Constancy is not the result of selection or of improvement. It is a
quality of its own. It can neither be constrained by selection if it is
absent [563] from the beginning, nor does it need any natural or
artificial aid if it is present. Most of my new species have proved
constant from the first. Whenever possible, the original mutants have
been isolated during the flowering period and artificially
self-fertilized. Such plants have always given a uniform progeny, all
children exhibiting the type of the parent. No atavism was observed and
therefore no selection was needed or even practicable.
Briefly considering the different forms, we may state that the full
experimental proof has been given for the origin of _gigas_ and
_rubrinervis_, for _albida_ and _oblonga_, and even for _nanella_, which
is to be considered as of a varietal nature; with _lata_ the decisive
experiment is excluded by its unisexuality. _laevifolia_ and
_brevistylis_ were found originally in the field, and never appeared in
my cultures. No observations were made as to their origin, and seeds
have only been sown from later generations. But these have yielded
uniform crops, thereby showing that there is no ground for the
assumption that these two older varieties might behave otherwise than
the more recent derivatives.
_Scintillans_ and _elliptica_ constitute exceptions to the rule given.
They repeat their character, from pure seed, only in part of the
offspring. I have tried to deliver the _scintillans_ from this [564]
incompleteness of heredity, but in vain. The succeeding generations, if
produced from true representatives of the new type, and with pure
fertilization, have repeated the splitting in the same numerical
proportions. The instability seems to be here as permanent a quality as
the stability in other instances. Even here no selection has been
adequate to change the original form.
IV. Some of the new strains are evidently elementary species, while
others are to be considered as retrograde varieties.
It is often difficult to decide whether a given form belongs to one or
another of these two groups. I have tried to show that the best and
strictest conception of varieties limits them to those forms that have
probably originated by retrograde or degressive steps. Elementary
species are assumed to have been produced in a progressive way, adding
one new element to the store. Varieties differ from their species
clearly in one point, and this is either a distinct loss, or the
assumption of a character, which may be met with in other species and
genera. _laevifolia_ is distinguished by the loss of the crinkling of
the leaves, _brevistylis_ by the partial loss of the epigynous qualities
of the flowers, and _nanella_ is a dwarf. These three new forms are
therefore [565] considered to constitute only retrograde steps, and no
advance. This conclusion has been fully justified by some crossing
experiments with _brevistylis_, which wholly complies with Mendel's law,
and in one instance with _nanella_, which behaves in the same manner, if
crossed with _rubrinervis_.
On the other hand, _gigas_ and _rubrinervis_, _oblonga_ and _albida_
obviously bear the characters of progressive elementary species. They
are not differentiated from _lamarckiana_ by one or two main features.
They diverge from it in nearly all organs, and in all in a definite
though small degree. They may be recognized as soon as they have
developed their first leaves and remain discernible throughout life.
Their characters refer chiefly to the foliage, but no less to the
stature, and even the seeds have peculiarities. There can be little
doubt but that all the attributes of every new species are derived from
one principal change. But why this should affect the foliage in one
manner, the flowers in another and the fruits in a third direction,
remains obscure. To gain ever so little an insight into the nature of
these changes, we may best compare the differences of our
evening-primroses with those between the two hundred elementary species
of _Draba_ and other similar instances. In doing so we find the same
main [566] feature, the minute differences in nearly all points.