KINETOGENESIS. 311 



Ryder sums up the evidence in two propositions, 

 as follows : l 



"I. The mechanical force used in locomotion dur- 

 ing the struggle for existence has determined the digits 

 which are now performing the pedal function in such 

 groups as have undergone digital reduction. 



"II. When the distribution of mechanical strains 

 has been alike upon all the digits of the manus or of 

 the pes, or both, they have remained in a state of ap- 

 proximate uniformity of development." 



The application of the impact, or strain, or both, 

 in progression, is easily understood. In recover (see 

 p. 299), the leg is bent on the foot as it rests on the 

 ground, and those digits which then leave the ground 

 last, sustain greater strain than those which leave it 

 sooner. In replacing the foot on the ground (planta- 

 tion), those digits which strike it first experience greater 

 force of impact than those which strike it later. Sup- 

 posing the five primitive digits to have been of equal 

 length, the distribution of the impact and of the strain 

 will depend on the angle at which the foot is directed 

 with reference to the direction of motion. If the feet 

 are pointed forwards, the middle digits will experience 

 strain and impact ; if outwards, the inner digits bear 

 the weight ; if inwards, the external digits receive it. 



Observation on five-toed plantigrade mammals 

 shows that their feet are turned neither inwards nor 

 outwards in progression, but straight forwards. It is 

 probable that the primitive Mammalia moved in the 

 same manner. This is also to be inferred from the 

 fact that they were plantigrade, so that the leverage 

 transversely in or out which results from the elevated 

 heel of the digitigrade leg was very much less in them. 



^American Naturalist, 1877, p. 607. 



