INTRODUCTION 15 



DiGiTiGRADisM, where the foot rests only on the rows of phalanges, as in 



the dog and cat. 

 Unguligradism, where the foot rests only upon the end phalanx, as in the 



horse and the deer. 

 Rectigradism, where the foot is immobile and the entire weight rests 



on a large pad, as in the elephant. 



Reduction of digits. — This led to the further generalizati-on that all 

 primitive types of mammals were pcntadactyl or five-fingered and planti- 

 grade, or with the sole of the hand and foot resting upon the ground. From 

 this it was an easy step to perceive that the raising of the wrist and ankle 

 joints from the ground in the passage from plantigradism to digitigradism 

 also tended to raise the shorter digits, namely, the thumb, or first digit in 

 the hand, and the big toe, or first digit in the foot, from the ground, to 

 render them useless in progression and to initiate their degeneration or 

 retrogression. It is, in fact, in this stage, where the inside digits of both 

 hand and foot are in process of disappearance, that we discover most hoofed 

 mammals of the early geological periods. The loss of one digit after another 

 occurs under what is known as the law of digital reduction. Thus the 

 passage from five to four, to three, to two, to a single digit is a gradual 

 process, not the work of a century or centuries, but of vast periods of time. 



Perfection of joints and facets. — Another and more intricate law in the 

 science of foot evolution concerns the changes in the articulations, or facets, 

 between the bones of the wrist and ankle and the bones of the metapodium 

 on which they rest. 



Kowalevsky first directed close attention to the fact that all these facets 

 and articulations are altered while the wrist or ankle is in process of be- 

 coming raised from the ground, while the digits are being reduced, and 

 while the weight is being concentrated more and more on the central digits 

 and taken from the lateral digits. This shifting of the joints or facets was 

 found by Kowalevsky,' Cope,^ Riitimeyer,^ and Osborn * to produce an 

 interlocking system, so that the bones are placed above each other like 

 rows of bricks with "struck" or alternating joints, and this alternation 

 of the joints with the closed surfaces is brought about by more rapid growth 

 cf some parts of the foot bones than of other parts, producing at every 

 stage a perfect mechanism, calculated to resist the enormous strains which 

 come upon the foot, especially in the rapid movements of swift running 



' Kowalevsky, Monographie der Gattung Anthracotherium Cuv. und Versuch einer 

 natiirUchon Classification der fossilen Hufthiere. Paldontographica, Vol. XXII, 1873. 



- Cope, The Vertebrata of the Tertiary- Formations of the West. Rept. U. S. Geol. Surv. 

 Terr., Vol. Ill, Washington, 1884; also, The Pcrissodactyla, Amer. A'atural., Vol. XXI, 1887. 



^ Riitinieyer, L., Uber einige Beziehungen zwischen den Siiugethierstammen Alter und 

 Neuer Welt. Ahh. Schweiz palaont. Ges., Vol. XV, pp. 1-151, Zurich, 1888. 



* Osborn, The Evolution of the Ungulate Foot, Pt. IV of The Mammalia of the Uinta 

 Formation by Scott and Osborn. Trans. Amer. Philos. Soc, n.s. Vol. XVI, Aug. 20, 1889, 

 pp. 531-569. 



