Experimentation has not so far played a sufficient part in the biological study of the invertebrates and it will not be easy to bring together many examples similar to the preceding, nor as good ones. But these suffice to place the role of memory of articulates, and especially of insects, in the full light. Most of these animals, if not all, are capable of receiving sensations, of graving them for a longer or shorter time on their nervous systems, and keeping them there, so much the better if they have been strong and repeated several times. By the association of these sensations, they can modify their habits, acquire new ones, and show themselves capable of apprenticeship. The cockroaches of Turner learned to remain in a lighted room by associating the notion of shock with that of darkness. The Pompilus of Nicolas immediately stored its spider, because its memory associated the recollection of the search with that of its capture, and, in the experiment of Drzewina on the crabs, visual sensation surely intervened,- the sensations of places and muscular sensations, perhaps still others.
Whether the sensations remain isolated or are associated in the memory, they do not impress themselves equally on all articulates. The examples just cited show us that from this point of view a very great diversity exists not only among the species but between the different individuals of the same species. What differences there were in the remembrance of impressions and facility in apprenticeship between the two species of Osmia studied by Ferton, between the individuals of the common Blatta submitted to the eiectric shock by Szymanski and Turner, between the different kinds of the yellow-winged sphex which Fabre studied ! The faculty of retaining, of learning by experiment, in association, shows in each individual a particular development, and merits, for this reason, the name individual memory. From the fact that the sensorial nervous organs of the representatives of the same species are very nearly alike, they ought to show a great similarity in their manner of receiving and holding sensations, but this similitude does not reach identity, and just as each individual has certain structural traits that distinguish it, each one is distinguished also by the development of its psychic faculties.
How these especial aptitudes of individuals separate us from species memory, where the response to stimulating variations is an adapted movement, always the same, variable simply in its amplitude, and the same way with all individuals ! Moreover, between the individual memory and the species memory one can see other differences still more striking and all as profound. The domain of the first involves all the influences of the environment ; that of the second is restricted to the variations of the external agents. The first may be extended and actually modified; the second is an immutable legacy of the past; with the first the association of the sensations and, consequently, apprenticeship are possible, in the second the vague original psychism has been transformed into a pure automatism.
In spite of the differences the individual memory and the species memory are not without contact. Both, in fact, are the result of nervous impressions ; both enter into the psychic domain, because at a given moment, more or less, they are manifest by choice ; finally, both have led to habits which have quickly 'turned to automatism.