io6 THE DARWINIAN THEORY 



of existence ; hence we find the external form of an 

 early ancestral stage preserved, while internally the 

 larva is passing through the later stages and gradually 

 working its way up to the adult form and structure. 

 A similar process occurs in many other animals from 

 various groups, the explanation in all cases being 

 found in considerations such as the above. 



Tests of Recapitulation. 



An important consideration is that, if the 

 developmental changes are to be interpreted as 

 a correct record of ancestral history, then, first, 

 the several stages must be all possible ones, the 

 history must be one that ■ could actually have 

 occurred — i.e., the several steps of the history 

 as reconstructed must form a series, all the stages 

 of which are practicable ones. Secondly, each stage 

 must be an advance of the preceding one, other- 

 wise it would not have been retained, for it must 

 constitute an advance so distinct as to confer on its 

 possessor an appreciable advantage in the struggle 

 for existence. It is not enough that the ultimate 

 stage should be more advantageous than the initial 

 ones, but each intermediate stage must also be a 

 distinct advance. Intermediate stages, which are not 

 and could not be functional, can form no part of an 

 ancestral series. 



A good example of an embryological series fulfil- 

 ling these conditions is afforded by the development 

 of the eye in the higher Cephalopoda. First let us 

 consider the evolution of eyes in the Mollusca. In 



