ADAPTATION 219 



the length of the foot. Later the foot and lower limb elongate 

 and the change of posture becomes more extreme, giving rise to 

 the unguligrade habit, in which only the extreme tips of the digits 

 touch the ground. With high specialization in this direction the 

 final step, reduction of the digits, occurs. Such a change becomes 

 possible through emancipation of the appendages from other 

 functions than locomotion; it is an advantage in reducing the 

 weight of the appendage with the reduction of its efficiency as a 

 lever. 



Vertebrates of the classes Reptilia, Aves and Mammalia have 

 developed cursorial powers, but none show any higher specializa- 

 tion than the horse, which we have already considered in detail. 

 Some existing lizards are cursorial, but the extinct dinosaurs 

 attained the highest speed adaptations in this class. Their adapta- 

 tions included also bipedality. The flightless birds, such as the 

 ostriches, and some of our common birds including the quail, are 

 excellent runners, although the power of flight, when not com- 

 pletely lost, is always a last resort when speed is necessary. Such 

 adaptations as are shown by the horse are developed in lesser 

 degrees in many mammals, consequently we are most familiar 

 with cursorial animals in this class. The ungulates including 

 dogs and cats are highly developed cursorial animals. 



Saltatory Adaptations. Saltatory, or jumping animals, are more 

 conspicuously different in the two groups. In the vertebrates the 

 hind legs are always predominant appendages for propulsion, 

 while the front legs act as supports for the anterior end of the 

 body, serve to catch the body at the termination of a leap, and in 

 a minor degree aid in propulsion. This predominance of the hind 

 legs is well illustrated by their earlier specialization in fossil 

 horses. Locomotion in cursorial vertebrates is therefore asso- 

 ciated with saltatory power, since a gallop is essentially a succes- 

 sion of jumps. In the rabbits and the kangaroos and wallabies 

 the development of the hind legs is carried to such a point that the 

 animals may be called truly saltatory (Fig. 129). This is espe- 

 cially true of the kangaroos, since they are able to move by a 

 series of jumps, without the aid of the fore-limbs. 



The insects show saltatory adaptations of a very different type. 

 The basis for such development is probably twofold. Structur- 

 ally, the presence of six legs affords an opportunity for walking, 

 even though one pair be modified to such an extent that they are 



