The observation of the production of mutants in the field at Hilversum,
and the subsequent cultivation of the new types in the garden at
Amsterdam, gives ample proof of the mutability of plants. Furthermore it
furnishes an analogy with the hypothetical origin of the swarms of
species of _Draba_ and _Viola_. Last but not least important it affords
material for a complete systematic and morphologic study of the newly
arisen group of forms.
The physiologic laws, however, which govern this process are only very
imperfectly revealed by such a study. The instances are too few.
Moreover the seeds from which the mutants spring, escape observation. It
is simply impossible to tell from which individual plants they have been
derived. The laevifolia and the brevistylis have been found almost every
year, the first always recurring on the same spot, the second on various
parts of the original field. It is therefore allowable to assume a
common [548] origin for all the observed individuals of either strain.
But whether, besides this, similar strains are produced anew by the old
_lamarckiana_ group, it is impossible to decide on the sole ground of
these field-observations.
The same holds good with the other novelties. Even if one of them should
germinate repeatedly, without ever opening its flowers, the possibility
could not be excluded that the seeds might have come originally from the
same capsule but lain dormant in the earth during periods of unequal
length.
Other objections might be cited that can only be met by direct and fully
controlled experiments. Next to the native locality comes the
experimental garden. Here the rule prevails that every plant must be
fertilized with pollen of its own, or with pollen of other individuals
of known and recorded origin. The visits of insects must be guarded
against, and no seeds should be saved from flowers which have been
allowed to open without this precaution. Then the seeds of each
individual must be saved and sown separately, so as to admit of an
appreciation, and if necessary, a numerical determination of the nature
of its progeny. And last but not least the experiments should be
conducted in a similar manner during a series of successive years.
[549] I have made four such experiments, each comprising the handling of
many thousands of individual plants, and lasting through five to nine
generations. At the beginning the plants were biennial, as in the native
locality, but later I learned to cultivate them in annual generations.
They have been started from different plants and seeds, introduced from
the original field into my garden at Amsterdam.
It seems sufficient to describe here one of these pedigree-cultures, as
the results of all four were similar. In the fall of 1886 I took nine
large rosettes from the field, planted them together on an isolated spot
in the garden, and harvested their seeds the next year. These nine
original plants are therefore to be considered as constituting the first
generation of my race. The second generation was sown in 1888 and
flowered in 1889. It at once yielded the expected result. 15,000
seedlings were tested and examined, and among them 10 showed diverging
characters. They were properly protected, and proved to belong to two
new types. 5 of them were _lata_ and 5 _nanella_. They flowered next
year and displayed all the characters as described in our preceding
lecture. Intermediates between them and the general type were not found,
and no indication of their appearance was noted in their parents. [550]
They came into existence at once, fully equipped, without preparation or
intermediate steps. No series of generations, no selection, no struggle
for existence was needed. It was a sudden leap into another type, a
sport in the best acceptation of the word. It fulfilled my hopes, and at
once gave proof of the possibility of the direct observation of the
origin of species, and of the experimental control thereof.
The third generation was in the main a repetition of the second. I tried
some 10,000 seedlings and found three _lata_ and three _nanella_, or
nearly the same proportion as in the first instance. But besides these a
_rubrinervis_ made its appearance and flowered the following year. This
fact at once revealed the possibility that the instability of
_lamarckiana_ might not be restricted to the three new types now under
observation. Hence the question arose how it would be possible to obtain
other types or to find them if they were present. It was necessary to
have better methods of cultivation and examination of the young plants.
Accordingly I devoted the three succeeding years to working on this
problem.
I found that it was not at all necessary to sow any larger quantities of
seed, but that the young plants must have room enough to develop into
full and free rosettes. Moreover I observed [551] that the attributes of
_lata_ and _nanella_, which I now studied in the offspring of my first
mutants, were clearly discernible in extreme youth, while those of
_rubrinervis_ remained concealed some weeks longer. Hence I concluded
that the young plants should be examined from time to time until they
proved clearly to be only normal _lamarckiana_. Individuals exhibiting
any deviation from the type, or even giving only a slight indication of
it, were forthwith taken out of the beds and planted separately, under
circumstances as favorable as possible. They were established in pots
with well-manured soil and kept under glass, but fully exposed to
sunshine. As a rule they grew very fast, and could be planted out early
in June. Some of them, of course, proved to have been erroneously taken
for mutants, but many exhibited new characters.
