NATURAL SELECTION. 389 



divert omnivorous forms into carnivorous or herbivo- 

 rous habits as abundance of food and competition of 

 rivals might dictate. 



Sexual selection is of two general classes, that in 

 which the male selects, and that in which the female 

 determines the result. In the former case the most 

 vigorous males, or those in which the mechanisms for 

 seizing the females are most effective, propagate their 

 kind most successfully. It is well known that in many 

 animals, especially the Arthropoda, the males are fur- 

 nished with especial organs of prehension. In verte- 

 brates similar organs are especially conspicuous in 

 some of the Batrachia Salientia. (See p. 65.) 



The species of Arcifera exhibit peculiar structures 

 during the breeding season ; either an extension of the 

 natatory membrane, or the development of corneous 

 plates or spurs, as aids to prehension. There is much 

 variety and efficiency displayed in this point (except 

 in Bufonidae), in especial contrast to the apparent ab- 

 sence of all but the weakest modifications among the 

 Ranidae. This is in compensation for the structure of 

 the sternum, whose lateral halves, being movable on 

 each other, offer a slighter basis of resistance for the 

 flexor and extensor muscles of the fore limbs of the 

 male. In the Firmisternia the halves of the shoulder 

 girdle do not overlap below on the middle line, but 

 abut against each other, thus preventing compression 

 (Fig. 51, page 198). 



While no appendages of the season have been ob- 

 served in some Cystignathidae, in several genera two 

 acute spurs appear on the superior aspect of the thumb 

 and more rarely spur-like tubercles on the breast ; the 

 body is sometimes shielded with hardened points on 

 the rugosities, or the lip surrounded by an arched 



