THE LIFE OF MAMMALS 
there; but white asses have always been regarded in the East 
as animals suitable for the great to ride upon (Judges iv. 10), 
and are still held in honor. From the Orient they have been 
scattered widely, but do serious service only in the warmer parts 
of the Old World, or under the similar conditions in Mexico. 
Excepting some differences in size, donkeys everywhere are 
practically the same. In part, as Shaler remarks, this lack of 
change may be due to neglect by unprogressive owners; but 
Shaler *? tells us that in Spain, where a long-continued effort 
has been made to develop the animal for interbreeding with 
the horse, the result shows that the form is relatively inelastic. 
“So long as pack animals were in general use, the qualities of 
the donkey have proved, and are still found, of value. The animal 
can carry a relatively heavy burden, being in such tasks, for its weight, 
more efficient than the horse. It is less liable to stampedes. It learns a 
round of duty much more effectively than that creature, and can subsist 
by browsing on coarse herbage, where a horse would become so far weak- 
ened as to become useless. In general, we may say that the 
donkeys belong to a vanishing state of human culture, to the time before 
carriage ways existed. Now that civilization goes on wheels, they seem 
likely to have an ever decreasing value.” 
While the evolution of the horse was proceeding in North 
America, another group of perissodactyls, from an apparently 
quite independent origin, the Lito pterna, evolved in 
South Amcrica a race adapted to the pampas, and 
singularly like the horse in many ways. These animals likewise 
lost the lateral toes one after another, and concentrated the step 
on the central toe; changed the form of the joint surfaces; 
lengthened the limbs and the neck, and gradually increased in 
size. The teeth were long and complicated the pattern, but no 
cement formed on them, so that they were not so efficient grind- 
ers as those of horses. This group of animals, which illustrates 
the principle of “parallelism” or “convergence,” became totally 
extinct in South America shortly before the migration thither of 
37° 
Litopterna. 
