This is a very curious fact. It embraces two minor points, viz: the
multitude of similar mutants in the same year, and the repetition
thereof in succeeding generations. Obviously there must be some common
cause. This cause must be assumed to lie dormant in the _Lamarckiana_s
of my strain, and probably in all of them, as no single parent-plant
proved ever to be wholly destitute of mutability. Furthermore the
different causes for the sundry mutations must lie latent together in
the same parent-plant. They obey the same general laws, become active
under similar conditions, some of them being more easily awakened than
others. The germs of the _oblonga_, _lata_ and _nanella_ are especially
irritable, and are ready to spring into activity at the least summons,
while those of _gigas_, _rubrinervis_ and _scintillans_ are far more
difficult to arouse.
These germs must be assumed to lie dormant during many successive
generations. This is especially evident in the case of _lata_ and
_nanella__, which appeared in the first year of the pedigree culture and
which since have been repeated yearly, and have been seen to arise by
mutation [567] also during the last season (1903). Only _gigas_ appeared
but once, but then there is every reason to assume that in larger
sowings or by a prolongation of the experiments it might have made a
second appearance.
Is the number of such germs to be supposed to be limited or unlimited?
My experiment has produced about a dozen new forms. Without doubt I
could easily have succeeded in getting more, if I had had any definite
reason to search for them. But such figures are far from favoring the
assumption of indefinite mutability. The group of possible new forms is
no doubt sharply circumscribed. Partly so by the morphologic
peculiarities of _lamarckiana_, which seem to exclude red flowers,
composite leaves, etc. No doubt there are more direct reasons for these
limits, some changes having taken place initially and others later,
while the present mutations are only repetitions of previous ones, and
do not contribute new lines of development to those already existing.
This leads us to the supposition of some common original cause, which
produced a number of changes, but which itself is no longer at work, but
has left the affected qualities, and only these, in the state of
mutability.
In nature, repeated mutations must be of far greater significance than
isolated ones. How [568] great is the chance for a single individual to
be destroyed in the struggle for life? Hundreds of thousands of seeds
are produced by _lamarckiana_ annually in the field, and only some slow
increase of the number of specimens can be observed. Many seeds do not
find the proper circumstances for germination, or the young seedlings
are destroyed by lack of water, of air, or of space. Thousands of them
are so crowded when becoming rosettes that only a few succeed in
producing stems. Any weakness would have destroyed them. As a matter of
fact they are much oftener produced in the seed than seen in the field
with the usual unfavorable conditions; the careful sowing of collected
seeds has given proof of this fact many times.
The experimental proof of this frequency in the origin of new types,
seems to overcome many difficulties offered by the current theories on
the probable origin of species at large.
VI. The relation between mutability and fluctuating variability has
always been one of the chief difficulties of the followers of Darwin.
The majority assumed that species arise by the slow accumulation of
slight fluctuating deviations, and the mutations were only to be
considered as extreme fluctuations, obtained, in the main, by a
continuous selection of small differences in a constant direction.
[569] My cultures show that quite the opposite is to be regarded as
fact. All organs and all qualities of _lamarckiana_ fluctuate and vary
in a more or less evident manner, and those which I had the opportunity
of examining more closely were found to comply with the general laws of
fluctuation. But such oscillating changes have nothing in common with
the mutations. Their essential character is the heaping up of slight
deviations around a mean, and the occurrence of continuous lines of
increasing deviations, linking the extremes with this group. Nothing of
the kind is observed in the case of mutations. There is no mean for them
to be grouped around and the extreme only is to be seen, and it is
wholly unconnected with the original type. It might be supposed that on
closer inspection each mutation might be brought into connection with
some feature of the fluctuating variability. But this is not the case.
The dwarfs are not at all the extreme variants of structure, as the
fluctuation of the height of the _lamarckiana_ never decreases or even
approaches that of the dwarfs. There is always a gap. The smallest
specimens of the tall type are commonly the weakest, according to the
general rule of the relationship between nourishment and variation, but
the tallest dwarfs are of course the most robust specimens of their
group. [570] Fluctuating variability, as a rule, is subject to
reversion. The seeds of the extremes do not produce an offspring which
fluctuates around their parents as a center, but around some point on
the line which combines their attributes with the corresponding
characteristic of their ancestors, as Vilmorin has put it. No reversion
accompanies mutation, and this fact is perhaps the completest contrast
in which these two great types of variability are opposed to each other.
The offspring of my mutants are, of course, subject to the general laws
of fluctuating variability. They vary, however, around their own mean,
and this mean is simply the type of the new elementary species.