406 OUTLINES OF EVOLUTIONARY BIOLOGY 



arthropods; in the huge labyrinthodont amphibians; in many 

 groups of reptiles of the Secondary period, some of which 

 attained a length of 80 feet or more, and amongst mammals in 

 the extinct Tinoceras (Fig. 150) and the still surviving elephants 

 and whales. Comparative anatomists are familiar with similar 

 phenomena exhibited by individual organs, such as the extra- 

 ordinary development of horns and spines in many of the 

 extinct reptiles referred to (Fig. 145), the immense tusks of 



FIG. 185. Head of Babirusa alfurus. (From Flower and Lyddeker's 

 " Mammals Living and Extinct.") 



the babirusa (Fig. 185), and the gigantic and grotesque beak 

 and " helmet " of the hornbill (Fig. 186). 



The exuberant development of some organs of this kind may 

 possibly be attributed to the action of sexual selection, and indeed 

 our daily experience of our own species seems to warrant us in 

 believing that there is no limit to the grotesque results which 

 may ensue from the unrestricted exercise of the aesthetic faculties 

 of either sex, but it seems hardly reasonable to attempt to 

 explain all such bizarre and monstrous productions in this 

 manner. 



In all the cases cited, and in many others which could be 

 adduced, either the entire body or some particular organ appears 

 to have acquired some sort of momentum, by virtue of which it 

 continues to grow far beyond the original limits of utility, although 

 perhaps in some cases a new use may be found which will assist 

 the species in maintaining itself in the struggle for existence. 



