54 EVOLUTION 



stream of living material, carrying along with 

 it the substance out of which the similar 

 embryonic forms are made. As the stream of 

 embryonic material divided into different 

 paths it has also changed many of the details, 

 sometimes even all; but, nevertheless, it has 

 often retained the same general method of 

 development that is associated with its par- 

 ticular 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 develop- 

 ment 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 condensation of ancestral 

 initiatives which are materially represented 

 hi the living substance and compel the 

 developing embryo to re-tread, to some extent 



