616 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



individuals within a certain species than in others. Age and sex functions may 

 also modify the strength of the reaction, which, in certain species, is appar- 

 ently much more developed in younger than in older birds. It seems that in 

 migratory birds environmental changes, arising from seasonal conditions, set 

 in motion a reaction system which expresses itself in restlessness, but the 

 mechanisms underlying these migratory tendencies are as yet only imperfectly 

 known. 



The instincts, as well as the kind and range of modifiability of behavior, 

 are inherited characteristics, and, on the whole, they are very similar in 

 closely related species and more dissimilar the more distant the species are; 

 these reactions resemble, in this respect, the organ systems and outer con- 

 figurations, for instance, in various species of birds which, on the whole, are 

 similar in nearly related species and differ in more distant species. However, 

 there may be differences here too between nearly related species ; as, for 

 instance, in behavior connected with courtship and parental care of the 

 young. There exists among birds, also, a well developed group life, the groups 

 consisting of individuals belonging to the same species, and members of one's 

 own group are distinguished from those belonging to strange groups. The 

 stimuli which hold certain groups together may be genetically determined in 

 one species and acquired by experience in another species. Voice, posture and 

 movements may regulate the activities in a certain group. There are indications 

 that even conditions comparable to what may be called suggestion in the 

 human species play a part in the group life, inasmuch as movements of one 

 individual are quickly transmitted to other members of the flock and elicit 

 in them a similar behavior. The movements and attitudes of a male may 

 initiate corresponding actions in the female ; in this case sense organs trans- 

 mit the mode of reaction and induce imitative behavior in another individual. 

 Similar effects of suggestions have been noted also in fishes. 



Also in other respects the species characteristics of behavior are to a 

 large extent genetically fixed, although there is a certain range of modifi- 

 ability in accordance with experience. Fixed species reactions are, for instance, 

 those of cowbirds, which return to their own species even if they have been 

 reared by foster parents belonging to different species. The flocking together 

 of birds of the same species, another fixed mechanism, depends on the in- 

 herited functions of organs and only in an indirect manner on the identity of 

 the organismal differentials of the individual birds, although in different 

 species of birds the degree of specificity in the tendency to gregariousness 

 seems to vary. The European and African stork, for instance, flocks each 

 with its own type and the two types do not mingle with each other; but in 

 certain other species such a strict segregation does not take place, members 

 of different species undertaking common flights. 



The superiority-inferiority relations may begin very early in the life of birds 

 among nestlings. These reactions depend upon inherited reflex systems, but 

 they may be influenced by experience gained in testing other individuals. As 

 stated above, a stranger in a certain territory tends to be inferior in authority 

 to the first-comer. In herons the male must have established superiority over 



