382 THE BIOLOGY OF BIRDS 



rabbit, which may be due to the meagre circulation of the 

 blood in an adult lens. But the issue is important : a 

 specific modification of the lens was induced in the young 

 in utero by means of a peculiarly sensitised serum, and the 

 deleterious effect on the lens was observed in a second generation 

 of rabbits developing without interference. 



§ 7. MODIFICATIONAL SPECIFIC CHARACTERS 



It is highly probable that some of the diff"erences 

 between species or varieties are purely modificational or 

 nurtural, that is to say, that they are impressed on each 

 individual by some peculiarity in environment, nutrition, 

 or function. The well-known newt, Proteus anguineus, 

 of the caves of Carinthia, Carniola, and Dalmatia is 

 destitute of pigment, and no specimens in the caves have 

 ever been found that were otherwise. It might be thought 

 that the wan white colour was a definite specific character, 

 an ingrained constitutional defect. But it is well known 

 that if specimens are brought out of the darkness and kept 

 in the light they rapidly put on pigment. The absence of 

 pigment in the cave specimens is not due to any constitu- 

 tional defect, it is due to the absence of light which is 

 necessary as a liberating stimulus for pigment formation. 



Linnets from California were introduced about half a 

 century ago into the Hawaiian Islands. In the United 

 States the usual colour of the males after the post-juvenal 

 moult is red, in those parts of the plumage which are brightly 

 coloured. There occur rather infrequently, irrespective of 

 locality, individuals of three other colours — yellow (most 

 rarely), orange, and red with yellow or orange feathers inter- 

 mixed. Now Mr. Joseph Grinnell points out (191 1) that 

 male linnets in Hawaii are all of the yellow or orange type 

 of coloration, and that is in some way due to the island 

 habitat. It is not possible to say what the stimulus is, 

 though Grinnell thinks that it is not a very obvious one, 

 or what is lacking in stimulus ; but it need not be anything 

 very momentous. For it seems that the diff'erence in colour 



