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SEXUAL DIMORPHISM 



sidered by some to support the selection theory. But it 

 must be noted that in the descriptions of the habits of court- 

 ship there is nothing to show that the males perform their 

 antics in rivalry with other males, their object is to excite 

 the female sufficiently to induce her to take up the sper- 

 matophores they deposit, and according to Gasco, the move- 

 ments themselves serve chiefly to excite the generative 

 organs of the males themselves. Thus, if there is a selection, 

 a superiority of some males over others, which I am not 

 concerned to deny, it is not the selection of the most 

 beautiful or the most ornamented by the females. Success 

 is to the most eager and virile male, and the particular 

 movements which express his virility have determined his 

 unisexual characters, as the physiological effect of mechanical 

 forces. The dorsal crest where it exists is thus the result of 

 the habits of courtship, but there is nothing to show that it 

 in any way contributes to the success of the courtship ; it is 

 the consequence of the behaviour of the sexes towards each 

 other, not a means by which their union is effected. 



On the other hand, amplexus occurs in other species 

 of Urodela. In Molge (Pleurodeles) waltlii of the south of 

 Europe, the male places himself beneath the female and 

 clasps her fore limbs with his own. His tail performs rapid 

 undulations, and the membrane of it is broadened in the 

 breeding season, but in addition to this the epidermis of his 

 fore limbs is thickened by the pressure and friction of the 

 amplexus, as in the frog. 



In Molge aspera the peculiarity of the male consists in a 

 somewhat greater size of the hind limbs, and a shorter, 

 thicker, and more muscular tail. There are no ornamental 

 colours or crests. Even the tail is without a fin-membrane. 

 This species lives in lakes at considerable altitudes in the 

 Pyrenees. The peculiarities of the male as usual correspond 

 to its mode of courtship. It seizes and holds the female by 



