PREFACE 



ONE hears nowadays in many quarters cries for 

 a freer exercise of the scientific imagination. 



I have, whether consciously or unconsciously I 

 do not know, responded to these repeated stimuli, 

 and have essayed to imagine protoplasm as en- 

 dowed with a second fundamental property, affect- 

 ability, of course, being the first. 



It is two hundred and thirty years ago since 

 Francis Glisson, M.D., of Cambridge, formally 

 gave us the concept of irritability of living matter, 

 more especially that of muscle. 



Biology has since expanded so as to be unrecognis- 

 able to the author of the " Tractatus de ventriculo " 

 and in particular to the author of its chap, (vii.), 

 " De irritabilitate fibrarum " ; but the potentiality 

 of its growth lay largely in the recognition of this 

 property of excitability (affectability) in living 

 matter. Careful analysis has shown me that a 

 theoretically complete account of the behaviour 

 of bioplasm with regard to the environment and also 

 in consequence of heredity, cannot follow from the 

 assumption in it of only one fundamental property. 



I have, for some years, seen that the recognition 

 of two physiologically opposite or complementary 



