrops. 



The hypothesis is advanced that, probably on account 

 of competition, closely related forms are antagonistic. 2 

 That is, when in touch geographically they tend to force 

 one another apart in superficial characters. If this 

 hypothesis, which seems to fit into certain known facts 

 extremely well, be accepted, it involves a centrifugal 

 force in Evolution opposed to the centripetal tendencies 

 of blood relationship. 



It is the main theme of the paper to advance the con- 

 cept of these two forces as the fundamental framework 

 of evolutionary control, the helm which is swayed by 

 natural selection or other forces. 



Larus marinus (the great black-backed gull), which 



1 Paper read before the Section of Biology of the New York Academy of 



