322 THE HEGINNINGS OF CAUSAL MORPHOLOGY 



function, in the second period the function forms the organ, 

 or at least completes its differentiation. It is characteristic 

 that in the first period functionally adapted structure appears 

 in the complete absence of the functional stimulus. 



The explanation of the difference between the two 

 periods is to be found in the different evolutionary history of 

 the characters formed during each. First-period characters 

 are inJierited characters, and taken together constitute the 

 historical basis of the organism's form and activity ; second- 

 period characters are those of later acquirement which have 

 not yet become incorporated in the racial heritage. 



Inherited characters appear in development in the 

 absence of the stimulus that originally called them forth ; 

 acquired characters are those that have not yet freed 

 themselves from this dependence upon the functional 

 stimulus. First -period characters were originally, like 

 second - period characters, entirely dependent for their 

 development upon the functional stimuli in response to 

 which they arose, and only gradually in the course of 

 generations did they gain that independence of the functional 

 stimulus which stamps them as true inherited characters. 

 Speaking of the formative stimuli which are active in second- 

 period development, Roux writes : " These stimuli can also 

 produce new structure, which if it is constantly formed 

 throughout many generations finally becomes hereditary, /.<'., 

 develops in the descendants in the absence of the stimuli, 

 becomes in our sense embryonic" (p. 180, 1881). Again, 

 "form-characteristics which were originally acquired in 

 post-embryonic life through functional adaptation ma}' be 

 developed in the embryo without the functional stimulus, and 

 may in later development become more or less completely 

 differentiated, and retain this differentiation without functional 

 activity or with a minimum of it. But in the continued 

 absence of functional activity they become atrophied . . . 

 and in the end disappear" (p. 201, 1881). 



This conception of the nature of hereditary transmission 

 is an important one, and constitutes the first big step towards 

 a real undrrstanding of the historical element in organic 

 form and activity. It supplies a practical criterion for the 

 distinguishing of "heritage" characters from acquired 



