FISHES 185 



such as to allow a very limited motion downward, but much 

 greater freedom of movement sideways. In the posterior 

 half of the organ a bend is made either to the right or the 

 left. Of 17 males, the bend was to the right on 11, to the 

 left on 6. There is also a small fleshy tubercle at the side 

 of the seventh or the sixth anal ray, at the middle of its 

 length. When this prominence is on the left side, the organ 

 bends to the right ; when it is on the right, the bend is to 

 the left. The urinogenital tube lies on the concave side of 

 the bend. In the female a large scale covers the opening of 

 the ovary and ureters. This scale may be called a shutter, 

 or foricula. It is attached at one side and free at the other, 

 the opening being to the right in some specimens, to the left 

 in others. Thirty-four females were lefts, twenty-one rights : 

 thus the proportion is the opposite of that found in the 

 males, in accordance with the requirement that right males 

 should copulate with left females, and vice versa. 



In all these adaptations everything is in harmony with 

 the theory that their evolution is due to gradual hyper- 

 trophy and modification due to the use made of the parts. 

 The selection theory assumes that the necessary slight modi- 

 fications have occurred, but we have no reason for believing 

 that such necessary variations ever came into existence until 

 the irritations caused by the habitual act of copulation pro- 

 duced them. 



Cyprinidce. In many of the Cyprinidse or Carp family, 

 wart-like tubercles are developed in the male during the 

 breeding season, chiefly on the head, but sometimes all 

 over the body and fins. These disappear when the annual 

 spawning has been accomplished. That these dermal ex- 

 crescences are dependent upon mechanical irritations there 

 can be no reasonable doubt, and there can also be no doubt 

 that they are now hereditary. Fatio 1 and several other 



1 Faune desvcrttbres de la tiuissc, vol. iv. 1882. 



