General Notes. 
Torsion, impact, and flexure in one plane. 
Tongue and groove joints in many orders. 
Flexure in two planes. 
Saddle-shaped cervical vertebrce in Quadrumana. 
Flexure in several directions. 
A A. Motion not in articulation. (Teeth.) 
7. Displacement of cusps of triconodont molars by crowding*. 
Tri tubercular molars. 
8. Transverse thrust. 
The Vs of molars teeth in various orders. 
9. Longitudinal thrust. 
The Vs of the Multituberculata. 
Obliquity of molars in many Rodentia. 
10. Stimulation of pressure and strain. 
Incisors of Rodentia, Multituberculata, etc. 
Prismatic molars of Diplarthra, Rodentia, etc. 
Confluence of cusps into crests generally. 
Sectorial teeth of Carnivora. 
Canine teeth in general. 
Incisors of Proboscidia, Monodon, Halicore, etc. 
As a general result we may assert that that it is a general law of 
animal as of other mechanics — viz., that identical causes produce 
identical results. The evidence for this law may be arranged under 
two heads, as follows : 
I. The same structure appears in distinct phyla which are sub- 
jected to the same mechanical conditions. Examples of this are : 
the identical character of the articulation of the limbs in Diplarthra 
and Rodentia which possess powers of rapid locomotion. The iden- 
tical structure of the head of the radius in Edentata and Quadru- 
mana which possess the power of complete supination of the manus. 
Identical reduction of the number of the digits under increased use 
of the limbs in many of the orders. Identical modification of den- 
tal cusps into longitudinal Vs and crescents under transverse strains 
in several orders, and into transverse crescents under longitudinal 
strains, in the Multituberculata. Identical modifications of the 
form and development of crests of the skull under identical condi- 
tions of use of the canine teeth for defence, in all the orders where 
the latter are developed. 
II. Different structures appear in different parts of the skeleton 
of the same individual animals in consequence of the different me- ^ 
chanical conditions to which these parts have been subjected. Ex- ' 
amples: the diverse modification of the articulations of the limbs m 
consequence of difference of the uses to which they have been put, 
in mammals which excavate the earth witli one pair of limbs only, 
as in the fossorial Edentata, Insectivora, and Eodentia. The reduc- 
tion of the number of digits in the posterior limb only, when this is 
exclusively used for rapid progression, as in leaping ; this is seen 
