552 Organic Physics. [July, 



of special tissues might result from the same cause, an excess or 

 lack of polar energy in the chemical constituents of the germ 

 which give rise to these tissues. 



We have so far considered protoplasm as consisting of mole- 

 cules of similar chemical constitution, and differing only in acid 

 or basic energy. But in the protoplasm of the higher animals a 

 special chemical differentiation takes place. The molecules of 

 the unit of each tissue have a special chemical constitution of 

 their own, which differs from that of any other tissue. 



The simplest organisms have, in all their parts, identical rela- 

 tions with the environment. But as development goes on, this 

 identity of relations ceases to exist. Fixed duties are assigned 

 to fixed parts of the body, until finally every region of the organ- 

 ism has its definite office to fulfill. Chemical variation necessa- 

 rily accompanies this functional variation. The special duties 

 given to special tissues are chemical duties, or motor duties de- 

 For their proper performance 

 chemical constitution. Such a 

 chemical divergence is a necessary result of divergent relations 

 with the environment. In the higher animals all the internal tis- 

 sues are removed from contact with external conditions. Their 

 environment is the nutrient fluid. But this differs in different 

 parts of its course. Certain organs take certain elements from it. 

 More internal organs must employ as nutriment other material 

 for which they may have a less vigorous affinity. But this assim- 

 ilation of varied nutriment produces a divergence in chemical 

 constitution, so that each tissue finally accepts from choice what it 

 first may have accepted from necessity. This process of differen- 

 tiation has gone steadily on, from the first to the last step of or- 

 ganic development, every change in the environment of a tissue 

 producing a change in its chemical organization. 



If such be the case we should look upon functional variation 

 not as causing but as caused by chemical spec 

 circumstances, having that undetermined orig 



the relations of the tissues to 

 nment. In consequence, their 

 chemical character changes. As a secondary result oi this change 

 their organic function varies. A new and perhaps more diverse 

 relation is established between the parts of the organif 

 sequence of specializations in its chemical constitute 



Many 

 ailed 



arising 



