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 onespecies 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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