384 ELEMENTARY ZOOLOGY. 



present time, whether of mollusks, of fishes, or of mam- 

 mals. The further back we go in the earth's history the 

 less similarity we find between the fossils and the present- 

 day life. The earlier strata show only invertebrate 

 remains; later the fishes appear, although much simpler 

 and more primitive than the fishes of to-day. Later still 

 appear amphibians, reptiles, mammals, and birds; and 

 last of all man's remains. 



The conditions are just what w r e should expect if life 

 appeared on the earth in its simpler forms and gradually, 

 by evolution through the ages, became complex and 

 modern. 



These things are not only true in a general way, but 

 have been found to be true of the special types of animals. 

 For example,' the fossil remains of modern horses have 

 been found in recent geological strata. The modern 

 horse has only one toe on each foot and walks on the end 

 of that toe. He has, however, some splints on either side 

 of this digit which point us to the history of his toes. In 

 the geological age preceding the present we find the 

 remains of an animal clearly like the horse, in which these 

 splints are larger and show more nearly the structure of 

 normal toes. By tracing the conditions backward in 

 geological times, links have been found which connect 

 the skeleton of the horse of the present day with an ani- 

 mal of the Eocene period, which had four toes on the fore- 

 feet and three toes on the hind feet and was little larger 

 than a fox. These steps are so complete that expert 

 students of fossils do not hesitate to regard that we have 

 a fair knowledge of the ancestry of the horse for perhaps 

 millions of years. Similar series of gradual changes are 

 shown among many species of fossil animals, as mollusks, 

 insects, fishes, birds, mammals. 



