No. 597] PRIMARILY UN ADAPTIVE VARIANTS 573 



of petrels (Genus Mstrelata) are indigenous to it. 

 These are almost identical in size and build but differ 

 markedly in color. JEstrelata arminjoniana is bicolored, 

 dark above and white below, JEstrelata trinitatis is uni- 

 formly dark colored, JEstrelata chionofara resembles the 

 former bird, but the white of the underparts spreads up 

 the sides of the neck and on to the back, which is largely 

 white, with dark shafts to the feathers. These birds are 

 obviously very closely related and it is sometimes debated 

 whether or not they are distinct species or merely color 

 phases of one and the same thing. The most convincing 

 evidence, in favor of the hypothesis of distinctness 

 perhaps being that arminjoniana and trinitatis, forms 

 which have long been known to science, seem to breed on 

 the islet at somewhat different times of the year. Chio- 

 nofara has only recently been described and is so far 

 known from the type specimen only. 



The writer has been particularly interested in these 

 birds and has studied them carefully with a view to 

 forming a definite opinion as to their relationship. The 

 bicolored type of plumage represented by arminjoniana 

 is perhaps the most common in the cosmopolitan pelagic 

 melius JEstrelata of which it is a member, but armin- 

 joniana is separated from most of the genus by the 

 greater development of dark color on the side of the neck 

 forming a sort of dark collar. Several uniformly dark 

 colored JEstrelata also occur in various parts of the 

 world, comparable to trinitatis. The third and recently 

 described form is complementary in color to t rut it at is, 

 being very white for the genus, and complementary in 

 pattern to arminjoniana, having the side of the neck 

 white instead of unusually dark. 



If we had three recently evolved forms breeding on the 

 same islet, complementary plumage is what would be ex- 

 pected, and that the three types of JEstrelata from there 

 show such plumages is good evidence that they are bona 

 fide forms, not color phases. 