All in all I had 334 young plants which did not agree with the parental
type. As I examined some 14,000 seedlings altogether, the result was
estimated at about 2.5%. This proportion is much larger than in the
yields of the two first generations and illustrates the value of
improved methods. No doubt many good mutations had been overlooked in
the earlier observations.
As was to be expected, _lata_ and _nanella_ [552] were repeated in this
third generation (1895). I was sure to get nearly all of them, without
any important exceptions, as I now knew how to detect them at almost any
age. In fact, I found many of them; as many as 60 _nanella_ and 73
_lata_, or nearly 5% of each. _Rubrinervis_ also recurred, and was seen
in 8 specimens. It was much more rare than the two first-named types.
But the most curious fact in that year was the appearance of _oblonga_.
No doubt I had often seen it in former years, but had not attached any
value to the very slight differences from the type, as they then seemed
to me. I knew now that any divergence was to be esteemed as important,
and should be isolated for further observation. This showed that among
the selected specimens not less than 176, or more than 1% belonged to
the _oblonga_ type. This type was at that time quite new to me, and it
had to be kept through the winter, to obtain stems and flowers. It
proved to be as uniform as its three predecessors, and especially as
sharply contrasted with _lamarckiana_. The opportunity for the discovery
of any intermediates was as favorable as could be, because the
distinguishing marks were hardly beyond doubt at the time of the
selection and removal of the young plants. But no connecting links were
found.
[553] The same holds good for _albida_, which appeared in 15 specimens,
or in 0.1%, of the whole culture. By careful cultivation these plants
proved not to be sickly, but to belong to a new, though weak type. It
was evident that I had already seen them in former years, but having
failed to recognize them had allowed them to be destroyed at an early
age, not knowing how to protect them against adverse circumstances. Even
this time I did not succeed in getting them strong enough to keep
through the winter.
Besides these, two new types were observed, completing the range of all
that have since been recorded to regularly occur in this family. They
were _scintillans_ and _gigas_. The first was obtained in the way just
described. The other hardly escaped being destroyed, not having showed
itself early enough, and being left in the bed after the end of the
selection. But as it was necessary to keep some rosettes through the
winter in order to have biennial flowering plants to furnish seeds, I
selected in August about 30 of the most vigorous plants, planted them on
another bed and gave them sufficient room for their stems and branches
in the following summer. Most of them sent up robust shoots, but no
difference was noted till the first flowers opened. One plant had a much
larger crown of bright blossoms than any of the others. [554] As soon as
these flowers faded away, and the young fruits grew out, it became clear
that a new type was showing itself. On that indication I removed all the
already fertilized flowers and young fruits, and protected the buds from
the visits of insects. Thus the isolated flowers were fertilized with
their own pollen only, and I could rely upon the purity of the seed
saved. This lot of seeds was sown in the spring of 1897 and yielded a
uniform crop of nearly 300 young _gigas_ plants.
Having found how much depends upon the treatment, I could gradually
decrease the size of my cultures. Evidently the chance of discovering
new types would be lessened thereby, but the question as to the repeated
production of the same new forms could more easily and more clearly be
answered in this way. In the following year (1896) I sowed half as many
seeds as formerly, and the result proved quite the same. With the
exception of _gigas_ all the described forms sprang anew from the purely
fertilized ancestry of normal _lamarckiana_s. It was now the fifth
generation of my pedigree, and thus I was absolutely sure that the
descendants of the mutants of this year had been pure and without
deviation for at least four successive generations.
Owing partly to improved methods of selection, [555] partly no doubt to
chance, even more mutants were found this year than in the former. Out
of some 8,000 seedlings I counted 377 deviating ones, or nearly 5%,
which is a high proportion. Most of them were _oblonga_ and _lata_, the
same types that had constituted the majority in the former year.
_Albida_, _nanella_ and _rubrinervis_ appeared in large numbers, and
even _scintillans_, of which I had but a single plant in the previous
generation, was repeated sixfold.
New forms did not arise, and the capacity of my strain seemed exhausted.
This conclusion was strengthened by the results of the next three
generations, which were made on a much smaller scale and yielded the
same, or at least the mutants most commonly seen in previous years.
Instead of giving the figures for these last two years separately, I
will now summarize my whole experiment in the form of a pedigree. In
this the normal _lamarckiana_ was the main line, and seeds were only
sown from plants after sufficient isolation either of the plants
themselves, or in the latter years by means of paper bags enclosing the
inflorescences. I have given the number of seedlings of _lamarckiana_
which were examined each year in the table below. Of course by far the
largest number of them were [556] thrown away as soon as they showed
their differentiating characters in order to make room for the remaining
ones. At last only a few plants were left to blossom in order to
perpetuate the race. I have indicated for each generation the number of
mutants of each of the observed forms, placing them in vertical columns
underneath their respective heads. The three first generations were
biennial, but the five last annual.
