[Chap. XL VARIATIONS AND DIVERSITY OF ORGANISMS 491 



Non-heritable variations. Non-heritable variations, or fluctuations, are 

 merely differences in the expression of hereditary potentialities in indi- 

 viduals of similar heredity growing in different kinds of environments. 

 They are the kinds of variations in the development of a plant that man 

 is trying to obtain when he adds water and fertilizers to the soil, or other- 

 wise tries to change the environment in a way that will be most favor- 

 able to the growth of a particular kind of plant. Any one of several 

 environmental factors may greatly alter the development and appearance 

 of a plant by affecting the rate of such processes as photosynthesis, 

 respiration, and transpiration, or by affecting the permeability or other 

 physical conditions of the protoplasm. In fluctuations, therefore, the 

 hereditary mechanism of the plant remains unchanged, and the heredity 

 of the progeny is not influenced by the environment of the parent plant. 



This fact is most easily tested experimentally with pure-line plants. 

 One example will be cited. In Norway a pure line of oats was cultivated 

 in experimental plots in several different types of soil and climate in 

 different parts of the country. When the growth and development of the 

 plants in the various plots were compared, several differences in yield 

 and other characters were evident. A careful record of all these diflFer- 

 ences was made in an experiment which was continued for 7 years. At 

 the end of that time seeds from five of the plots in different parts of the 

 country were planted in one plot in the same conditions of soil and 

 climate. The resulting plants were all alike. That is, there were no last- 

 ing effects of the variations in development that occurred in the plants of 

 the preceding seven generations in the diverse environments to which 

 they had been exposed. In all, seven different pure lines of oats and two 

 pure lines of barley were similarly tested and no changes occurred in the 

 heredity of the progeny. 



Experiments of this sort do not prove that a change in environment 

 never causes a heritable change in a plant. But they do show that the 

 variations we have called fluctuations are non -heritable, that they have 

 no influence on subsequent progeny, and that they are not the means by 

 which improved varieties were obtained through domestication, or by 

 which evolution occurs. Incidentally, these experiments also show some 

 of the fallacies in the popular belief in the inheritance of acquired 

 characters. 



Fluctuations are, however, of great economic importance. It is to se- 

 cure the best possible expressions of the hereditary potentialities in our 

 present crop plants that so much labor is annually expended in cultivat- 



