192 OUTLINES OF EVOLUTIONARY BIOLOGY 



a skilled musician, playing a composition for perhaps the 

 hundredth time, is quite unconscious of the individual actions 

 which he performs by memory in striking the notes, while a 

 beginner, who has only played the same piece once or twice 

 before, may be acutely conscious of every note which he strikes. 

 The development of an individual organism from the egg is, 

 according to the mnemic theory, merely an unconscious repetition, 

 by memory, of acts which have been performed many thousands 

 of times by its ancestors, and, as in the performance of a piece of 

 music, each successive act constitutes the stimulus which calls 

 forth the next. 



The fertilized ovum may be looked upon as being charged with 

 the latent memories of past generations. The unconscious memory 

 of what it did last time it was a fertilized ovum prompts it to divide 

 into two cells, this re-arrangement of its constituents supplies 

 another stimulus which prompts it to a further division, and so on 

 through all the stages of ontogenetic development. In the words 

 of Professor Francis Darwin, "the rhythm of ontogeny is actually 

 and literally a habit." * 



Progressive evolution takes place owing to the fact that each 

 successive generation may add a little bit to the record on its own 

 account, this addition being the result of some new experience 

 due to some difference in its environment as compared with the 

 environment of its predecessors. 



There are, then, two distinct sets of factors which determine 

 the individual development of any organism, first}the inherited 

 tendencies, or " engrams," and second^ the stimuli provided by 

 the environment during its own life-time. An egg placed under 

 very unusual conditions, as, for example, too low or too high a 

 temperature, may be unable to develop at all, but if the 

 change of conditions be but slight the organism may adapt 

 itself, educate itself and furnish itself with a new store of 

 experiences, which in course of time may be impressed upon the 

 germ plasm and thus added to the record and handed on to 

 future generations. 



We have already had occasion to notice, in the case of 

 artificially produced cyclopean fish larvae, how an alteration of the 

 environment not sufficient altogether to prevent development 

 may so far overcome the inherited tendencies of the organism 

 as to give rise to the production of monstrosities (p. 157). 



1 Presidential Address to the British Association, Dublin, 1908 



