244 ANIMAL ACTIVITIES. 
Fossil Forms. Much light has been thrown on the 
subject of animal kinships by a study of the animals 
which have previously lived on the earth. The history 
of these animals is written in the rocks where their 
rossil remains are found. From a study of these 
femains it is generally believed that the first animals 
existing on the earth were Protozoa. In general, we 
may say that the older the rocks the simpler are the 
fossil forms found in them. 
Were it not for the evidence furnished by fossils, we 
should hardly suspect that the first-known horses were 
Fic. 197.—-Feet of Ancestors of Horse. The figures indicate the num- 
bers of the digits in the five-fingered hand of most mammals. 
five-toed animals about the size of foxes. America 
when discovered had no living horses, yet wild horses 
are now found in Central and South America. These 
wild horses are in part the descendants of domestic 
animals brought to this continent by man; but some 
are also, doubtless, descendants of animals like those 
whose remains are found in American rocks. 
Among the fossil forms, too, we find the remains of 
many animals having characteristics shared by widely 
differing groups of animals of the present day. Such 
animals are supposed to be the ancestors of the differ- 
