DISCUSSION 179 



motion, but no useful work is directly performed. Opposi- 

 tional contractions are somewhat similar, and equally neces- 

 sary in daily life. They are needed in controlling the actions 

 of other muscles. 



This brings us to the question of group variation in 

 oppositional and inhibitional stimuli, touched upon pre- 

 viously. It falls under the general head of coordination 

 between muscles and groups of muscles. Such variation 

 must be great, but its satisfactory investigation in the 

 lower Mammalia seems almost impossible. The Impalla 

 (yEpyceros) of Africa makes frequent leaps of as much as 

 thirty-five feet, considerably surpassing those made by 

 any other of its relatives. There is probably no gross, 

 anatomical feature to account for this ability that is not 

 shared by similar species of like conformation. This phe- 

 nomenal leaping ability is likely attributable to an unusually 

 efficient quality of stimuli transmitted to the motor endings 

 of the nerves, superior coordination betw^een all muscles that 

 can be of possible aid in violent extension of the hind limbs, 

 or a greater facifity in inhibitional and oppositional control 

 of the muscles which might hinder such movements. The 

 last point is probably not sufficient in itself to account for 

 such great leaps, and the reason is probably a harmonious 

 combination of the three features mentioned. 



The head muscles of a mammal have developed in response 

 to phylogenetic stimuli and those concerned with food habits 

 and dentitional causes. These, of course, vary widely with 

 the group to which a particular mammal belongs, and usually 

 are negligibly subject to the influences which have been 

 brought to bear in the case of mammals which exhibit 

 paralleHsm and convergence for best coping with a particular 

 mode of life especially as regards progression. To a lesser 

 extent is this the case with the neck muscles. Burrowing 

 mammals of widely dissimilar affinities may use the head as 

 a modified shovel, for instance, which markedly affects the 



