876 TEXTBOOK OF ZOOLOGY 



foot both in front and behind but with the side toes much reduced 

 and a trace of the fifth digit in the forefoot, appeared during the 

 Oligocene epoch. Merychippus of Miocene times and Pliohippus of 

 the Pliocene epoch show a continuation of this reduction to a one- 

 toed type which leads to Equus cdballus of modern times. The 

 modern horse walks on the tip of the middle toe of each foot with 

 the vestiges of digits 2 and 4 persisting as splints. 



Arthur Dendy, in 1911, wrote that the horse is an example of the 

 adaptation of a lowland type to become a plains type, as the ex- 

 tensive, dry, grass-covered, plains developed. The adaptation has 

 proceeded along two lines. The limbs have become elongated by 

 the elevation of the heel, thus putting the animal on tiptoe ajid 

 fitting it for rapid flight from its enemies on the grass-covered open 

 prairie. At the same time the neck and head became elongated to 

 enable the animal to graze the ground without bending the legs. 

 Along with this the teeth changed from a carnivorous form to a 

 complex, broad grinding type for feeding on grass. In addition 

 to these changes the brain advanced. 



These changes took place gradually through millions of years and 

 the intermediate forms give the paleontologist a graphic picture of 

 the history of this modern species. The sequence of these stages 

 seems to fit in exactly with the theory that each has been derived 

 from the preceding by a continued adaptation to the changing con- 

 ditions of life. The horse's pedigree is essentially similar to that 

 of numerous other forms, such as the elephant, camel, and certain 

 birds, which have been worked out. 



The molluscan, echinoderm, and arthropod fossils give similar 

 stories among invertebrates. The numerous fossils of fish, Amphibia, 

 Reptilia, particularly dinosaurs, and birds with the famous Arcliae- 

 opteryx, all have their testimony to contribute. 



Distributional Evidence.— The paleontological distribution of ani- 

 mals is considered vertically while geographic distribution is in 

 horizontal plane. It is impossible to separate these two completely. 

 The study of geographical distribution gives essentially a cross 

 section of the vertical distribution, thus giving a sort of "still pic- 

 ture" of the complex developmental relations of animal groups at 

 one moment in geologic time. In studying this subject, it is neces- 

 sary to have in mind two fundamental conceptions. The first is that 

 the ancestors of related genera first appeared or originated in a 



