282 PLANT PHYSIOLOGY 



conceivable slowness if at all, as well as to changing influ- 

 ences. 



The dominant power of the unchanging influences is shown 

 by man's inability to make fundamental changes in living 

 organisms by experiment, and by the absence of any proof 

 of the so-called "inheritance of acquired characters." If 

 man could eliminate the force of gravity in experimenting 

 on an organism, he might obtain other results than now, 

 when all he can do is to expose the organism on all sides to 

 the force, or to oppose gravity by centrifugal or other force. 

 If he could change the composition of water, or find a sub- 

 stitute for it, he might make some fundamental change in 

 the organism under experiment. He cannot remove the air 

 from around a plant or animal without at once changing 

 the conditions of his experiment in other ways also, and the 

 result of such experiment is of little value in the subject 

 under discussion, because it is produced by a combination 

 of changes. 



We may safely conclude, then, that the unchanging factors 

 in its environment, to which it irritably responds, are 

 among the most powerful, if they are not the only, influ- 

 ences which make the offspring like its parents. The chang- 

 ing factors cause it to vary. Heredity is possible only 

 because of the irritability and continuity of living proto- 

 plasm, and the continuity of certain influences acting upon 

 this. 



Heredity is a mystery, but so is the form of a crystal of 

 common salt. Spirogyra plants are no more and no less 

 alike than are crystals of common salt. Does common salt 

 "inherit" its crystalline form? Crystals of common salt 

 represent the reaction of NaCl molecules to their environ- 

 ment, of which some factors are constant and others chang- 

 ing. Spirogyra cells represent the reaction of the molecules 

 and combination of molecules of which its living protoplasm 

 consists, to their environment, of which some factors are 

 constant and others changing. Given the continuity of the 

 irritable substance (protoplasm) from parent to offspring, 

 heredity and variation are the inevitable results of the 

 constant and of the changing factors respectively, which. 



