by the individual and the reproduction of characteristics in ontogeny

Semon reaches the conception of Mneme from the conception of stimulus. No action is a stimulus which does not produce some corresponding physiological change. Thus there can be no stimulus in the case of an inorganic body, nor in that of an organism after extinction of life.' (The Mneme. Trans. 1921, p. I.) In relation to the stimulus as exciting cause the ensuing change is effect. The excitation effect dies away, sometimes rapidly, sometimes more slowly, but there is an abiding effect of the stimulus which remains; this is the after-effect or engram. The contention is that without this hypothesis of an after-effect which manifests itself in later responses, being brought forth (ecphory) side by side with the primary effect of the operating stimulus, one cannot adequately explain animal or plant behaviour, and in particular one cannot account for the facts of ontogeny and heredity. On the other hand, given such a hypothesis, Semon claims that all these facts are brought into line and are harmonized with the psychological facts of memory. The hypothesis is also a specific against 'Vitalism'. It is a plank of rescue for the thinker who is baffled by his inability to explain physiological effects in terms of the stimuli operative at the time when the effect occurs.

The nature of the engraphic effect of stimuli, Semon admits, is not explained, but in the third edition of Die Mneme he repudiates the suggestion that anything immaterial or metaphysical is intended.

'On the contrary, I have conceived and definitely described it as a material alteration. I made the point clear in describing the engram as a change left behind in the irritable substance after the excitation has died down. As the altered state of a substance, the engram must necessarily be substantial or material, and may therefore be quite correctly described as a material alteration.' (ibid., p. 275). 'We are not in a position to say more than that after a vigorous stimulation has run its course the change which is produced is to be conceived

or

as a change in the excitable substance, that is, as a substantial material change.' (Die mnemischen Empfindungen, pp. 138, 139.)

In other words that which persists is structural.

Now one may raise two questions: first, how far is ' Mneme' a necessary postulate for the explanation of the phenomena referred to; second, how far is it true that it brings biological facts into line and harmonizes them with the psychological facts of memory, and, further, saves the thinker from relapse into vitalism? For the first point we must consider 'mnemic' causation.

An application of the law of a fixed quantitative relation between cause and effect entitles Semon to say, 'the existence of an engram is recognised by the fact that the unaltered original stimulus is no longer requisite for the production of the corresponding reaction.' (Die Mneme, p. 69.) Since the reaction (effect) is quantitatively the same, the weakened stimulus (cause) must be augmented by something, and that something is the engram.

An engram can be called forth (i) by a repetition of the original stimulus in a qualitatively or quantitatively identical, or partially identical, form; (ii) by the ecphory of an engram which was generated at the same time or immediately before the engram in question; (iii) by the lapse of a given period of time, a cycle, which brings about a given energetic condition in the organism.

Taking the first set of conditions, Semon cites the cases where a stimulus produces the same response though it is applied for a much shorter time, or is of lessened intensity, and the cases where it produces upon repetition a much greater response than at first. The relation between stimulus and response is said in each case to involve the engraphic effects of previous stimulation. Cases of simple summation of stimuli, or of fatigue, both of which may give a relation between stimulus and response of the kind indicated, are ruled

out of court. In fatigue, Semon considers that a second indifference state has not been established before the new application of the stimuli. In summation of stimuli, he considers that the interval between the stimuli is too short to allow of engraphic effects. One excitation has not died away before another is received. As an illustration of engraphic effects of the type contemplated, Semon refers to the experiments by Davenport and Cannon on Daphniae. These show that at the third application of a strong light-stimulus the heliotropic movements of Daphniae take place in about half the time of the original response.

Slight qualitative deviations from the original stimulus do not, however, invalidate the ecphoric action of a stimulus on the engram belonging to the original stimulus. Thus, it is sufficient to see the picture of a landscape in order to be able to ecphorize the engram belonging to the landscape itself; to hear a tune hummed in order to ecphorize the engram belonging to the original full orchestral performance. If the smell of Selene gas ecphorizes in us the olfactory engram of rotten radish, it is clear that one ecphoric stimulus has vicariously replaced another which from a chemical point of view is altogether different.' (ibid., pp. 45, 46.)

