EVIDENCES OF EVOLUTION 53 



has also changed many of the details, some- 

 times even all; but, nevertheless, it has often 

 retained the same general method of devel- 

 opment that is associated with 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 stage and the neces- 

 sary condition for the following." '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 which are mate- 

 rially represented in the living substance and 

 compel the developing embryo to re-tread, 

 to some extent at least, the path taken by 

 the embryos of its ancestors. 



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 



