228 AMPHIBIA 



habits and development, but here they are to be considered 

 specially from the point of view of evolution. In all cases 

 where an amplexus of the sexes takes place the male is distin- 

 guished by modifications of the parts which are used in holding 

 the female. A typical example of this occurs in the common 

 frog, the male having a swollen tubercle on the inner side of 

 the fore-foot covered with black skin, while the muscles of the 

 fore-leg and the leg generally are larger and stronger than 

 those of the female. The tubercle is pressed into the flanks of 

 the female and the greater strength of the limb muscles enables 

 the male to hold the female with sufficient power. The sexual 

 instinct in male frogs is so great that occasionally they fix on 

 to fresh-water fishes and blind them by pressing the tubercles 

 into their eyes. These secondary sexual characters develop to 

 a maximum in the breeding season and diminish very much 

 afterwards, they are therefore sometimes termed nuptial organs. 

 It has been recently shown that their development in the male 

 and absence in the female depends on a chemical secretion 

 produced by the testis and present in the blood. In a castrated 

 frog these characters fail to develop, but if portions of testis 

 from another frog are introduced under the skin of a castrated 

 specimen the organs develop as in an uninjured frog. This fact 

 supplies a new argument for the Lamarckian theory of the 

 evolution of such secondary sexual characters, for it is possible 

 that modifications originally due to friction of the skin or exer- 

 cise of the muscles may produce internal secretions which act 

 in the opposite direction on the reproductive organs. In all 

 Anura which practise an embrace similar to that of the common 

 frog there are similar modifications of the anterior limbs. In 

 Urodela various modes of amplexus occur with corresponding 

 modifications of structure. In Diemyctylus viridescens, an 

 American newt, the male clasps the female with his hind legs 

 either just before or just behind her fore-legs, and the hind legs 

 in the male are larger than in the female, and in the breeding 

 season hard rough black warts are developed on the inner sur- 

 faces of these legs. In the Pyrenean newt, Triton asper, the 

 male holds the female by twisting his tail round the hinder part 

 of her body, and the part of the tail used in this way by the 

 male is larger and more muscular than in the female. 



The most extraordinary of the adaptations for protecting 



