® 
1871.) 249 
[Cope. 
bart, tends to determine an increased amount of nutritive fluid to it, in 
other words to localize growth nutrition, and when this has attained com- 
plex 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 repeti- 
tion.) 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 ex- 
cessive use, exostoses appear on the bones surrounding the tibio-tarsal 
articulation. 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 dis- 
tinct types of animals. (Herbivora have no (dental) weapons and need 
horns). Itisseeninthe Batrachia Stegocephala in the extinct genus Cerater- 
peton ; among Anura it is approached by Triprion and Hemiphractus. 
Among Reptilia it is well marked in Phrynosoma, a Lacertilian genus. In 
Mammatia 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 aware that when attacked and unable to escape, they 
defend themselves by presenting the top of the head forwards 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 de- 
sree of ossification of the superior cranial walls, the longest being that of 
the Hylidx, which embraces six terms, viz: IHylella, Hyla, Scytopis, Osteo- 
cephalus, Trachycephalus and Triprion. The two last have the head 
thoroughly shielded, and 77tprion has projecting angles which appear in 
Some South American forms lately described by M. Espada, to be devel- 
Sped into short horns. That this excessive ossification is associated with 
the habit of protecting the whole body with the front, seems likely. 
In the case of Phrynosoma we know that precisely the same habit is as- 
Sociated with the presence of the sharp horns; and that some genera 
Without horns possess it also. Phrynosoma is an exceptionally sluggish 
senus in a family of mostactive forms, and must necessarily resort to this 
mode of defence more than they. 
In the case of Ruminants, we also know that defence is accomplished 
by throwing the head down with the horns thrown forwards. But this 
1Shot confined to this group. That generalized suborder, the Artiodactyla 
Ordinaria, represented by the hog, which were no doubt the genetic pre- 
decessors of the Ruminants in time, also throw the head down in defence 
in the same way, having thus a manner totally distinct from that seen in 
*Origin of Genera, 1868, p. 14. 
A. P, S— VOL, xII.—2F. 
