DOCTRINE OF EVOLUTION AND RELATED IDEAS 479 



development in the animals of the great continental and other 

 natural divisions of the earth. 



483. Evidences from Geological History. In the rock 

 strata of the crust of the earth we find abundant plant and ani- 

 mal remains in the form of fossils. In the most recent strata 

 we find remains similar to the species of the present time,, 

 whether of mollusks, of fishes, or of mammals. The further 

 back we go in the earth's history the less similarity we find be- 

 tween 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 we should expect if life ap- 

 peared 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 animal of the Eocene period, 

 which had four toes on the forefeet and three toes on the hind 

 feet and was little larger than a fox. These steps are so com- 

 plete 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, 

 Crustacea, fishes, birds, and mammals. 



All this is entirely without explanation if animals have not 

 been subject to gradual evolution through the ages. 



