GREGORY, QUADRUPEDAL LOCOMOTION 271 



diverse lines of evolution, — in the elephant no less than in the antelope. 

 The general factors of long-distance travelling power may be grouped 

 broadly under the headings : (A) Endurance and (B) Speed. 



A. ENDURANCE 



Endurance may be measured either by ( 1 ) the length of time an ungu- 

 late can keep in motion without rest or refreshment, or by (2) the. 

 amount of reserve strength left after a stated expenditure of energy, or 

 by (3) the relative quickness of recuperation. Endurance increases with 

 practice. As metabolism increases, the muscles, lungs, heart and other 

 organs of the thorax become stronger and larger. 



More rapid metabolism requires more food and larger digestive ap- 

 paratus. Although the Lamarckian hypothesis in its crude form is very 

 probably untenable, it is a fact that herbivorous animals have longer and 

 heavier digestive tracts than carnivores. Moreover, as fast as the denti- 

 tion has become adapted for the harder, less nutritious kinds of food, the 

 digestive apparatus must have become more complex and much heavier. 



While the enlarging thorax and abdomen have made available a great 

 increase in energy, the3^ have caused an even more rapid increase in total 

 weight. 



With increasing total weight, the internal and external resistance to 

 be overcome in locomotion also rises, and hence the margin between the 

 total energy available and the energy required in progressing a given dis- 

 tance is lessened. In other words, endurance, the first great factor of 

 long-distance travelling power, is directly proportional to the efficiency 

 of the adaptations for minimizing the waste of energy. 



ADAPTATIONS FOB MINIMIZING WASTE OF ENERGY 



Higher efficiency in locomotion has doubtless been attained, first, by 

 advantageous modifications of the organs of propulsion (such as are de- 

 scribed below), secondly, by improvements in the supporting frame- 

 work, thirdly, by improvements in the methods (a) of conserving the 

 inertia of forward motion, (&) of taking up shock, (c) of preventing dis- 

 location and (d) of minimizing lost motion. 



Momentum. — The shocks and strains to which the •locomotive apparatus 

 is subject vary with the momentum of the body in motion. Hence as 

 momentum is the product of mass by velocity," the shocks and strains 

 experienced by heavy animals in rapid motion are very great, and devices 

 for lessening them become conspicuous. 



