CHAPTER XL 



TYPES OF VARIATIONS AND DIVERSITY OF ORGANISMS 



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In all his attempts to arrive at a scientific basis of improving plants and 

 animals by selection and controlled breeding, and in his efforts to inter- 

 pret the diversity of living organisms in the world about him, man has 

 been confronted v^ith the problem of understanding the nature of organic 

 variation. His various ansv^ers to the problem are voluminously illus- 

 trated throughout recorded history, as are also the various social impli- 

 cations he attached to any particular answ^er. For a long time the prob- 

 lem was attacked by casual observations accompanied by philosophical 

 and metaphysical speculation. As a consequence we have socially in- 

 herited all sorts of contradictory explanations and their associated 

 phrases and slogans. 



It is most important for us today, therefore, to try to recognize estab- 

 lished facts and see what inferences may reasonably be drawn from 

 them. The numerous experimentally established facts included in previ- 

 ous chapters may be used as a basis for ( 1 ) distinguishing types of varia- 

 tions, ( 2 ) considering the causes and consequences of the different types, 

 and ( 3 ) understanding the uses one may make of a knowledge of these 

 variations. Other data will be included in the chapters on the great 

 groups of plants; but we shall have a better appreciation of certain 

 features of the different types of plants in the world today if we can 

 approach a study of them with certain basic ideas about how they came 

 to be. 



Solely on the basis of heredity, and without any reference to causes, 

 the variations in living organisms may be classified as either heritable or 

 non-heritable. Having made such a classification of variations, one may 

 then attempt to distinguish their underlying causes and consequences, 

 as we have tried to do in many of the foregoing chapters. 



The first part of this chapter is primarily a review and summary of 

 several previously described facts about variations. Some of the uses 

 that may be made of these facts in evaluating various ideas about evolu- 

 tion and adaptation are briefly outlined in the remainder of the chapter. 



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