186 EVOLUTION AND GENETICS 



logical explanation. A transition by gradual steps from a rela- 

 tively primitive state, such as that of Eohipjms, to the highly 

 specialized modern horse, correlated with geological time succes- 

 sion and evidence of chmatic change, is a part of that natural 

 record which, if complete, would be the whole story of evolution. 

 Summary. The fossil remains of many vertebrates show a 

 gradual transition in structure from primitive ancestors to existing 

 species. The evolution of the elephants, horses, and camels is 

 especially well demonstrated by these records. Evolution in the 

 elephants is chiefly linked with the development of great bulk and 

 browsing habits, and is shown by the development of pillar-like 

 limbs, finely ridged grinding teeth, and characteristically high 

 short heads. Their length of liml? is accompanied by the develop- 

 ment of a trunk. Horses are more lightly built. Their feet are 

 elongated, their leg muscles bunched, and their toes reduced to 

 one on each foot. Every characteristic of the limbs shows adapta- 

 tion for rapid locomotion over hard ground. Length of limb is 

 compensated by elongation of the neck and the teeth are adapted 

 for grazing. The camels are differently adapted for life in arid 

 regions. In all three groups primitive ancestral species of the 

 Eocene are known which can be linked with the existing animals 

 through a chain of other species occurring in the intervening 

 epochs. The structures of these species in all cases show a gradual 

 transition leading up to the highest stage. Divergent species are 

 also recorded which became extinct without leaving known de- 

 scendants. The correlation of structural transition, chronology, 

 and environmental conditions is significant evidence of evolution. 

 These cases are, in fact, fragments of the actual record of past 

 evolution. 



REFERENCES 



Scott, W. B., A History of Land Mammals in (he Western Hemisphere, 1913. 

 Lull, R. S., Organic Evolution, 1917. 



Matthew, W. D., "The Evolution of the Horse. A Record and Its Interpre- 

 tation." Quarterly Review of Biology, 1, 139-185, 1926. 



