Clarence Day Jr. - "This Simian World"

Harold Howland - "Theodore Roosevelt and His Times,"

William Shakespeare - "Measure for Measure"

William Shakespeare - "The life and death of King Richard the Second"

Species and Varieties, Their Origin by Mutation

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




Of course we are far from a decision between these views, on the sole
ground of the facts as known at present. Mutations under observation are
as yet very rare; enough to indicate the possible and most probable
ways, but no more. On the other hand the accumulation of fluctuations
does not transgress relatively narrow [9] limits as far as the present
methods of selection go. But the question remains to be solved, whether
our methods are truly the right ones, and whether by the use of new
principles, new results might not cause the balance of opinion to favor
the opposite side.

Of late, a thorough and detailed discussion of the opposing views has
been given by Morgan in his valuable book on evolution and adaptation.
He has subjected all the proposed theories to a severe criticism both on
the ground of facts and on that of their innate possibility and logical
value. He decides in favor of the mutation theory. His arguments are
incisive and complete and wholly adapted to the comprehension of all
intelligent readers, so that his book relieves me entirely of the
necessity of discussing these general questions, as it could not be done
in a better or in a clearer way.

I intend to give a review of the facts obtained from plants which go to
prove the assertion, that species and varieties have originated by
mutation, and are, at present, not known to originate in any other way.
This review consists of two parts. One is a critical survey of the facts
of agricultural and horticultural breeding, as they have accumulated
since the time of Darwin. This body of evidence is to be combined with
some corresponding experiments [10] concerning the real nature of
species in the wild state. The other part rests on my own observations
and experiments, made in the botanical garden of the University of
Amsterdam.

For many years past I have tried to elucidate the hereditary conditions
of species and varieties, and the occasional occurrence of mutations,
that suddenly produce new forms.

The present discussion has a double purpose. On one side it will give
the justification of the theory of mutations, as derived from the facts
now at hand. On the other hand it will point out the deficiencies of
available evidence, and indicate the ways by which the lacunae may
gradually be filled. Experimental work on heredity does not require vast
installments or costly laboratory equipment. It demands chiefly
assiduity and exactitude. Any one who has these two qualities, and who
has a small garden at his disposal is requested to take part in this
line of investigation.

In order to observe directly the birth of new forms it is necessary, in
the first place, to be fully clear concerning the question as to what
forms are to be expected to arise from others, and before proceeding to
a demonstration of the origin of species, it is pertinent to raise the
question as to what constitutes a species.

Species is a word, which always has had a [11] double meaning. One is
the systematic species, which is the unit of our system. But these units
are by no means indivisible. Long ago Linnaeus knew them to be compound
in a great number of instances, and increasing knowledge has shown that
the same rule prevails in other instances. Today the vast majority of
the old systematic species are known to consist of minor units. These
minor entities are called varieties in systematic works. However, there
are many objections to this usage. First, the term variety is applied in
horticulture and agriculture to things so widely divergent as to convey
no clear idea at all. Secondly, the subdivisions of species are by no
means all of the same nature, and the systematic varieties include units
the real value of which is widely different in different cases. Some of
these varieties are in reality as good as species, and have been
"elevated," as it is called by some writers, to this rank. This
conception of the elementary species would be quite justifiable, and
would at once get rid of all difficulties, were it not for one practical
obstacle. The number of the species in all genera would be doubled and
tripled, and as these numbers are already cumbersome in many cases, the
distinction of the native species of any given country would lose most
of its charm and interest.

[12] In order to meet this difficulty we must recognize two sorts of
species. The systematic species are the practical units of the
systematists and florists, and all friends of wild nature should do
their utmost to preserve them as Linnaeus has proposed them. These units
however, are not really existing entities; they have as little claim to
be regarded as such as genera and families. The real units are the
elementary species; their limits often apparently overlap and can only
in rare cases be determined on the sole ground of field observations.
Pedigree-culture is the method required and any form which remains
constant and distinct from its allies in the garden is to be considered
as an elementary species.

