PALEOXTOL OGY 1 09 



lower we find shells of a gradually simpler and simpler 

 form, less corrugated and with less irregular aperture and 

 less elongated from mouth to apex. \Ye have here in these 

 fossils a most complete record of the several steps in the 

 evolution of the irregular, rugose shells of this species of 

 pond snail. Such a series points almost indisputably to 

 the theory of descent with modification for its explanation. 

 There are many indications of close resemblance between 

 birds and reptiles, but the descent of the former from the 

 latter is most clearly shown by the numerous fossil forms 

 which bridge the gap between the two groups. Notice the 

 accompanying drawings of three of these intermediate forms: 

 Arch&opteryx (Plate 44); Hespcrornis (Plate 45, A}; and 

 Ichthyornis (Plate 45, B}. Compare these drawings with 

 Plate 45, C which represents the skeleton of one of the 

 ancient flying reptiles, and with the skeleton of a bird as 

 shown in Fig. 27. The intermediate forms first figured so 

 approach the character of the flying reptiles as to strongly 

 indicate that they are descended from the latter, but they 

 are true birds. The fact of the development of the birds 

 from the reptiles is very clearly indicated in the discovered 

 fossils which are intermediate in structure between the two 



One further illustration will be sufficient. The record 

 of the origin of the horse, worked out by American paleon- 

 tologists from American fossils, is probably the best example 

 of paleontological evidence of evolution. The horse is 

 especially peculiar in the character of its feet and teeth, and 

 we will direct our attention to these points as shown in the 

 accompanying illustrations. In the lower Eocene rocks 

 we find an animal, Phenacodus, about the size of a fox, 



