Hayes and Garber - Cucurbit Breeding

INTRODUCTION 11
character may be produced there must be a modification of the
germ plasm. The real question, then, is what causes germinal
change? In considering this question we must keep in mind the
importance from the
possibility that agencies which are of little
standpoint of the plant breeder may be of profound importance
in evolution.
DeVries' Mutation Theory. The more recent theory of evolution
developed by DeVries attacks the question of the sort
of variations which furnish the basis for evolution. DeVries
gives only slight value to the small continouus variations and
advances the hypothesis that large variations are of primary
value. He believes in periods of mutation when from some unknown
cause a species is
producing many new forms, and other
periods when stability of the species is the rule. DeVries recognized
three sorts of mutations; (1) progressive, when an entirely
retrogressive, when an
new character appears ; (2) degressive, the appearance of a partially
latent or hidden character; and (3)
active character becomes latent. The cause, or causes, of these
sudden changes was not known. Mutations are frequently not
large but small.
All sudden heritable changes which cannot be
explained by the laws of segregation and recombination are
called mutations.
The Pure-line Theory. The studies of Johannsen are of particular
value from the standpoint of the plant breeder. He
worked with self-fertilized crops and found that while the progeny
of a single self-fertilized plant varied quite widely, these variations
were not inherited. From single commercial varieties he
found it possible to isolate numerous lines which in their means
differed slightly from each other and which bred true. Johannsen
considered a pure line to be the progeny of one or more selffertilizations
from a single homozygous ancestor. Selection
within such a pure line was of no practical value. Numerous
investigations with self-fertilized crops have been made and corroborate
the results of Johannsen. Isolated cases of mutations in
these pure lines have been reported, and while these are of much
scientific interest they occur far too infrequently to be used as a
basis for a system of breeding.
Johannsen 's pure-line theory has been extended to cover
clonal or asexual propagation in both plants and animals. At its
proper place evidence will be given to show that in heterozygous
organisms which are asexually propagated there sometimes occur