EVIDENCES OF EVOLUTION 53 



fcas also changed m^ny of the details, some- 

 times even all ; but, nevertheless, it has often 

 retained the same general ir^-hod of deveU 



>vith its particular 



composition. We find the likeness, in the 

 sense of similarity of plan, accounted for by 

 the inheritance of the same sort of substance; 

 the differences in the development must be 

 accounted for in some other way." 



In thinking of the repetition or recapitula- 

 tion, there are two,distinct ideas to be kept 

 in mind. On the one hand, each stage "in 

 embryonic development is, as Professor His 

 put it long ago, "(the physiological conse- 

 quence of the preceding st^ge and the neces.- 

 sary condition for the^jQlIjowing." "If the 

 embryo is to reach the complicated end- 

 forms, it must pass, step by step, through 

 the simpler ones." On the other hand, the 

 inheritance of a living creature is, in some 

 manner that we cannot image, .a condensa- 

 tion of ancestral initiatives whic^ are mate- 

 rially represented in the 



Compel the developing ^p^hrya to re-tread. 

 to some extent at . jeast^ the path taken by 

 he embryos of its ancestoxs . 



Let us take the particular case of the 

 notochord, a supporting axial rod, present for 

 some time at least in all Vertebrate embryos, 

 and always arising in the same way as a fold 



