62 GENERAL PHYSIOLOGY 



cell, invariably perishes. In every case, therefore, the cell remains 

 the elementary organism. 



If the above considerations be summarised, it is found that five 

 stages of individuality can be distinguished in the organic world, 

 and can be characterised as follows : 



1. Individuals of the first order are cells. They represent 



elementary organisms that are not composed of lower 

 units capable of life. An example is the unicellular, 

 ciliate infusorian Stentor (Fig. 6). 



2. Individuals of the second order are tissues. The tissues 



are associations of individuals of the first order, each 

 one of which is like the others. An example is the 

 flagellated spherical alga, Eudorina (Fig. 5). 



3. Individuals of the third order are organs. The organs 



are associations of various kinds of individuals of 

 the second order. An example is Hydra (Fig. 2), 

 the body of which consists of only two layers of 

 tissues. 



4. Individuals of the fourth order are persons. The persons 



are associations of various individuals of the third 

 order. An example is man, whose body consists of 

 various organs united. 



5. Individuals of the fifth order are communities. The 



communities are associations of individuals of the 

 fourth order. Examples are communities of ants and 

 bees. 



This scheme requires one more remark. It shows that every 

 individual of a higher order consists of an assemblage of individuals 

 of the next lower order, but the constituents of an individual of 

 the higher order are not always real individuals, i.e., they are 

 capable of self-preservation when living in union with, but not 

 when separated from, their fellows ; in other words, they are only 

 virtual individuals. A person or individual of the fourth order, 

 for example a man, consists of single organs, which are equal to 

 individuals of the third order. These organs, however, are virtual, 

 not real, individuals, for they perish when separated from their 

 fellows. It is the same with individuals of all orders. E.g., the 

 cell of an animal tissue, if separated from its fellows, is in itself 

 incapable of life; in the tissue, therefore, it is only a virtual 

 individual. In other cases, however, the constituents of an in- 

 dividual of a higher order, when separated from their fellows, can 

 become real individuals of the next lower order, as is shown, e.g., 

 by Eudorina, in which the single cells when separated are in them- 

 selves capable of life. 



From these considerations the important facts follow that in the 

 end all living individuals of whatever order either are composed of 

 cells as the elementary structural components or are themselves 



