When the embryo becomes differentiated to such an extent as to have specialized organs for producing movement its capacity for making responsive movements to stimuli becomes much increased. If the responses of animals and plants to stimuli are of such a sort that the organism turns or moves toward or away from a source of stimulus they are termed tropisms; if the responses are very complicated, one response calling forth another and involving many reflexes, as is frequently the case in animals, they are known as instincts. In the embryo the rhythmic contractions of heart, amnion and intestine are early manifestations of reflex motions. These appear chiefly in the involuntary muscles before nervous connections are formed, the protoplasm of the muscle cells probably responding directly to the chemical stimulus of certain salts in the body fluids, as Loeb has shown. Reflexes which appear later are the random movements of the voluntary muscles of limbs and body, which are called forth by nerve impulses. Tropisms are manifested only by organisms capable of considerable free movement and hence are absent in the foetus though present in many free living larvæ. Some instincts are present immediately after birth, such as the instinct of sucking or crying, though these are so simple when compared with some instincts which develop later that they might be classed as reflexes; it is doubtful whether any of the activities before birth could properly be designated as instincts. Reflexes, tropisms and instincts have had a phylogenetic as well as an ontogenetic origin, and consequently we might expect that they would in general make for the preservation of the species, and as a matter of fact we usually find that they are remarkably adapted to this end. For instance the instincts of the human infant to grasp objects, to suck things which it can get into its mouth, to cry when in pain, are complicated reflexes which have survived in the course of evolution probably because they serve a useful purpose.
Very much has been written on the nature and origin of instincts, but the best available evidence strongly favors the view that instincts are complex reflexes, which, like the structures of an organism, have been built up, both ontogenetically and phylogenetically, under the stress of the elimination of the unfit, so that they are usually adaptive.
3. Memory.—Another general characteristic of protoplasm is the capacity of storing up or registering the effects of previous stimuli. A single stimulus may produce changes in an organism which persist for a longer or shorter time, and if a second stimulus occurs while the effect of a previous one still persists, the response to the second stimulus may be very different from that to the first. Macfarlane found that if the sensitive hairs on the leaf of Dionea, the Venus fly-trap (Fig. 20, SH), be stroked once, no visible response is called forth, but if they be stroked a second time within three minutes the leaf instantly closes. If a longer period than three minutes elapses after the first stimulus and before the second no visible response follows, i.e., two successive stimuli are neces-
