Nature of Heredity. 3-5 



in the case of ihe shrimp-like crustacean Hippolytc varians ot the 

 British Coast. This animal adapts itself in a marvellous manner by 

 colour changes to its environment, evidently for purposes of con- 

 cealment, and, moreover, undergoes a periodical nocturnal and 

 (hurnal variation in colour. This periodical variation has been found 

 to continue after the normal daily alternation between light and 

 darkness has bpen artifically suspended. 



Further examples of the same class of phenomena could easily 

 be atlduced, while, in the domain of psychology, the phenomena of 

 memory may be regarded as affording another illustration of such 

 after-effects, for we must regard these phenomena as being a kind 

 of response to stimuli which were primarily exerted by the envircjn- 

 ment. and subsequentlv stored up in the brain for use on future 

 occasions. 



We thus see that the stimulus of changed en\ ironment may 

 make such a deep impression upon the organism as to shew itself in 

 " after-affects " when the original stimulus has ceased to operate. If 

 one seeks for a purely physical analogy one can scarcely help calling 

 to mind the phenomena of luminosity as exhibited by various sub- 

 stances which have the power of absorbing light rays when exposed 

 to the light and emitting them again subsequently in darknes.s. 



Judging from human experience in regard to memory the depth 

 and permanence of the impression received by the organism frcjm 

 the environment will usually depend upon the length of time for 

 which the original stimulus was acting. The details of a walk whic-h 

 we ha\e taken every day for a month are much more deeply and 

 permanently impressed upon the memory — in other words, upon cer- 

 tain cells of the brain — than those of a walk which we have onlv 

 taken once or a few times. 



Thus we see that there appear t(j be two verv distinct kinds 

 of what we ma\ call " memory " a purel\ phvsical and un- 

 conscious memory, exhibited, ff)r example, iji the after-affects 

 of dail\ periodicity, and a mental memor\ associated with con- 

 sciousness ; and though we may. for the sake of convenience, be 

 allowed to speak of them as " two kinds," yet ultimatelv thev are 

 probably identical in nature, consisting in a storing up of stimuli bv 

 the organism, to be utilised on future occasions. 



The plant-physiologist, Detmer. was. so far as 1 am aware, the 

 first to call attention to the bearing of the remarkable phenomena 

 of "after-effects"" upon the theory of heredity. Weismann. in 

 criticising Detmers views, asks " what connection there is between 

 these facts and the transmission of acquired characters."'* 



'■ .W\ these peculiarities produced b\ external influences," he 

 says, " remain restricted to the individual in which they arose ; most 

 of them disappear comparatively soon, and long before the death 

 of the individual. No example of the transmission of such a 

 peculiarity is known, "f 



* c/ at, p. 404. t hoc at. 



