No: 2. THE HARD PARTS OF THE MAMMALIA. 151 
a. Increase of Length by Impact. 
The increase in the length of the legs has not been always 
due to increase in length of the same segment. In a majority 
of the higher mammals, the increase has been principally in the 
foot, and especially in the metapodials and digits, producing 
digitigradism. In the forms which have remained plantigrade, 
the femur (Proboscidia), or femur and tibia (Quadrumana), or 
all three segments (Tarsius), have been the seat of the elonga- 
tion. We can again trace these especial elongations to special 
uses. In animals which leap, the distal segments of the limbs 
are elongated; in those which do not leap, but which merely 
run or walk, it is the proximal segments of the 
limbs which are elongated. 
Animals which run by leaping are divided into 
those which run and leap with all fours, as Dip- 
larthra ; and those which run and leap with the 
posterior limbs only, as the jerboas and kan- 
garoos. In both types, the 
distal segments of the hind 
limb are elongated, and in 
the Diplarthra, those of 
the fore limb also. 
Animals which do not 
leap in progression (ele- 
phants, Quadrumana, bears) 
are always plantigrade, and 
have very short feet, but 
elongate thighs, and most- 
ly, tibias. 
These? factsi’rendér it 
highly probable that those 
elements which receive the 
principal impact in pro- 
gression are those which 
increase in length. In dig- 
Figure 2.— Pes of (A) Merychocherus montanus ; (B) Bos taurus, much reduced. 
Ca. Calcaneum; As. Astragalus; Ma. Navicular; Wcé. Naviculocuboid; Cz. JWec., 
Ecto-mesocuneiform; J/¢. Metatarsals (cannon bone); Zc, Entocuneiform. 
