Facts and Factors of Development 43 



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 in other cases. 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 move- 

 ment and hence are absent in the foetus though present in many 

 free-living larvae. 



Development of Instincts. Some instincts are present imme- 

 diately after birth, such as the instinct of sucking or crying in the 

 human infant, 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 ; 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 compli- 

 cated reflexes which have survived in the course of evolution, 

 probably becauSfe they serve a useful purpose. 



Very much has been written on the nature and origin of in- 

 stincts, 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 phylo- 

 genetically, under the stress of the elimination of the unfit, so that 

 they are usually adaptive. 



3. Summation of Stimuli, Memory, Another general character- 

 istic 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 

 stimulus still persists, the response to the second stimulus may be 



