NA TURE 



371 



THURSDAY, JANUARY 18, 1912. 



THE MNEMIC THEORY OF HEREDITY. 

 Die Mneme als erhaltendes Prinzip in Wechsel des 



Organischen Geschehens. By R. Semon. Dritte 



Auflage. Pp. xviii + 420. (Leipzig: W. Engel- 



mann, 191 1.) Price 10 marks. 

 '"PHE theory of heredity which is associated more 

 J- especially with the names of Ewald Hering, 

 Samuel Butler, and Richard Semon, and endeavours 

 to explain the phenomena of inheritance as due to 

 a kind of unconscious memory, on the part of the 

 developing organism, of the experiences of past gene- 

 rations, has not, at any rate in this country, met 

 with a very large measure of acceptance. It is a 

 noteworthy fact, however, that in 1908 Mr. Francis 

 Darwin made it the subject of his presidential address 

 to the British Association at Dublin, and expressed 

 views which correspond very closely with those of 

 ihe authors mentioned. 



The mnemic theory, which is based upon a belief 

 in the inheritance of acquired characters, naturally 

 does not appeal to those who deny the possibility of 

 such inheritance. The position taken up with regard 

 to this question by Prof. Weismann and his fol- 

 lowers, however, can scarcely be maintained much 

 longer in face of the rapidly accumulating evidence 

 which, it must be confessed, their unbelief has been 

 perhaps the chief agent in eliciting. 



Prof. Semon quotes several instances of this evidence 

 in the work before us, the most convincing of which 

 appears to be that obtained by M. E. Bordage in the 

 case of peach trees. It has long been known that 

 European trees, when grown in tropical or semi- 

 tropical countries, tend to lose their deciduous char- 

 acter and become evergreen. This is the case in the 

 Island of Reunion, where M. Bordage conducted his 

 experiments. He found that in the course of twenty 

 years peach trees imported from Europe become almost 

 completely evergreen. This, of course, is an indi- 

 vidual somatogenic, or so-called "acquired" character; 

 but when seeds of these modified trees are sown in 

 certain mountain districts where they are exposed to 

 a considerable amount of frost, they produce young 

 [leach trees which are also evergreen, although seeds 

 imported from Europe and sown in similar situations 

 j)roduce normal deciduous trees. 



It is true that an attempt has been made with 

 regard to such cases to uphold the Weismannian 

 position by suggesting that the stimulus of changed 

 f'nvironment affects both the somatic cells and the 

 i^orm cells of the parent simultaneously, by what is 

 called "parallel induction," so that the germ cells 

 are modified in a corresponding fashion to the somatic 

 cells, and will therefore give rise to similarly modified 

 offspring; but this certainly looks very like a last 

 attempt to maintain an untenable position, and, in 

 my case, as Sumner and Semon have pointed out, it 

 makes no difference as regards the importance of the 

 action of the environment as a factor in organic evolu- 

 tion whether we suppose the effect upon the germ cells 

 to be produced by " parallel " or by " somatic induction." 



The mnemic theory assumes that the germ cells are 

 NO. 2203, VOL. 88] 



not, as Weismann would have us believe, shut off 

 from the influence of the body or soma, but that they 

 are, like other cells, and more especially the nerve 

 cells, capable of responding to stimuli by some definite 

 alteration in their condition. The stimuli to which 

 they respond are changes in what Semon calls the 

 " energetic situation " of the whole organism. The 

 effects of such changes are supposed to be impressed 

 upon the germ cells as "engrams," and are of a 

 lasting character. 



We may here translate Semon 's two mnemic 

 " laws " (" Hauptsatze ") :■ — 



First Mnemic Law {Law of Engraphy). — All simul- 

 taneously acting stimuli within an organism form a 

 coherent simultaneous stimulation-complex, which acts 

 engraphically as such, i.e. which leaves behind a 

 coherent and to that extent a unified en gram-complex. 



Second Mnemic Law {La7v of Ekphory). — The 

 partial recurrence of the energetic situation, which has 

 previously acted engraphically, acts ekphorically upon 

 a simultaneous en gram-complex. To speak more 

 precisely : the partial recurrence of the stimulation- 

 complex which left behind it the engram-complex 

 acts ekphorically upon a simultaneous engram-com- 

 plex, whether it be a recurrence in the form of the 

 original stimuli or of mnemic stimuli. 



It is perhaps scarcely necessary to point out that 

 by " Ekphorie " Semon means the calling forth again 

 of the latent engrams by some appropriate stimulus, 

 whereby they become manifested in the organism. 



The condition of the germ cells is thus supposed 

 to be more, or less permanently modified by changes 

 in the "energetic situation" of the parent 1mh1\ , and 

 such modifications affect the djtfvelopment of the germ 

 cells because the engrams are called forth in due 

 sequence by appropriate stimuli and express them- 

 selves in corresponding modifications of the body of 

 the offspring. 



The germ cells are thus stored with the latent 

 "memories" of past generations, and they may con- 

 tain many engrams that may never get the chance 

 to express themselves in any particular individual 

 ontogeny. Thus a number of alternative routes are 

 open to each individual at the commencement of its 

 life-history, and the particular route followed will 

 depend upon the nature of the stimuli which the 

 developing organism happens to encounter. 



Semon finds confirmation of these views in the 

 experiments of Kammerer upon the toad, .llytcs 

 obstctricans. It is well known that most frogs and 

 toads deposit their eggs in water, where the male 

 embraces the female and squeezes the eggs from her 

 by the pressure of his arms. As the eggs pass out 

 they are fertilised, their gelatinous envelopes swell 

 up, and they adhere together to form the spawn. 

 In adaptation to this habit the forefinger of the male 

 exhibits a characteristic pad or swelling, and the 

 musculature of the forearm is hypertrophied. 



In the obstetric toad, on the other hand, sexual 

 union, followed by the fertilisation and deposition of 

 the eggs, takes place on land. The eggs are larger 

 and contain more yolk, and arc produced in much 

 smaller numbers. The male assists in the removal 

 of the eggs from the female by means of his hind 



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