2 THE ORIGIN OF THE NERVOUS SYSTEM 



concerned. The crystal is not a purely fortuitous aggre - 

 gation of molecules, the house is not simply a pile of 

 lumber,.~bricks or stone, mortar, etc., nor is the organism 

 simply a kaleidoscopic mixture of substances. In all 

 these types of individuality the relations of the con- 

 stituent materials are ordered and controlled in accord- 

 ance with some sort of pattern. Pattern then, in the 

 broad sense, is the particular kind of order, unity, or 

 wholeness which constitutes the entity or individual, 

 whether it be crystal, house, or organism. Moreover, 

 in the organism as elsewhere, the distinction between 

 pattern and material is clearly indicated by the fact 

 that very similar patterns may exist in very different 

 materials. For example, certain hydroids resemble 

 very closely certain plants as regards the order of 

 budding and the growth relations of branches. As 

 regards general axial relations certain bilaterally sym- 

 metrical plants, e.g., Marchantia, are very similar to 

 certain bilaterally symmetrical animals, such as Planaria, 

 and very similar radiate arrangements of parts are 

 found in many different plants and animals. Again, 

 the so-called law of anteroposterior development calls 

 attention to a feature of developmental pattern common 

 to many widely different organisms, and finally the 

 receptor-conductor-effector pattern in some form, ran- 

 ging from a momentary relation in which the three com- 

 ponents are completely interchangeable, to the reflex arc 

 associated with a permanent nervous structure, is appar- 

 ently a physiological pattern characteristic of living pro- 

 toplasm in general in relation to an external environment. 

 Pattern develops in or is imposed upon material of 

 some sort, and the problem of the constitution of this 



