576 GENERAL PHYSIOLOGY 



arising from the continued division of the egg-cell are in harmony 

 with the special conditions under which they appear. All in 

 which this is not the case must perish in the struggle for existence 

 through the action of selection. But the most complete harmony 

 is reached when the individual labours of the different cells so fit 

 into one another that, although every cell or cell-group has de- 

 veloped a different labour for its own specialty, this labour is for 

 the good of all the other cells, is, indeed, necessary to all the 

 others. Thus, the extraordinarily far-reaching differentiation and 

 surprisingly detailed division of labour of the individual cells and 

 tissues in the cell-community become comprehensible. 



As a result of the division of labour, every kind of cell, every 

 tissue, every organ in the multicellular community undertakes 

 a special task, and since early times physiology has termed this 

 task the " physiological function " of the cell-complex in question. 

 All elementary vital phenomena which, in the lowest organisms, 

 take place in the individual cell, in multicellular organisms 

 are developed in a special degree as specific functions of 

 definite cell-groups and become adapted very perfectly to specific 

 purposes. Thus, in the higher animals, by the special development 

 of contractility, movement becomes the specific function of the 

 muscle-cells. The capacity of appreciating stimuli is developed in 

 an especially high degree as the function of the sense-organs. The 

 capacity of conducting stimuli is augmented to a surprising extent 

 as the function of nerves. Secretion undergoes its greatest per- 

 fection in the function of gland-cells. Every kind of cell retains 

 all the elementary vital phenomena, but the one becomes pre- 

 eminently developed as its specialty. The more the specialties of 

 the individual cells and cell-groups come to act for the good of all 

 cells and assist their vital processes, the more highly evolved does 

 the cell-community become. It represents a mechanism in which, 

 m spite of its extraordinary extent and its excessive complication, 

 as exhibited especially in the bodies of the higher animals, all its 

 parts co-operate as a unit. 



C. CENTRALISATION OF ADMINISTRATION 



If the last point, namely, the development of a unity in the co- 

 operation of the cells and tissues of the cell-community be de- 

 veloped more in detail, it is found that in addition to the principles 

 of dependence and cell-differentiation, a third principle comes into 

 consideration, namely, that of centralisation of administration. 

 This principle is connected very closely with the two others ; con- 

 sidered from the point of view of natural selection, it is in a certain 

 sense a necessary result of those, and it is developed pari passu 

 with them. 



The farther the differentiation of the cells goes and the closer 



