674 DR. J. C. MOTTRAM ON SECONDARY 



other reasons — namely, possession of terrestrial and arboreal 

 habits, of no defensive weapons, of non-diving habits, of no ill- 

 flavour to man, of small size, of continental habitat, of solitary 

 disposition, and of non-carnivorous habits — have been considered 

 to be relatively liable to the attack of enemies. These two 

 factors thus appear to be correlated, but it is obvious that, though 

 great sexual dimorphism is only to be found amongst birds liable 

 to attack, nevertheless the converse does not hold, for some 

 birds which do not appear to possess any special characters 

 which would protect them from atta,ck, none the less fail to 

 present any secondary sexual dimorphism, e.g. ibises, mound- 

 birds, mouse-birds, rollei'S, barbets, and coursers. 

 Conclusions may be briefly stated : — 



(1) Among birds specially liable to the attack of enemies, 



secondary sexual dimorphism is chiefly to be observed. 



(2) Among birds not specially liable to attack of enemies, 



secondary sexual dimorphism is i-arely found. 



(3) In both cases, birds presenting no sexual differences are 



repi'esented , but much more commonl}^ in bii'ds more or 

 less free from attack of enemies. 



Having come to these conclusions, it will not be out of place 

 to examine briefly the more important theories put forward to 

 account for secondary sexiial dimoi'phism, in order to discover 

 whether any of them be substantiated or not. 



Exuberance of Vital Energy in Males. — Wallace, in ' Tropical 

 Nature,' believed that natural selection could account for second- 

 ary sexual dimorphism, by supposing that only the strongest and 

 most virile males gained mates, and that these mates thus came 

 to possess an excess of vital energy which became manifest in 

 bright plumes, etc. If this be so, it is difficult to see the reason 

 Avhy birds vulnei'al)le to enemies should be specially picked out. 

 One would think that it would be dangerous for these birds to 

 make themselves thus conspicuous, and would have anticipated 

 that birds not liable to attack could have done so with more 

 impunity. 



Sexual Selection (D-a^vwin., '■Tfescent of Man'). — The argument 

 used above applies similarly against Darwin's theory, which in 

 no way can account for the correlation under examination. One 

 would have thought that female selection would have had fi'eer 

 play among birds more or less free from enemies than among 

 birds frequently subiect to their attack. 



Nesting -Habits Theory (Wallace and othei's). — It has been noted 

 that birds liable to the attack of enemies, and making open nests, 

 more frequently present secondary sexual dimoiphism than birds 

 making closed nests. If, however, birds liable to the attack of 

 enemies more often present secondary sexual dimorphism than 

 birds in general, then it follows that birds making open nests, 

 and also liable to the attack of enemies, must do likewise. 



It is thus obviously necessarv to decide which is the more 



