124 THE EVOLUTION OF MAN. 



which occurs for the first time in Amphibians, was developed 

 from this multi-digitate form. This reduction in the number 

 of the digits from ten to five occurred in those Dipneusta 

 which must be regarded as the parent-forms of the Amphibia, 

 probably as early as the latter half of the Devonian Period 

 or, at latest, in the immediately subsequent Carboniferous 

 Period. Several fossil Amphibia with five digits have already 

 been found in the strata of the latter period. Fossil foot- 

 prints of the same animals are very numerous in the 

 Trias (Ckerotheriuri). 



The great significance of the five digits depends on the 

 fact that this number has been transmitted from the 

 Amphibia to all higher Vertebrates. It would be impossible 

 to discover any reason why in the lowest Amphibia, as well 

 as in Reptiles and in higher Vertebrates up to Man, 

 there should always originally be five digits on each of the 

 anterior and posterior limbs, if we denied that heredity 

 from a common five-fingered parent-form is the efficient 

 cause of this phenomenon : heredity can alone account for 

 it. In many Amphibia, certainly, as well as in many higher 

 ; Vertebrates, we find less than five digits. But in all these 

 cases it can be shown that separate digits have retrograded, 

 and have finally been completely lost. 



The causes which effected the development of the five- 

 fingered foot of the higher Vertebrates in this Amphibian 

 parent-form from the many-fingered foot, must certainly be 

 found in the adaptation to the totally altered functions 

 which the limbs had to discharge during the transition from 

 an exclusively aquatic life to one which was partially 

 terrestrial. While the many -fingered fins of the Fish had 

 previously served almost exclusively to propel the body 



