198 GENERAL EVOLUTION". 



cialized of all snakes, standing at the head of the order, and as 

 such, on the principle of acceleration, present the greatest amount 

 of grade-nutrition. 



Now it appears to me that the constant habit of violent vibra- 

 tion in a part tends to determine an increased amount of nutritive 

 fluid to it, in other words to localize growth-nutrition, and when 

 this has attained complex repetition or grade-nutrition, to result 

 in new grade-structure. (The segments of the rattle being nearly 

 all alike, it is a case of simple repetition.) This view appears to 

 be as reasonable as that generally entertained with regard to the 

 cause of spavin in the horse's leg. Here, owing to excessive use, 

 exostoses appear on the bones surrounding the tibio-tarsal articula- 

 tion. As to the reason of the structure in question not appearing 

 in forms lower in the scale than the rattlesnake, it is explained 

 below, if the law of accumulation of grade-nutrition be true. (See 

 Sec. B.) This is, that repetition (or acceleration) is only possible 

 where the animal has an excess of growth-force at its disposal, or 

 can abstract it from some portion which is unused or useless. 



y. On horns. — The possession of horns on the posterior part 

 of the cranium, as defenses against enemies, is a character found 

 in many distinct types of animals. (Herbivora have no (dental) 

 weapons and need horns.) It is seen in the Batrachia Stego- 

 cephala in the extinct genus Ceraterpeton ; sawoi^g Anura it is 

 approached by Triprion and Hemipliractus. Among Eeptilia it 

 is well marked in Phrynosoma, a Lacertilian genus. In Mamma- 

 lia the Artiodactyla Ruminantia are the horned animals of the 

 order. We have opportunities of observing the habits of these 

 representatives of the Frogs, the Lizards, and the Mammals. 



In the first case, any one who has kept ordinary toads and 

 tree-toads in confinement, is aAvare that when attacked and unable 

 to escape, they defend themselves by presenting the top of the 

 head forward and using it as a shield. Now I have already 

 pointed out * that in both toads, tree-toads, and frogs there are 

 natural series of genera, measured by the degree of ossification of 

 the superior cranial walls, the longest being that of the HylidiB, 

 which embraces six terms, viz. : Hylella, Hijla, Scytopis, Osteo- 

 cephalus, Trachyceplialus, and Triprion. The two last have the 

 head thoroughly shielded, and Hemipliractus has projecting an- 

 gles which appear in some South American forms, described by M. 



* " Origin of Genera," 1868, p. 14. This work, article I. 



