56 EVOLUTION 



chord supplies the stimulus, the necessary 

 developmental condition, for the formation 

 of the backbone when suitable materials arc 

 forthcoming. Of course we require to know 

 more precisely how the old-fashioned structure 

 prepares the way for and stimulates the 

 growth of its future substitute, but the general 

 idea of one organ leading on to another is 

 suggestive. It is consistent with our general 

 conception of development that each stage 

 supplies the necessary condition for the next; 

 it helps us to understand more clearly how 

 new structures, too incipient to be functional, 

 and old structures, too transitory to be of 

 direct use, may persist ; in short, it makes the 

 process both of development and of evolution 

 more intelligible. But to the idea of the 

 architectural utility of the notochord as a 

 piece of scaffolding, we must add, unless the 

 recapitulation is simply metaphorical, the idea 

 that the notochord is laid down to-day in the 

 development of a higher Vertebrate because of 

 a continuity of germinal material since the 

 days of the ancestral forms which had no 

 backbone at all. 



It must be admitted that the recapitulation 

 doctrine has been often stated in somewhat 

 crude and exaggerated form, so that many 

 saving-clauses are necessary. The human 



