574 CHORDATA. 
Hence we find that gradational adaptive structure in living forms is 
mainly due to ‘ gradational” environments, and that in fossil forms it 
is due to gradual change of environment. 
SUB-ORDER V.—ARTIODACTYLA. 
The Artiodactyla form a large branch or assemblage of 
Ungulata, which in many respects show parallel evolution to 
the Perissodactyla. They follow, however, rather different 
lines:—(1) The molar teeth are bunodont or selenodont. 
(z) The dorso-lumbar vertebrae are nineteen. (3) The 
femur has no third trochanter. (4) No alisphenoid canal 
and small nasals. (5) The carpus and tarsus are alternate 
and the toes are four or two; the main axis is between the 
third and fourth toes. (6) The stomach may be simple or 
complex and the placenta diffuse or cotyledonary. 
One division of the Artiodactyla retain the bunodont 
teeth (Bunodonta), only multiplying the number of the 
tubercles, whilst the other division (Se/enodonta) have the 
tubercles twisted into crescents or curves and worn down, 
thus producing the selenodont type. As in the Pevisso- 
dactyla, there is the addition of cement and the heightening 
of the crowns. 
The femur appears to have lost its third trochanter very 
early in the history of this sub-order. The toes show the 
same gradational reduction as in Perissodacty/a, but on a 
different plan. The third and fourth toes are always equal 
and larger than the second and fifth. These latter are 
hoofed and touch the ground in pigs, but are greatly re- 
duced in sheep and oxen and disappear altogether in the 
camel. It follows from the main axis passing between the 
third and fourth toes that the cuboid and ectocuneiform 
tend to be more or less equally developed, and that the 
astragalus articulates equally with the cuboid and navicular 
whilst the fibula, or its distal end, still remains in articula- 
tion with the calcaneum. The cuboid often fuses across 
the middle line with the navicular. 
Family I.—Hippopotamidz.—The Hippopotamus is confined to 
the rivers of Africa. Its canines and incisors are large and grow from 
persistent roots. The molars are of a slightly modified bunodont type 
and each tubercle wears into a three-lobed crown. The stomach is 
complex and the diet herbivorous. All four toes (first is absent) are 
