208 SEXUAL DIMORPHISM 



has been the cause of the hypertrophy of the fins in the 

 male. I do not pretend to know why the muscular exertion 

 of erecting the fin should increase the length of the fin-rays. 

 It might be urged that it would only increase the size of the 

 muscles concerned. But the muscles are attached to the 

 fin-rays, and the erection of the fin causes a constant stretch- 

 ing and folding of the fin-membrane. The movement is, 

 therefore, an irritation of the tissues of both fin-rays and 

 membrane, and, in my opinion, we are forced to conclude 

 that the increased size which exists has been the result of 

 this irritation. Again, in the copulatory vertical ascent to 

 the surface of the water, the motion is principally due to 

 the vibration of the caudal-fin and of the hinder parts of the 

 second dorsal and ventral fins of the male, and it is just 

 these parts which are larger in the male than in the female. 

 Again, in the display with which the male invites the female, 

 the jaws and gill-covers are protruded, and corresponding to 

 this movement there is a hypertrophy, at any rate of the 

 snout region, in the male as compared with the female. 

 Lastly, the greater size and strength of the male, as a whole, 

 corresponds to the greater activity and exertion he exhibits 

 in the process of courtship and copulation. 



Mr. Holt points out that the male dragonet makes the 

 gestures observed in courtship also, to a certain extent, under 

 other circumstances. He erects his dorsal fins when rushing 

 to seize a worm, and if young males are present they flee 

 before him. He also raises his fins and increases the bright- 

 ness of his blue bands when chased about the tank with a 

 net, and was observed to do the same when alarmed by a 

 turbot introduced into the tank. He does not, however, 

 under these circumstances assume the full courting attitude, 

 and sometimes preferred to remain still in order to escape 

 observation. The gesture by no means prevented males 

 from being seized and devoured by predaceous fish, and may 



