CHAPTER XXI 



VARIATION AS THE BASIS FOR HEREDITARY 

 DISTINCTIONS 



Variation is a common phenomenon among all organisms and includes 

 many kinds and degrees of difference. Variations in size, in number of 

 parts, in form, in color, and in physiological functions may be either large 

 or small. They may be abrupt and discontinuous or, within large popula- 

 tions, may form minutely graded series. The difference in height between 

 the tall and the dwarf peas of Mendel's experiment, for instance, was 

 both great and abrupt ; but we know that differences in height among men 

 may be either large or small and that if we compare the heights of 1,000 

 men selected at random, we find an almost perfectly graded series be- 

 tween the tallest and shortest individuals of the whole group. 



Practically all early attempts to classify variations were based upon 

 some scheme of distinguishing between the degrees, or visible kinds, of 

 differences among individuals. None of these schemes was very satisfac- 

 tory, because no clear-cut distinction could be made between the proposed 

 classes of variation. With the introduction of experimental breeding, 

 however, it soon became evident that some variations are inherited and 

 that others are not, but appear to be determined by the type of environ- 

 ment in which the organism is placed. This distinction, although often 

 difficult to test, has proved to be a very fruitful one and has made possible 

 a much more adequate knowledge of organic variation. 



Environmentally produced variations. Once a number of clearly 

 noninherited variations were known and could be studied as a group, it 

 was seen that they were all produced by some environmental influence. 

 Literally thousands of experiments and careful observations have shown 

 that variations, slight or marked, may be produced in the form, size, 

 color, functioning, habits, and longevity of organisms by modifying their 

 environment. Changes in temperature, in the kind, duration, or intensity 

 of light, in humidity, nutrition, water supply, amount of exercise, amount 

 of crowding, etc., have all been used singly and in combination to produce 

 such variations. But none of these environmentally produced variations is 

 ever inherited beyond the generation in which the soma is directly affected. 



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