GENESIS, HEREDITY, AND VARIATION. 371 



we have just seen, the hypothesis of physiological units, does 

 not relieve us from the need for kindred suppositions. 



One more assumption seems necessary if we are to imagine 

 how changes of structure caused by changes of function can 

 be transmitted. Keverting to 54d, where an unceasing cir- 

 culation of protoplasm throughout an organism was inferred, 

 we must conceive that the complex forces of which each con- 

 stitutional 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 modifica- 

 tions which have arisen in those structures. Whence is to be 

 drawn the corollary that in course of time all the circulating 

 units, physiological, or constitutional if we prefer so to call 

 them visiting all parts of the organism, are severally made 

 bearers of traits expressing local modifications; and that 

 those units which are eventually gathered into sperm-cells 

 and germ-cells also bear these superposed traits. 



If against all this it be urged that such a combination of 

 structures and forces and processes is inconceivably involved, 

 then the reply is that so astonishing a transformation as 

 that which an unfolding organism displays cannot possibly be 

 effected by simple agencies. 



97 g. But now let it be confessed that none of these hypo- 

 theses serves to render the phenomena really intelligible ; and 

 that probably no hypothesis which can be framed will do this. 

 Many problems beyond those which embryology presents have 

 to be solved ; and no solution is furnished. 



What are we to say of the familiar fact that certain small 

 organs which, with the approach to maturity, become active, 

 entail changes of structure in remote parts that after the 

 testes have undergone certain final developments, the hairs 

 on the chin grow and the voice deepens? It has been con- 

 tended that certain concomitant modifications in the fluids 

 throughout the body may produce correlated sexual traits; 



