354 BIOLOGY AND ITS MAKERS 
After this kind of revelation in reference to lower animals, 
we turn with awakened interest to the fossil bones of the 
higher animals. 
Evolution of the Horse.—When we take into account the 
way in which fossils have been produced we see clearly that 
it is the hard parts, such as the shells and the bones, that will 
be preserved, while the soft parts of animals will disappear. 
Is it not possible that we may find the fossil bones of higher 
animals arranged in chronological order and in sufficient 
number to supplement the testimony of the shells? There 
has been preserved in the rocks of our Western States a very 
complete history of the evolution of the horse family, written, 
as it were, on tablets of stone, and extending over a period 
of more than two million vears, as the geologists estimate 
time. Geologists can, of course, measure the thickness of 
rocks and form some estimate of the rate at which they were 
deposited by observing the character of the material and com- 
paring the formation with similar water deposits of the 
present time. Near the surface, in the deposits of the 
quarternary period, are found remains of the immediate 
ancestors of the horse, which are recognized as belonging 
to the same genus, Equus, but to a different species; thence, 
back to the lowest beds of the tertiary period we come 
upon the successive ancestral forms, embracing several dis- 
tinct genera and exhibiting an interesting series of trans- 
formations. 
If in this way we go into the past a half-million years, we 
find the ancestors of the horse reduced in size and with three 
toes each on the fore and hind feet. The living horse now 
has only a single toe on each foot, but it has small splint-like 
bones that represent the rudiments of two more. If we go 
back a million years, we find three toes and the rudiments 
of a fourth; and going back two million years, we find four 
fully developed toes, and bones in the feet to support them. 
