. 
ry 
MAMMALIA. 509 
possibly able on occasion to scratch or burrow, to run and climb. 
From this it will be seen that the later mammals have become special- 
ised in varying degrees for special habitats, the five principal of which 
we will notice. 
A study of man will show that he does not agree with the true or 
specialised group of any of these types, but that he would really fall 
into the incidental, if not the transition, group of all but the erial. To 
this adaptability to all environments without a corresponding modifica- 
tion involving loss of organs and specialisation, man probably owes his 
position at the head of the mammalian world. In other words, as the 
environment is ever inconstant and specialisation means a modification 
for one particular temporary form of environment, it also means certain 
extinction of the type, sooner or later. True evolutionary progress is 
effected by an acquired reactivity to a varzety of environmental surround- 
ings and not by an adaptation to a “‘ special” environment, which checks 
further progress and culminates in extinction of the type. 
2 and 3.—THE Horse (Zguus caballus) and Ox (Bos 
taurus).—CURSORIAL. 
The horse and ox represent two culminating points in 
the evolution of the large herbivorous cursorial type, the 
former belonging to the sub-order ertssodactyla and the 
latter to the Artodactyla, which together comprise the order 
Ungulata or hoofed animals. 
Both the horse and ox stand high on their four legs and 
walk only on their toes (digitigrade). In each case the 
legs and neck are long. As they obtain their food from the 
level of the ground, or graze, the elongation of the neck and 
head must keep pace with that of the limbs. They are 
surrounded in natural conditions by the carnivorous types, 
the large “cats” and the “dogs,” and they are endowed 
with keen senses. The sense of hearing is assisted by the 
large. external ear or pinna which can be turned in any 
direction to catch the sound. That of sight is mainly 
assisted by the long neck which adds considerably to the 
field of vision. The sense of smell is also highly developed. 
The effects of these developments in the Ungulafa will be 
seen in corresponding modifications of the carnivorous types 
(see Dog and Cat). 
- Both-types are covered with dense hair which is particu- 
larly long upon the tail. This organ is mainly used for 
protection against the attacks of certain flies. ‘The horses 
and their close allies, the zebras and asses (£guide), 
