THEORIES OF HEREDITY 113 



duced by the crystallisation of one given substance is 

 invariably the same and nothing could cause a sub- 

 stance which crystallises in straight prisms to crystal- 

 lise in oblique prisms. The polarity of physiological 

 units is more delicate, more easily affected. It pre- 

 supposes very definite conditions and can be in- 

 fluenced by many factors. Those factors may pro- 

 duce some slight modifications in the whole without 

 destroying it, without even altering its general plan. 



The result is that, while only one physiological unit 

 corresponds to one species, differences may be ob- 

 served between the individuals constituting that spe- 

 cies. Organic plasticity explains individual variations 

 and there is no need of supposing that to each and 

 every individual there corresponds a physiological 

 unit. 



Apart from the differences observable between in- 

 dividuals, there are differences between histological 

 characters. They arise from the fact that the units, 

 all alike in polarity (specific characters) and in vari- 

 ability of polarity (individual characters), are af- 

 fected differently while in the embryonic state by the 

 action of incidental forces ( from the fact of the loca- 

 tion of each cell in space) and thus slightly modified 

 in their nature. The units which constitute muscular 

 tissue are not identical with those constituting osseous 

 tissue. They present the same shape, but they are 

 like the crystals of two different substances which al- 



