Hayes and Garber - Cucurbit Breeding

32 BREEDING CROP PLANTS
a conception of the mode of transmission of these size characters
and there seems no good reason for believing that a different
mechanism is involved than in the inheritance of color characters.
Environmental conditions probably play a larger role in the
modification of the appearance
characters.
of size characters than for color
Stability of Inherited Factors. That sudden changes in the
appearance of a character are sometimes found is a well-known
fact. The causes of these sudden changes are not so easily
determined.
Whether these are more logically explained as due
to changes in certain inherited factors or due to a new recombination
of factors or by other causes is an unanswered question.
The pure-line theory of Johannsen was a result of an experimental
attack on the question of the stability of a character. A few
sudden changes in characters have been observed.
Nevertheless,
plant characters of self-fertilized crops exhibit remarkable
uniformity. Many of the inherited sudden changes which
have been noted are most logically explained as the result of a
natural cross.
Others appear to be due to a sudden change in
the hereditary factors of the organism or to the loss of a genetic
factor.
The view of factor stability which seems most helpful for the
plant breeder has been clearly stated by East and Jones (1919).
"For these and other reasons which might be given,
could further
space be devoted to the subject, we believe there should be no hesitation
in identifying the hypothetical factor unit with the physical unit factor
of the germ cells. Occasional changes in the constitution of these
factors, changes which may have great or small effects on the characters
of the organism, do occur; but their frequency is not such as to make
necessary any change in our theory of the factor as a permanent entity.
In this conception biology is on a par with chemistry, for the practical
usefulness of the conception of stability in the atom is not affected by the
knowledge that the atoms of at least one element, radium, are breaking
down rapidly enough to make measurement of the process possible."