DOCTRINE OF EVOLUTION AND RELATED IDEAS 485 



other side. In cases like this, which are by no means infre- 

 quent, it is beheved that the animals found in "b" and "c" 

 have migrated into their respective regions and in becoming 

 adapted to their new condition of life have so changed or evolved 

 as to become different from their parent stock and from each 

 other. 



In a similar way, but on a much larger scale and in a form 

 too cornplex to discuss here, we find evidences of change and 

 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 moUusks, 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 more primitive and generalized 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 modem 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 structtire 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, 

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



