Chap. XV. Birds — Development of Spurs. 449 



early in life are apt to be transmitted to both sexes ; not that all 

 the cases of sexually-limited transmission can thus be accounted 

 for. It was further shewn that if a male bird varied by be- 

 coming brighter whilst young, such variations would be of no 

 service until the age for reproduction had arrived, and there 

 was competition between rival males. But in the case of birds 

 living on the ground and commonly in need of the protection of 

 lull colours, bright tints would be far more dangerous to the 

 young and inexperienced, than to the adult males. Conse- 

 quently the males which varied in brightness whilst young- 

 would suffer much destruction and be eliminated through 

 natural selection ; on the other hand, the males which varied in 

 this manner when nearly mature, notwithstanding that they 

 were exposed to some additional danger, might survive, and 

 from being favoured through sexual selection, would procreate 

 their kind. As a relation often exists between the period of 

 variation and the form of transmission, if the bright-coloured 

 young males were destroyed and the mature ones were suc- 

 cessful in their courtship, the males alone would acquire bril- 

 liant colours and would transmit them exclusively to their 

 male offspring. But I by no means wish to maintain that the 

 influence of age on the form of transmission, is the sole cause of 

 the great difference in brilliancy between the sexes of many 

 birds. 



When the sexes of birds differ in colour, it is interesting to 

 determine whether the males alone have been modified by sexual 

 selection, the females having been left unchanged, or only partially 

 and indirectly thus changed ; or whether the females have been 

 specially modified through natural selection for the sake of pro- 

 tection. I will therefore discuss this question at some length, 

 even more fully than its intrinsic importance deserves ; for various 

 curious collateral points may thus be conveniently considered. 



Before we enter on the subject of colour, more especially in 

 reference to Mr. Wallace's conclusions, it may be useful to 

 discuss some other sexual differences under a similar point of 

 view. A breed of fowls formerly existed in Germany 6 in which 

 the hens were furnished with spurs ; they were good layers, but 

 they so greatly disturbed their nests with their spurs that they 

 could not be allowed to sit on their own eggs. Hence at one time 

 it appeared to me probable that with the females of the wild 

 Gallinacese the development of spurs had been checked through 

 natural selection, from the injury thus caused to their nests. 

 This seemed all the more probable, as wing-spurs, which would 

 not be injurious during incubation, are often as well-developed 

 • Bechstein, ' Natuvgesch. Deutschlands,' 1793, B. iii. s. 339. 



