220 THE BIOLOGY OF BIRDS 



it should occur in related types in the males only and be used 

 only in their combats, it would illustrate Tandler's theory. 

 And it should be noticed that in the pugnacious extinct 

 Solitaire of Rodriguez there is a walnut-like excrescence (an 

 exostosis) developed on the lower end of the radius and a 

 similar tubercle on the upper end of the carpo-metacarpus — 

 in the males only. 



It must be borne in mind that in many cases where the 

 sexes are practically identical, there are special features that 

 appear in mature birds at the breeding season, such as the 

 extra horny plates at the root of the puffin's brightly coloured 

 bill, the white hair-like feathers occurring in the dark 

 plumage of cormorants on head, neck, and shank, the long 

 horny orange-coloured up-growth at the root of the bill in 

 Chimerina cornuta, one of the auks of the Bering Sea. In 

 the case last mentioned the process is moulted off — like the 

 puffin's accessory plates — in the autumn, and developed 

 afresh in spring. 



The thesis that : "All secondary sex-characters were 

 at first specific characters," appears to us to be an exaggera- 

 tion of a sound idea, A specific character may be trans- 

 formed into a secondary sex-character, so that it has a 

 different expression in the two sexes, as when the tail-coverts, 

 present in both peacock and peahen, are transformed into 

 the male's gorgeous tail. But an exclusively male character 

 may arise de ?iovo, apart from any previous counterpart 

 among the specific characters. And similarly for an 

 exclusively female character. 



There are, it seems to us, numerous peculiarities of one 

 sex or the other which cannot be readily derived from specific 

 characters supposed to be common to both sexes. And if it 

 be said that the cases we would adduce are not fair samples 

 of sex-characters, we would reply that it is very difficult to 

 draw a line round " secondary sex- characters," separating 

 them from other sex- differences. This is especially difficult 

 among Invertebrate animals where we have little knowledge 

 of glands of internal secretion connected with the essential 

 gonads, and are therefore bereft of that useful criterion of a 



