No. 2.] THE HARD PARTS OF THE MAMMALIA. 273 
Confluence of cusps into crests generally. 
Canine teeth in general. 
Incisors of Rodentia, Multituberculata, etc. 
Incisors of Proboscidia, of Monodon, Halicore, etc. 
The relation of stimulus to nutrition is as yet so little under- 
stood, that there is plausible ground for the assertion that the 
hypothesis that use develops structure is “not proven.” What 
evidence there is, however, mostly supports the hypothesis, 
but the proof of the theory of kinetogenesis (¢.e. the origin 
and development of structure through motion) is not in the 
least indefinite or inconclusive. I point especially to the his- 
tory of the articulations, as described in the preceding pages. 
And the general principles which we derive from the investi- 
gation are applicable to the entire animal kingdom. 
The general law which we may derive from the preceding 
evidence is, that in biological growth, as in ordinary mechanics, 
Identical causes produce tdentical results. The evidence may be 
arranged under two heads, viz. : — 
I. The same structure appears in distinct phyla which are 
subjected to the same mechanical conditions. Examples: the 
identical character of the articulations of the limbs in Diplarthra 
and Rodentia which possess powers of rapid locomotion. The 
identical structure of the head of the radius in Edentata and 
Quadrumana which possess the power of supination of the manus. 
Identical reduction of the number of the digits under increased 
use of the limbs in many of the orders. Identical modifications 
of the form and development of the crests of the skull under 
identical conditions of use of the canine teeth for defence in all 
the orders where the latter are developed. Identical modifica- 
tion of dental cusps into longitudinal V’s and crescents under 
transverse thrusts in several orders, and into transverse cres- 
cents under longitudinal thrusts in Multituberculata. 
II. Different structures appear in different parts of the skele- 
ton of the same individual animal, in direct correspondence 
with the different mechanical conditions to which these parts 
have been subjected. Examples: the diverse modifications of 
the articulations of the limbs in consequence of the uses to 
which they have been put, in mammals which excavate the 
earth with one pair of limbs only; as in the fossorial Edentata, 
