50
Influence of
mass-selec¬
tion.
as starting points for new races cannot depend exclusively upon apparent
morphological differences.
In commenting upon the influence of mass-selection, in a case such as
this, Nilsson-Ehle (42 p. 128) says: “Were a mass-selection of plants over
125 c.m. in height to be made from this old mixed sort, plants from types
which normally produce a short straw would be taken as would also those from
types which normally produce a taller growth. The latter would naturally
preponderate whereby a certain advance in the desired direction would likely
be made.” It is quite possible, however, that an advance in one direction
may be made at the expense of some more valuable quality, hence the
danger which is associated with this form of selection.
The Origin of Aberrant Forms as Quantitative Hereditary Variations.
Apart from the mass of apparently related individuals which go to make
up the greater part of a plant population, there may occasionally arise
strange forms which at first sight do not seem traceable to any definite
parentage. There may arise bearded heads of wheat in a bald sort, brown-
chaffed individuals in a white-chaffed sort, white-kernelled forms in a red-
kernelled sort, etc. In oats, white and grey kernelled individuals have been
found in black-grained sorts and vice versa, while side oat types have been
found in sorts characterized by spreading panicles. Formerly these aberrant
forms were commonly regarded as Atavists or Reversions, being looked upon
as the sudden reappearance of certain ancestral characters. More recently
they have received the name Mutation. Experience at Svalöf and elsewhere
has shown that the majority of these so-called novelties which thus suddenly
appear in cultivated crops may be produced artificially by cross-hybridiza¬
tion and may therefore be regarded in most cases, simply as new combina¬
tions of already existing units. Apart from the great scientific interest which
surrounds the appearance of these aberrant individuals there is an interest
for the practical breeder which cannot be denied. If these forms represent
mutations by which apparently new characters are suddenly acquired, it
would clearly be the breeder’s main duty to watch carefully for their appear¬
ance in his fields with a view to isolating and propagating them and perchance
obtaining something better than the old sort. On the other hand, if they
represent the results of natural crossings between different sorts, as they are
now believed to do, it is of much less importance to spend time in seeking for
things which can be produced artificially with much greater assurance of
obtaining an advance. Thus where formerly, striking natural crosses found
in the experimental plots at Svalöf were eagerly isolated and studied they
are now very largely ignored unless the marks by which they are characterized
point to a certain parentage of known value. Instead it is preferred to make
crossings artificially between known sorts whose values have already been
proven.