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Species and Varieties, Their Origin by Mutation

H >> Hugo DeVries >> Species and Varieties, Their Origin by Mutation




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.


II. New forms spring laterally from the main stem.

The current conception concerning the origin of species assumes that
species are slowly converted into others. The conversion is assumed to
affect all the individuals in the same direction and in the same degree.
The whole group changes its character, acquiring new attributes. By
inter-crossing they maintain a common line of progress, one individual
never being able to proceed much ahead of the others.

The birth of the new species necessarily seemed to involve the death of
the old one. This last conclusion, however, is hard to understand. It
may be justifiable to assume that all the individuals of one locality
are ordinarily intercrossed, and are moreover subjected to the same
external conditions. They might be supposed to vary in the same
direction if these conditions were changed slowly. But this could of
course have no possible influence on the plants of the [561] same
species growing in distant localities, and it would be improbable they
should be affected in the same way. Hence we should conclude that when a
species is converted into a new type in one locality this is only to be
considered as one of numerous possible ones, and its alteration would
not in the least change the aspect of the remainder of the species.

But even with this restriction the general belief is not supported by
the evidence of the evening-primroses. There is neither a slow nor a
sudden change of all the individuals. On the contrary, the vast majority
remain unchanged; thousands are seen exactly repeating the original
prototype yearly, both in the native field and in my garden. There is no
danger that _lamarckiana_ might die out from the act of mutating, nor
that the mutating strain itself would be exposed to ultimate destruction
from this cause.

In older swarms, such as _Draba_ or _Helianthemum_, no such center,
around which the various forms are grouped, is known. Are we to conclude
therefore that the main strain has died out? Or is it perhaps concealed
among the throng, being distinguished by no peculiar character? If our
_gigas_ and _rubrinervis_ were growing in equal numbers with the
_lamarckiana_ in the native field, would it be possible to decide [562]
which of them was the progenitor of the others? Of course this could be
done by long and tedious crossing experiments, showing atavism in the
progeny, and thereby indicating the common ancestor. But even this
capacity seems to be doubtful and connected only with the state of
mutability and to be lost afterwards. Therefore if this period of
mutation were ended, probably there would be no way to decide concerning
the mutual relationship of the single species.

Hence the lack of a recognizable main stem in swarms of elementary
species makes it impossible to answer the question concerning their
common origin.

Another phase of the opposition between the prevailing view and my own
results seems far more important. According to the current belief the
conversion of a group of plants growing in any locality and flowering
simultaneously would be restricted to one type. In my own experiments
several new species arose from the parental form at once, giving a wide
range of new forms at the same time and under the same conditions.


III. New elementary species attain their full constancy at once.

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.


V. The same new species are produced in a large number of individuals.

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.

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