224 THE THEORY OF EVOLUTION 



approvingly and with a detailed confirmation by 

 0. Hertwig. 1 ' From the fact that the ontogenesis of 

 the plant and animal species usually begins with a 

 simple cell stage, the fertilized egg, it has been con- 

 cluded that all organisms have descended from common 

 unicellular indifferent ancestors : the hypothesis of 

 a monophyletic pedigree has been put forward. How 

 improbable, however, must this appear to us if we 

 start from the point of view above set forth that ac- 

 cording to the ontogenetic causal law the fertilized 

 egg cells of the different species of animal vary in 

 their being quite as much from each other and are 

 quite as good bearers of the various specific differences 

 as are, at the end of their ontogenesis, the perfected 

 individuals upon whose characters we base our animal 

 system/ 



The same considerations suggest themselves when 

 it is sought to be concluded, by reason of the ' similarity ' 

 of many embryonic stages of the Mammals to tho&e 

 of adult fishes or the larvae of Amphibia, ' that the 

 Mammals descend from Amphibia or Fishes/ 2 ' The 

 recapitulation theory in the old sense ' cannot therefore 

 * be longer maintained/ 3 



Hacked basal principle therefore involves a perfect 

 miscomprehension of the nature of embryogeny. The 

 life of a mammal begins with an egg which a parent 

 animal of the same species has formed as an extract 



1 AUgemeine Biologie, p. 674. 



2 Ibid. p. 675. 



Ibid. 



