202 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



The means which the animal organism possesses for the regulation of dis- 

 turbances, initiating reactions tending to reestablish a state of equilibrium 

 between the parts of an individual, are relatively limited. Against a great 

 variety of interferences which may affect an organism, the latter reacts in a 

 very similar manner making use of the small number of reaction patterns 

 which are at its disposal, in accordance with its inherited constitution. The 

 variations in the environmental agents which act on the organism are very 

 much greater than the various modes of reaction which the affected indi- 

 vidual can initiate, but a gradation in the kind and intensity in the individu- 

 ality differential reactions which may take place against these interfering ele- 

 ments can be noted. No reaction occurs against living parts of the animal's 

 own body, to which a complete adaptation exists. Likewise, against non-living 

 constituents of the environment, other than protein substances, no general 

 or specific local reaction occurs. Against dead protein substances the lympho- 

 cytes in the circulation, and presumably those of the lymphatic organs, react 

 in about the same way as against homoiogenous individuality differentials, al- 

 though locally there is a distinction between the reactions against living 

 homoiogenous tissues and against dead material. The local reaction against 

 all foreign bodies, whether of protein or non-protein nature, is in principle 

 the same, tending to destroy, to transform and to incorporate the strange ma- 

 terial into the body in a way which is least injurious to the organism. However, 

 certain heterogenous material, non-living but containing formerly living ele- 

 ments such as blood clots, if introduced into the organism may call forth a 

 local reaction corresponding to that seen after implantation of heterogenous 

 living tissues. Also, the general blood-cell reaction is stronger against more 

 complex heterogenous proteins than against the simpler ones. Thus injections 

 of heterogenous blood sera elicit merely a lymphocytic reaction, whereas, sub- 

 cutaneous implantation of more complex proteins, such as heterogenous fibrin- 

 ogen induce a typical heterogenous blood-cell reaction. 



There are thus increasing intensities and specificities of reactions noted if 

 non-living substances of different degrees of chemical complexity are intro- 

 duced into the body; but the maximal specificity in reaction is attained only 

 if living tissues, bearing strange organismal differentials, are transplanted. It 

 is therefore those substances which are most nearly related to the characteris- 

 tic constituents of living tissues namely the most complex proteins which call 

 forth reactions most similar to those elicited by the tissues themselves. 



We see, then, that the reactions against strange individuality and species 

 differentials are not entirely disconnected and newly created responses of the 

 organism against interferences, but they represent the endstage of a series of 

 interactions which are graded in specificity in accordance with the increasing 

 complexity in the structure of the strange environmental elements, and in ac- 

 cordance with the increasing similarity between their constitution and the 

 constitution of living tissues. 



