182 SEXUAL DIMORPHISM 



unless we get over the difficulty by supposing that the adult 

 males change their diet. 



CmMiEROiDS. In Chimccra the sexual peculiarities of the 

 males are highly specialised. The hinder part of the ventral 

 fin forms a complicated intromittent organ of large size, and 

 in addition to this there is in front of the same fin a fiat 

 projection provided with two curved spines, which is evi- 

 dently used for holding the female. On the head there is 

 further a remarkable knobbed and curved spine projecting 

 forwards, and covered at its extremity with sharp denticles. 

 There can be no doubt that this also is used for holding the 

 female, although the exact mode in which it is used has not 

 been observed. 



The intromittent appendages of Elasmobranchs, and the 

 various sexual appendages of male Chiimeroids above de- 

 scribed, differ from most of the cases previously discussed 

 among the higher vertebrates in their antiquity and their 

 uniformity throughout the orders. At the same time, there 

 is no insuperable obstacle to regarding them as probably 

 evolved in accordance with Lamarckian principles. The use 

 of part of the pelvic fins as intromittent organs may well 

 have been, at the very distant period when these fishes 

 acquired their characters, the origin of the so-called claspers 

 or pterygopodia. It has been suggested that the frontal 

 clasper of the male Chimoeroid is a modification of an an- 

 terior fin-spine ; but as it is covered with skin and separate 

 denticles, it is quite possible that it was formed independ- 

 ently by the use of the skin in this region. In this case we 

 cannot reason with such a good basis as in that of the antlers 

 of stags, because the habits of the living Chimoera are so 

 little known. 



Teleostei Sub-order Physostomi Siluridce. Darwin 

 refers to the sexual differences in Plecostomus harhatus, which 

 lives in the fresh waters of South America, as described by 



