MAMMALIA. 509 



jx'^sibly able on occasion to scratch or burrow, to run and climb. 

 i- rom 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 aerial. 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 va^-iety 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 {Equus caballus) and Ox {Bos 

 faurus). — CuKSORiAL. 



The horse and ox represent two culminating points in 

 the evolution of the large herbivorous cursorial type, the 

 former belonging to the sub-order Ferissodactyla and the 

 latter to the Artiodactyla, 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 Ufigiilata 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 {Equidcs), 



