KINETOCENESIS. 309 



while the feet beyond this point receive none, and do 

 not support the body, except very partially at the mo- 

 ment of leaving the earth. 



b. The Number of the Digits. 



The reduction in the number of toes is supposed to 

 be due to the elongation of those which receive the 

 greater number of strains and impacts in rapid pro- 

 gression, and the complementary loss of material 

 available for the growth of those not subject to this 

 stimulus. This is rendered probable from the fact 

 that the types with reduced digits are dwellers on dry 

 land, and those that have more numerous digits are 

 inhabitants of swamps and mud, or are more or less 

 aquatic. That this inequality is due to these mechan- 

 ical causes is still further indicated by the fact that in 

 those forms where the soles are thickly padded (Car- 

 nivora, Proboscidia) the reduction has either not taken 

 place, or has made little progress, amounting to the 

 loss of only one digit. (An apparent exception in the 

 case of the camels will be mentioned later.) A still 

 more important body of evidence which shows that 

 the inequality in size and number of digits is due to 

 impacts and strains unequally distributed, has been 

 brought forward by Ryder. He points out that defi- 

 nite results are to be observed in those limbs of a given 

 type of animal which experience correspondingly defi- 

 nite influences ; while in the limbs where the strains 

 are equal, the modifications do not appear. Examples 

 of this kind are to be found in the unguiculate Mam- 

 malia and in the Marsupialia. Thus in the jerboas 

 which use the hind limbs in leaping, these only dis- 

 play reduced digits, the fore limbs remaining of prim- 



