On account of the irritability of living matter, we understand the sensitiveness which makes organisms respond to the excitation of stimuli and to react to them by movements and by more or less complex chemical phenomena. This sensitiveness is an essential property of living matter. It is found in all organisms, but it is especially apparent in those animals which respond by motion reactions.

With the lower animals and with the motile plants which form the contact between the two kingdoms this sensitiveness may be very delicate, but it is always diffuse, without special localization in organs where it is strongest, without nerve conductors which propagate its efforts. Also, tropisms play a predominant role with those beings with which the tropic manifestations seem simply due to internal physiological changes and to differences of excitants. But this subordination gives to the motive reactions of lower organisms a very characteristic suppleness which one especially notices when he examines one of these animals under the microscope. The infusorian advances, recoils, describes a circle, evolutes like a vortex, stops, or shivers, to go away immediately. One divines in this being an active sensitiveness always awake and, above all, infinitely complex. Without doubt, tropic manifestations belong to the domain of automatism, but animal automatism does not present the rigidity of mechanical automatism. Always complex in its nature, as in its manifestations, it shows a delicate play which indicates the complexity.

The subordination of tropisms becomes greater in the higher animals and notably in the articulates, where the sensitiveness is localized for the greatest part in special sensorial organs and the propagation of its effects in a well-differentiated nervous system. But it is not always easy to distinguish tropic excitations from sensorial impressions or sensations; and, in the same way that we can distinguish all the intergrades between diffuse sensitiveness and its localization in sense organs, it is probable that all the degrees may be established between these two kinds of phenomena. But there is more. We shall see in what follows (Chapter VI) that sensations can be graven on the memory and that these associated can induce new acts. From this conflicts often result between tropisms and the associative memory. Then the intellect and the automatism are in disaccord, and in this struggle of such an especial kind it is often the memory which carries the day. Bees go to the light, but this does not prevent them from working in the obscurity of the hive or from penetrating into the darkest places in search of their favorite honey. The house fly also loves the light, but it has passed its apprenticeship in our houses, and, less stupid than the big blow fly, does not find its end in our artificial lights.