372 THE BIOLOGY OF BIRDS 



bears considerable resemblance to a Moorhen), a great 

 portion of the body feathers were in this condition. The 

 feathers of the Apteryx and Cassowary are also partially 

 destitute of barbules. Mr. Gurney was informed of a single 

 case of a Grey Brahma hen which showed the same 

 peculiarity, which appears otherwise to be without parallel. 

 The case of the Silky Fowl is similar in the absence of most 

 of the barbules, but in it the tip of the shaft is produced to a 

 delicate point, and the barbs are fine and sometimes bifid or 

 trifid at the apex. The colour of the skin and bones is 

 purplish blue." The " hairy moorhen " and the " silky 

 fowl " may serve as good examples of mutations. 



Professor Bateson (191 3, p. 42) gives another interesting 

 case among the Honey-creepers or Sugar-birds (Coereba 

 or Certhiola) of certain West Indian islands. They are 

 small birds about the size of a nuthatch, with a general 

 colouring of black, yellow, and white. Many of the islands 

 have types peculiar to themselves, as is usual in such cases. 

 From the island of St. Vincent the Smithsonian Institution 

 received in the late seventies of last century several com- 

 pletely black specimens in addition to two specimens of the 

 usual yellow and white " type." The black bird conformed 

 to " the type " except that the yellow pigment was obscured 

 by black ; it was a " melanic " mutation. It is interesting 

 to find that, as is happening with some melanic moths, the 

 dark mutation is replacing " the type." The yellow type 

 is now nearly or perhaps actually extinct ; the black form is 

 one of the commonest birds on the island. This case 

 illustrates the meaning of a mutation, and it also shows how 

 a mutation may succeed. 



There are many hints of species and varieties arising by 

 mutation, but the fact must not be overlooked that there are 

 other series of species among birds which strongly suggest 

 that evolution may sometimes have occurred by small steps 

 and slow stages, that is to say, without mutation. Thus 

 Dr. C. B. Davenport refers to the wide-ranging species of 

 the North American song-sparrow (Melospiza) which form 

 a graduated series, very different at the extremes. 