As an instance of the second set of cases Semon refers to observations by Prof. Lloyd Morgan. Young birds when newly hatched peck at any and all objects of convenient size. Prof. Lloyd Morgan threw before some young chickens caterpillars which are conspicuous for their marking of black and gold rings, and which presumably are unpleasant in flavour to the chickens' palate, for after picking them up the chickens at once dropped them. And although subsequently they ate readily of other caterpillars, they always avoided those of the first variety. According to Semon one experience sufficed to establish an association of optic and chemical engrams. Whenever the sight of the caterpillars called forth the optic engram, the chemical engram came with it and the caterpillar was left alone.

As instances of the third set of conditions Semon gives the rhythmic fall of the leaf in autumn and the bursting of the buds in spring,

'A central European beech, which stands in full vegetation from May until September, reaches in the latter month an organic state which acts ecphorically on that engram whose successive reactions consist of food circulation from leaves into branches and root-stems, and of the fall of leaves. This ecphory takes place in the beech in autumn irrespective of those specific influences of temperature which rightly are expected to act at that time of year, and which by abnormal mildness fail to come into play.... We conclude that the time period as such does not act ecphorically, but that the ecphory is due to the appearance of a definite state associated with the respective engram, and that this appearance is determined in time, in so far as it takes place on the conclusion of a definite number of life processes which may be estimated from the moment chosen as the starting-point. . . . I term such engrams and their ecphory "chronogeneous "." (ibid., p. 55.) "migration impulse", innate in so many species of birds, is the motor reaction of an inherited engram, a reaction produced by chronogeneous ecphory.' (ibid., p. 72.)

The

Now; how far is a special theory of 'mnemic' causation essential in these three types of cases? We find a writer who champions the mechanistic conception of life, making no use of Mneme in accounting for just such cases as those dealt with under the first set of conditions. We may parallel the response of the Daphniae, cited by Semon, with this account given by Prof. Loeb of the reaction of winged aphids to light. Two factors are regarded as determining the movement of the aphids: (1) the structure of their bodies, (2) the photochemical action of the stimulus. If light fall on the aphids from one side only, the processes in the muscles on that side are accelerated; first the head and then the body turns towards the light. After having gone through heliotropic reactions a few times, the aphids respond more quickly than at first.

This might be interpreted as a case of "learning". In so far as it is not a case of a lessening of the stickiness of the feet or the removal of some other purely mechanical factor which retards the rate of movement, it may be brought about by the carbonic or lactic acids produced through the muscular activity.' (The Mechanistic Conception of Life, pp. 40, 45.)

The important point to notice here is that there is no reference to anything which can be regarded as a 'second indifference' state. The change in reaction is a necessary consequence of a change in the nature of the beast. Is there any fundamental difference between Dr. Semon and Prof. Loeb? Both agree that on the second occasion different factors are at work, and hence the effects are different. It is true that some of the factors owe their presence to the operations of past stimuli, but does this render them different in character from members in any other physical chain of causal sequence? What is the distinction between earlier and later effects in an instance such as this, and their relation in an inorganic series of events? The effects of previous blows of a cold chisel make possible the cleavage of a rock at the nth blow, but the effect of the nth blow would not be described by Dr. Semon as due to the ecphory of the engrams of the n-I and n-2, &c., blows. Why then should the effects of past stimulation which are actually represented in the physicochemical state of the organism be regarded as a special variety of causation requiring a new 'mnemic' principle?

We will for the moment leave the second set of conditions, and consider the third, chronogeneous ecphory. As before we may find parallels to the instances given by Dr. Semon if we turn to the pages of Prof. Loeb. The following example of the role of tropisms in self-preservation supplies an instance intermediate between that of the fall of the leaf in the beech tree and that of the migration of birds. The butterfly Porthesia chrysorrhoea lays its eggs on a shrub. The larvae hatch out in autumn, and remain on the lower branches of the shrub during the winter. When in spring the warm weather comes,