3 go PRIMARY FACTORS OF ORGANIC EVOLUTION. 



series of corneous rugae. In the Leptodactylus penta- 

 dactylus Laur. a huge acute process of the metacarpal 

 of the thumb, projects inwards. Its apex is covered by 

 a horny cap, and it is a formidable grappling-hook to 

 aid the male in retaining his hold. There is added to 

 this in the same species a horny plate on each side of 

 the thorax of the male, from which project three acute 

 points. With these fixed in her back and the thumb 

 spikes in her breast the females cannot escape. Struc- 

 tures like this do not appear in the Firmisternia. 

 Here the inferior elements of the scapular arch abut 

 against each other, so that the thoracic cavity does 

 not contract on pressure, and the possibility of the 

 male retaining a firm grip on the female is thereby 

 greatly increased. In the Cystignathus pachypus the 

 males exhibit a permanent enlargement of the bra- 

 chium, dependent on largely developed anterior and 

 posterior alas of the humerus. (Vide Gimther, Ann. 

 Mag. N. H., 1859.) 



Another kind of male selection is accomplished by 

 the combats of males for the possession of the females. 

 This is usual in polygamous birds and Mammalia, and 

 in some promiscuous species of both. Of the birds the 

 Gallinae form the best known example ; and of the 

 mammals, most Ungulata, and the eared seals (Otari- 

 idae), are illustrations. In this way the weak males 

 are eliminated either by death, or by exclusion from 

 the opportunity of reproduction. The males in such 

 species are armed with spurs, horns, or large teeth, 

 except in some of the Perissodactyla, which have 

 neither. 



Female selection is seen in another direction. Here 

 the male attracts by the superior brilliancy of his colors 

 or peculiarity of physical appearance, as well as by 



