CHAPTER XXV 

 VARIATION' 



ERNEST BROWN BABCOCK AND ROY ELWOOD CLAUSEN 



Organic differences, their nature and causes, have furnished 

 abundant material for speculative enquiry since time immemorial. 

 The great significance of the fact of organic individuality was not fully 

 grasped until Lamarck founded his theory of evolution which postu- 

 lated the progressive, imperceptible change of one species into another. 

 It remained for Darwin to scrutinize all phases of organic life, past 

 and present, wild and domesticated, in his search for a guiding prin- 

 ciple which should explain the course of evolution. Darwin's hypothe- 

 sis of natural selection assumes variability without enquiring into 

 its causes, but this does not mean that Darwin was not concerned 

 with the problem of causes. In both his Origin of Species and 

 Variation in Animals and Plants under Domestication the causes of 

 variability are often referred to and he suggested among others, the 

 kind and amount of food, climatic changes and hybridization. Our 

 respect for the great naturalist's keen perception deepens when we 

 realize that very Httle has been added as yet to our knowledge of the 

 causes of variation. 



The universality of variation. — Individuality is common to all 

 organisms. No two trees, no two leaves, no two cells in a leaf are 

 identical in every respect. Individuals sometimes appear exactly 

 alike but even identical twins will be found to differ in some features. 

 The shepherd knows his sheep individually and the orchardist his trees. 

 Were there no differences in individuals there would be no changes in 

 species and there could be no improvement of cultivated plants'. 

 "Variation is at once the hope and despair of the breeder," the hope 

 because without it no improvement would be possible, the despair 

 because very often, when improvement has been made, variation 

 results in a tendency to fall below the standard previously reached. 

 In the sugar beet, for example, a high percentage of sugar has been 

 maintained by continually testing and selecting the "mother" beets 

 for the next crop of seed. However, this necessity for continual 



• From E. R. Babcock and R. E. Clausen, Genetics in Relation to Agriculture 

 (copyright 19 18). Used by special permission of the publishers, The McGraw- 

 Hill Book Company. 



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