No. 2.] THE HARD PARTS OF THE MAMMALIA. 159 
the extremities of the metapodial bones, and the phalanges take 
but little share in it. They are turned forwards and are nearly 
useless. Their great reduction in dimensions in these forms 
appears to me to have followed disuse, and this is then the 
probable cause of it. 
2. THe NuMBER OF THE DIGITs. 
The reduction in the number of toes is supposed to be due to 
the elongation of those which receive the greater number of 
strains and impacts in rapid progression, and the complementary 
loss of material available for the growth of those not subject to 
this stimulus. This is rendered probable 
from the fact that the types with reduced 
digits are dwellers on dry land, and those 
that have more numerous digits are inhabit- 
..\ ants of swamps and mud, or are more or less 
\ aquatic. That this inequality is due to these 
: mechanical causes is still further indicated by 
\ 4 the fact that in those forms where the soles 
$ are thickly padded (Carnivora, Proboscidia) 
the reduction has either not taken place, or 
has made little progress, amounting to the 
loss of only one digit. (An ap- 
parent exception in the case of 
the camels will be mentioned 
Figure 9.— Anterior feet of primitive Ungulata, reduced. A, Phenacodus pri- 
mavus. B, Coryphodon elephantopus. C, Hyracotherium venticolum. 
