234 THE LESSON OF EVOLUTION 



refute this argument. But, as it is occasionally re- 

 produced, it may be advisable to point out that there 

 is truly a fundamental difference between a crystal 

 and an organism, in that the molecules of the former 

 are all fixed in position with their axes parallel. If 

 this were not the case, the substance would not be 

 crystalline. It is true that the starch and cellulose 

 in the grain of wheat affect polarised light, in the 

 same way as a doubly-refracting crystal ; but the 

 starch and cellulose are not the living part of the 

 wheat. They are merely secretions, and the similar 

 action on polarised light merely shews that in both 

 crystal and starch grain the molecules are not spheri- 

 cal and are arranged with their longest axes parallel. 

 So also starch and cellulose grow from the outside, 

 like a crystal, and not from the inside, like proto- 

 plasm. This, again, is because starch and cellulose 

 are not alive. The living protoplasm of the embryo 

 in the grain of wheat is quite different in constitution 

 from the starch grain. 



An analogy has sometimes been drawn between 

 the definite shapes assumed by crystals and by or- 

 ganisms respectively ; but the analogy is a false one. 

 When crystals develop irregularly, the new faces are 

 always formed parallel to the regular faces, so that 

 the fundamental shape does not change ; there is no 

 true variation. And there is nothing in organic 

 variation which answers to the law of rationality of 

 intercepts in crystals, and nothing but physical con- 

 ditions limit the size of crystals. Even symmetry 

 obeys quite different laws in crystals and in or- 

 ganisms. In crystals the symmetry depends en- 



