FISHES 227 



incipient outgrowths of the ventral edges of the tail occurred 

 apart from the presence of the eggs, and such incipient out- 

 growths must have occurred on the selection theory. We do 

 not find such incipient outgrowths even in the females of 

 the same species, and the theory of selection makes no 

 attempt to explain the origin of the outgrowths. 



In the genus Solenostoma, which occurs on the east coast 

 of Africa and in the East Indies, according to Giinther, the 

 pouch for containing the eggs is in the female instead of in 

 the male. We have thus in this order a striking illustration 

 of the truth that special growths are due not to any general 

 properties of male or female constitution, but to special 

 mechanical stimulations. Such stimulations act on the 

 nerves and give rise to habitual reflex actions in muscles ; 

 the strains produced by such habitual exertions of the 

 muscles result in growth in particular directions of the 

 parts affected. It would be desirable to investigate in detail, 

 from the point of view here indicated, the structure and 

 development of the egg-pouch in these fishes. 



According to Giinther, as quoted by Darwin, the female 

 Solenostoma is more vividly coloured and spotted than the 

 male, although a difference of colour between the sexes is 

 not generally characteristic of the Lophobranchii. It is 

 probable that in this case the female is in the habit of 

 courting the male to induce him to fertilise her eggs, in 

 which case her more brilliant colouring, as in the males 

 of other species, might well be the result of nervous ex- 

 citement. 



