264 president's address. — section d. 



In view of the difficulties of Weismann's system it is not surprising 

 that theories of heredity according to which development proceeds 

 on lines comparable to those of mental phenomena have an increasing 

 number of adherents. It must be admitted frankly that mnemic 

 theories are open to the objection that they seek to illuminate one 

 phenomenon that is not understood by another that is not fully under- 

 stood. But although we do not pretend to a full understanding of all 

 the details of the process by which memory is achieved, biologists 

 certainly cannot view with favour any theory that postulates an 

 elaborate cell architecture of innumerable structural units. We 

 clearly recognise that an after effect of a stimulus long past is achieved, 

 and that the process is certainly one that does not involve the elaborate 

 parcelling out of an inconceivable number of material particles. The 

 great danger of mnemic theories of heredity is that we may be led by 

 what seems their very simplicity to suppose that we understand what 

 we do not understand, and therefore need not further investigate. 

 The most that we may regard as certain is that the phenomena of 

 memory and of development can be described in similar, if not identical, 

 terms. " We know positively," says Francis Darwin, " that by making 

 a dog sit up and then giving him a biscuit, we build up something 

 within his brain in consequence of which a biscuit becomes a stimulus 

 to the act of sitting. The mnemic theory assumes that the deter- 

 minants of morjjhological change are of the same type as the structural 

 alteration wrought in the dog's brain." 



A mnemic theory lends itself well to the explanation of the 

 inheritance of acquired characteristics. I have indeed referred to 

 mnemism here mainly on account of one or two forms of mnemic 

 theory that have lately been put forward especially to explain how 

 somatogenic characters may become blastogenic. The most notable 

 of these is the theory of Centro-Epiger.esis, put forward by Eugenio 

 Rignano. (Upon the Inheritance of Acquired Characters, a Hypothesis 

 of Heredity, Development, and Assimilation, 1912.) 



Rignano's theory pays special regard to the difficulties of what we 

 used to call Recapitulation, Jbut nmy now, perhaps, call, in the cautious 

 phrase of Hunt Morgan, the Repetition of Juvenile Forms. He seeks 

 especially to show how engrams are deposited in the germ cells as the 

 result of somatic change, and how these engrams later find expression 

 in the transmission of that change. 



Rignano holds that there is a special vital energy, transformable 

 into other kinds of energy, other kinds being transformable into it. 

 This energy circulates partly by aid of the nervous system, partly by 

 the inter-celiular bridges, the current constantly passing from cell to 

 cell. When environmental stimulus is leading to functional modi- 

 fication, in other words, when a new characteristic is being acquired. 



