CHAPTER II 

 VARIATION 



Organic differences, their nature and causes, have furnished abundant 

 material for speculative enquiry since time immemorial. The great sig- 

 nificance of the fact of organic individuality was not fully grasped until 

 Lamarck founded his theory of evolution which postulated 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 domes- 

 ticated, in his search for a guiding principle which should explain the 

 course of evolution. Darwin's hypothesis 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 percep- 

 tion deepens when we realize that very little has been added as yet to 

 our knowledge of the causes of variation. 



The Universality of Variation. Individuality is common to all or- 

 ganisms. 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. How- 

 ever, this necessity for continual selection does not exist in respect to 

 all important field crops although they are subject to the general law 

 of variation. That this must be so is clear when we realize that many 

 natural species as well as cultivated varieties of plants are really mix- 

 tures of sub-species, varieties, or races and that upon being isolated 

 these distinct forms reproduce their own particular type. This is most 

 easily demonstrated in plants normally self-fertilized, yet in all naturally 



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