356 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 w^hich 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 years, as the geologists estimate 

 time. Geologists can, of course, measure the thickness of 

 rocks and form some estimate of the rate at which thev 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 vears, 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. 



