INTRODUCTION 13 



Primitive and progressive foot structure. — It is surprising how little 

 attention was given to the feet of mammals between the time of Cuvier and 

 that of Gaudry, Kowalevsky, Huxley, and Cope. Cuvier himself had 

 assembled a lot of wholly unrelated animals as "Les Pachydermes" because 

 of the conmion possession of a thick skin; this was virtually a new desig- 

 nation for the assemblage termed 'Multungula' by Storr (1780). In this 

 unnatural assemblage consorted the thick-skinned rhinoceroses, hippo- 

 potami, and other quadrupeds. De Blainville (1816) founded the modern 

 classification of the ungulates by observing the number of digits in the 

 feet and separating certain of the Herbivora into two classes, "a doigts 

 pairs" and "a doigts impairs," or Avith an even and an odd number of toes 

 respectively. This suggested to Owen (1847) the separation of the Artio- 

 dactyla (apnos, even in numl)er, Sa/<rvXos, finger) from the Perissodactyla 

 (TTcpio-o-o's, odd in number, SoiktuAo?, finger), including the sharp separation 

 between hippopotami with their even toes and rhinoceroses with their odd 

 number, into two distinct orders. Thus attention was concentrated upon 

 the numerical changes in the evolution of the feet, and Huxley, Kowalevsky, 

 and Cope more or less independently reasoned that hoofed animals with 

 one, two, three, and four toes must have sprung from more primitive forms 

 with five toes. The number of digits, therefore, became an important 

 means of distinguishing the adaptive stages of evolution in many differ- 

 ent lines of mammals, as follows: 



Pentadactyl, primitive five-toed mammals such as Phenacodus. 

 Tetradactyl, mammals with four digits such as Hippopotamus and dog. 

 Tridactyl, mammals with three digits such as Rhinoceros and early stages 



in the evolution of the horse. 

 DiDACTYL, mammals with two digits such as the deer and cattle. 

 MoNODACTYL, mammals with one digit, typified by the horse. 



It was also observed that the simple number of digits does not tell the 

 whole story because of differences of proportion related to the amount of 

 service which each digit renders to the animal. Thus in an early stage in 

 the evolution of the horse there are three full-sized digits and a short ad- 

 ditional digit in process of degeneration. The former rest on the ground 

 and are functional or in active service; the latter is suspended at the 

 side of the foot, has lost contact with the ground, and is becoming vestigial. 

 This gave rise to a distinction between functional tridactylism, in which 

 all three toes are of service, and numerical tridactylism, as in Hipparion, 

 where the middle toe is doing all the work, and the two side toes are dan- 

 gling above the ground. De Blainville's (1816) and Owen's (1847) sepa- 

 ration of the even- and odd-toed hoofed animals led to the observation 

 that the main weight of the animal either passes through the central digit, 

 as in the rhinoceros and horse (Perissodactyla) or between the two cen- 



