200 TRANSMISSION OF ACQUIRED CHARACTERS 



each constitutional unit is the centre, and by which it acts on other 

 units while it is acted on by them, tend continually to re-mould 

 each unit into congruity with the structures around, superposing 

 on it modifications answering to the modifications which have arisen 

 in these structures. Whence is to be drawn the corollary that 

 in the course of time all the circulating units physiological, or 

 constitutional, if we prefer so to call them visit all parts of the 

 organism ; are severally bearers of traits expressing local modifica- 

 tions ; and that those units which are eventually gathered into 

 sperm-cells and germ-cells (i.e. egg-cells), also bear these superposed 

 traits." 



Thus the constitutional units are supposed to circulate and to 

 visit one another throughout the body. When they come to a 

 modified structure and visit its modified constitutional units, they are 

 supposed to be themselves impressed ; thus impressed, they are 

 supposed to be gathered into the germ-cells, which thus come to bear 

 the " superposed traits " resulting from modifications. 



If we were sure that modifications were ever transmissible, 

 we might be glad of this hypothetic interpretation of the business. 

 But it is a difficult hypothesis to think out, and it would hardly 

 be tolerable even if there were facts which it was needed to 

 interpret. In particular, the conception of "an unceasing 

 circulation of protoplasm," so that " each portion of protoplasm 

 visits every part of the body," seems not only unwarranted, 

 but contradicted by well-established facts. 



3. A Mechanism may exist though it remains Unknown. 

 In the third place, we must recall Prof. Lloyd Morgan's warning 

 that although we cannot imagine how a modification might, 

 as such, saturate from body to germ-cells, this does not exclude 

 the possibility that it may actually do so. Oscar Hertwig 

 also maintains that our ignorance of any mechanism which 

 could secure the transmission of an acquired character is not 

 a good argument against the possibility of its occurrence. There 

 are, he says, many facts in biology which are quite secure, 

 though no causal nexus can be worked out at present (All- 

 gemeine Biologie, 1906, p. 621). It must be noted, however, 



