46 ANIMAL PHYSIOLOGY. 
time in America by forms with a greater number of toes, the 
latter increasing according to the antiquity of the group. 
Fic. 48.—Bones of the feet of the different genera of noe (after Marsh). a, foot of Oro- 
hippus (Eocene); b, foot of Anchitherium (Lower Miocene) ; c, foot of Hipparion (Plio- 
cene) ; d, foot of the recent genus Hquus. 
These forms occur in succeeding geological formations. It is 
impossible to resist the conclusion that they are related gene- 
alogically (phylogenetically). : 
8, Progression.—Inasmuch as any form of specialization that 
would give an animal or plant an advantage in the struggle for 
existence would be preserved, and as in most cases when the 
competing forms are numerous such would be the case, it is 
possible to understand how-the organisms that have appeared 
have tended, on the whole, toward a most pronounced pro- 
gression in the scale of existence. This is well illustrated 
in the history of civilization. Barbarous tribes give way be- 
fore civilized man with the numberless subdivisions of labor 
he institutes in the social organism. It enables greater num- 
bers to flourish as the competition is not so keen as if activities 
could be exercised in a few directions only. 
9, Domesticated Animals.—Darwin studied our domestic ani- 
mals long and carefully, and drew many important conclusions 
from his researches. He was convinced that they had all been 
derived from a few wild representatives, in accordance with the 
principles of natural selection. Breeders have, both consciously 
and unconsciously, formed races of animals from stocks which 
the new groups have now supplanted ; while primitive man had 
tamed various species which he kept for food and to assist in 
the chase, or as beasts of burden. It is impossible to believe 
that all the different races of dogs have originated from dis- 
tinct wild stocks, for many of them have been formed within 
recent periods; in fact, it is likely that to the jackal, wolf, and 