PEDIGREE OF A MUTATING FAMILY
OF _OENOTHERA LAMARCKIANA_ IN THE
EXPERIMENTAL GARDEN AT AMSTERDAM
Gener: O.gig. albida obl. rubrin. Lam. nanella lata. scint.
VIII. 5 1 0 1700 21 1
VII. 9 0 3000 11
VI. 11 29 3 1800 9 5 1
V. 25 135 20 8000 49 142 6
IV. 1 15 176 8 14000 60 73 1
III. 1 10000 3 3
II. 15000 5 5
I. 9
It is most striking that the various mutations of the evening-primrose
display a great degree of regularity. There is no chaos of forms, no
indefinite varying in all degrees and in all directions. Quite on the
contrary, it is at once evident that very simple rules govern the whole
phenomenon.
I shall now attempt to deduce these laws from [557] my experiment.
Obviously they apply not only to our evening-primroses, but may be
expected to be of general validity. This is at once manifest, if we
compare the group of new mutants with the swarms of elementary forms
which compose some of the youngest systematic species, and which, as we
have seen before, are to be considered as the results of previous
mutations. The difference lies in the fact that the evening-primroses
have been seen to spring from their ancestors and that the _drabas_ have
not. Hence the conclusion that in comparing the two we must leave out
the pedigree of the evening-primroses and consider only the group of
forms as they finally show themselves. If in doing so we find sufficient
similarity, we are justified in the conclusion that the _drabas_ and
others have probably originated in the same way as the
evening-primroses. Minor points of course will differ, but the main
lines cannot have complied with wholly different laws. All so-called
swarms of elementary species obviously pertain to a single type, and
this type includes our evening-primroses as the only controlled case.
Formulating the laws of mutability for the evening-primroses we
therefore assume that they hold good for numerous other corresponding
cases.
[558] I. The first law is, that new elementary species appear suddenly,
without intermediate steps.
This is a striking point, and the one that is in the most immediate
contradiction to current scientific belief. The ordinary conception
assumes very slow changes, in fact so slow that centuries are supposed
to be required to make the differences appreciable. If this were true,
all chance of ever seeing a new species arise would be hopelessly small.
Fortunately the evening-primroses exhibit contrary tendencies. One of
the great points of pedigree-culture is the fact that the ancestors of
every mutant have been controlled and recorded. Those of the last year
have seven generations of known _lamarckiana_ parents preceding them. If
there had been any visible preparation towards the coming mutation, it
could not have escaped observation. Moreover, if visible preparation
were the rule, it could hardly go on at the same time and in the same
individuals in five or six diverging directions, producing from one
parent, _gigas_ and _nanella_, _lata_ and _rubrinervis_, _oblonga_ and
_albida_ and even _scintillans_.
On the other hand the mutants, that constitute the first representatives
of their race, exhibit all the attributes of the new type in full
display at once. No series of generations, no selection, [559] no
struggle for existence are needed to reach this end. In previous
lectures I have mentioned that I have saved the seeds of the mutants
whenever possible, and have always obtained repetitions of the prototype
only. Reversions are as absolutely lacking as is also a further
development of the new type. Even in the case of the inconstant forms,
where part of the progeny yearly return to the stature of _lamarckiana_,
intermediates are not found. So it is also with _lata_, which is
pistillate and can only be propagated by cross-fertilization. But though
the current belief would expect intermediates at least in this case,
they do not occur. I made a pedigree-culture of lata during eight
successive generations, pollinating them in different ways, and always
obtained cultures which were partly constituted of _lata_ and partly of
_lamarckiana_ specimens. But the _lata_s remained _lata_ in all the
various and most noticeable characters, never showing any tendency to
gradually revert into the original form.
Intermediate forms, if not occurring in the direct line from one species
to another, might be expected to appear perhaps on lateral branches. In
this case the mutants of one type, appearing in the same year, would not
be a pure type, but would exhibit different degrees of deviation from
the parent. The best would then have to [560] be chosen in order to get
the new type in its pure condition. Nothing of the kind, however, was
observed. All the _oblonga_-mutants were pure _oblongas_. The pedigree
shows hundreds of them in the succeeding years, but no difference was
seen and no material for selection was afforded. All were as nearly
equal as the individuals of old elementary species.