FACTS AND FACTORS OF DEVELOPMENT 27 



When the embryo becomes differentiated to such an extent as to have 

 specialized organs for producing movement its capacity for making re- 

 sponsive movements to stimuli becomes much increased. If the re- 

 sponses 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 rhyth- 

 mic 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. Eeflexes 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 larvae. 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. Eeflexes, 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- 



