No.2.] THE HARD PARTS OF THE MAMMALIA. 161 
fore limbs remaining of primitive character. The same is true 
of the kangaroos. In digging genera the fore limbs experience 
the modifications, while the hind limbs are more normal, as in 
Chrysochloris and various Edentata. 
Ryder sums up the evidence in two propositions as fol- 
lows : 1— 
“JT. The mechanical force used in locomotion during the 
struggle for existence has determined the digits which are now 
performing the pedal function 
in such groups as have under- 
gone digital reduction. 
“TI. When the distribution 
of mechanical strains has been 
alike upon all the digits of the 
manus or of the pes, or both, 
they have remained in a state 
of approximate uniformity of 
development.” 
The application of the im- 
pact, or strain, or both, in pro- 
gression, is easily understood. 
In recover (see p. 166), the leg 
is bent on the foot as it rests 
on the ground, and those dig- 
its which then leave the 
ground last, sustain greater 
strain than those which leave 
it sooner. In replacing the 
foot ,on the ground (planta- 
tion), those digits which strike Figure 11. — Equus caballus, fore and 
it first, experience greater force ping feet; from Cuvier. 
of impact than those which 
strike it later. Supposing the five primitive digits to have been 
of equal length, the distribution of the impact and of the strain 
will depend on the angle at which the foot is directed with 
reference to the direction of motion. If the feet are pointed 
forwards, the middle digits will experience strain and impact ; 
if outwards, the inner digits bear the weight; if inwards, the 
external digits receive it. 
1 American Naturalist, 1877 (Oct.), p. 607. 
