CHAP. XX 



Heredity 335 



influences which touch the very heart of the organism ? Is 

 it unreasonable to presume that some influences of habit and 

 conditions, of training and control, saturate the organism 

 thoroughly enough to affect every part of it ? 



A slight change of food affects the development of the 

 reproductive organs in a bee-grub, and makes a queen out of 

 what otherwise would have been a worker. A difference of 

 diet causes a brood of tadpoles to become almost altogether 

 female. There is no doubt that some somatic changes 

 affect the reproductive cells in some way. Is it incon- 

 ceivable that they affect them in such a precise way that 

 bodily changes may be transmitted ? 



It must be admitted that it is at present impossible to 

 give an explanation of the way in which a modification 

 of the brain can affect the cells of the reproductive organs. 

 The only connections that we know are by the blood, by 

 nervous thrills, by protoplasmic continuity of cells. But 

 there are many indubitable physiological influences which 

 spread through the body of which we can give no rationale. 

 Because we cannot tell how an influence spreads, we need 

 not deny its existence. 



It is at least conceivable that a deep functional or 

 environmental change may result in chemical changes 

 which spread from cell to cell, that characteristic products 

 may be carried about by the blood and absorbed by the 

 unspecialised reproductive cells, that nervous thrills of 

 unknown efficacy may pass from part to part. Nor do we 

 expect that more than a slight change will be transmitted 

 in one generation. 



Weismann traces all variations ultimately to the action 

 of the environment on the original unicellular organisms. 

 These are directly affected by surrounding influences, and 

 as they have no "body" nor specialised reproductive 

 elements, but are single cells, it is natural that the char- 

 acters acquired by a parent-cell should also belong to the 

 daughter-units into which it divides. And if so, is it not 

 possible that the reproductive cells of higher animals, being 

 equivalent to Protozoa, may be definitely affected by their 

 immediate environment, the body? Moreover, if it were 



