3 i 4 PRIMARY FACTORS OF ORGANIC EVOLUTION-. 



eater (Cycloturus) but one principal toe and two rudi- 

 ments. The excessive strain and impact experienced 

 by certain digits in leaping, accounts for the digital 

 reduction in the hinder foot of the kangaroos and jer- 

 boas, precisely as in the perissodactyle ungulates. 



c. The Horns. 



Horns are developed in Mammalia and other Ver- 

 tebrata on similar parts of the skull, principally on the 

 posterior lateral angles, as in various Batrachia, Rep- 

 tilia, and Mammalia, and on the nose, as in a few 

 Mammalia and several reptiles, recent and extinct. 

 These parts are the ones which are especially brought 

 into contact with resisting bodies ; the nose in pushing 

 a path or way for the head and body; the lateral occi- 

 pital region in defence and assault, when the sensitive 

 nose and eyes are protected by being held near the 

 ground. In the latter position the posterolateral an- 

 gles, when present, receive more frequent collision 

 with, and vigorous stimulation from, a body attacked 

 or resisted, and in accordance with the observed re- 

 sults of irritation on dermal and osseous tissues, addi- 

 tional matter has been deposited. In Lacertilia and 

 Batrachia Salientia there are distinct posteroexternal 

 cranial angles ; in Batrachia Urodela such angles are 

 less prominent. In unguiculate Mammalia and in all 

 others with a sagittal crest there are no such angles ; 

 hence this type of skull has never developed posterior 

 horns. The rhinoceros has developed the dermal 

 nasal horn, and the Elasmotherium, a median osseous 

 horn, since posterolateral angles of the skull are want- 

 ing or close together. In the Dinocerata and the Ar- 

 tiodactyla, where the temporal crests are lateral, leav- 

 ing a wide fronto-parietal plane with posterior lateral 