In the following lectures we shall consider this point at length, to
show the compound nature of systematic species in wild and in cultivated
plants. In both cases, the principle is becoming of great importance,
and many papers published recently indicate its almost universal
acceptation.

Among the systematic subdivisions of species, not all have the same
claim to the title of elementary species. In the first place the cases
in which the differences may occur between parts of the same individual
are to be excluded. Dividing an alpine plant into two halves and [13]
planting one in a garden, varietal differences at once arise and are
often designated in systematic works under different varietal names.
Secondly all individual differences which are of a fluctuating nature
are to be combined into a group. But with these we shall deal later.

Apart from these minor points the subdivisions of the systematic species
exhibit two widely different features. I will now try to make this clear
in a few words, but will return in another lecture to a fuller
discussion of this most interesting contrast.

Linnaeus himself knew that in some cases all subdivisions of a species
are of equal rank, together constituting the group called species. No
one of them outranks the others; it is not a species with varieties, but
a group, consisting only of varieties. A closer inquiry into the cases
treated in this manner by the great master of systematic science, shows
that here his varieties were exactly what we now call elementary
species.

In other cases the varieties are of a derivative nature. The species
constitutes a type that is pure in a race which ordinarily is still
growing somewhere, though in some cases it may have died out. From this
type the varieties are derived, and the way of this derivation is
usually quite manifest to the botanist. It is ordinarily [14] by the
disappearance of some superficial character that a variety is
distinguished from its species, as by the lack of color in the flowers,
of hairs on stems and foliage, of the spines and thorns, &c. Such
varieties are, strictly speaking, not to be treated in the same way as
elementary species, though they often are. We shall designate them by
the term of "retrograde varieties," which clearly indicates the nature
of their relationship to the species from which they are assumed to have
sprung. In order to lay more stress on the contrast between elementary
species and retrograde varieties, it should be stated at once, that the
first are considered to have originated from their parent-form in a
progressive way. They have succeeded in attaining something quite new
for themselves, while retrograde varieties have only thrown off some
peculiarity, previously acquired by their ancestors.

The whole vegetable kingdom exhibits a constant struggle between
progression and retrogression. Of course, the great lines of the general
pedigree are due to progression, many single steps in this direction
leading together to the great superiority of the flowering plants over
their cryptogamous ancestors. But progression is nearly always
accompanied by retrogression in the principal lines of evolution, [15]
as well as in the collateral branches of the genealogical tree.
Sometimes it prevails, and the monocotyledons are obviously a reduced
branch of the primitive dicotyledons. In orchids and aroids, in grasses
and sedges, reduction plays a most important part, leaving its traces on
the flowers as well as on the embryo of the seed. Many instances could
be given to prove that progression and retrogression are the two main
principles of evolution at large. Hence the conclusion, that our
analysis must dissect the complicated phenomena of evolution so far as
to show the separate functions of these two contrasting principles.
Hundreds of steps were needed to evolve the family of the orchids, but
the experimenter must take the single steps for the object of his
inquiry. He finds that some are progressive and others retrogressive and
so his investigation falls under two heads, the origin of progressive
characters, and the subsequent loss of the same. Progressive steps are
the marks of elementary species, while retrograde varieties are
distinguished by apparent losses. They have equal claim to our interest
and our study.

As already stated I propose to deal first with the elementary species
and afterwards with the retrograde varieties. I shall try to depict them
to you in the first place as they are seen in [16] nature and in
culture, leaving the question of their origin to a subsequent
experimental treatment.

The question of the experimental origin of new species and varieties has
to be taken up from two widely separated starting points. This may be
inferred from what we have already seen concerning the two opposing
theories, derived and isolated from Darwin's original broad conception.
One of them considers mutations as the origin of new forms, while the
other assumes fluctuations to be the source of all evolution.

As mentioned above, my own experience has led me to accept the first
view. Therefore I shall have to show that mutations do yield new and
constant forms, while fluctuations are not adequate to do so. Retrograde
varieties and elementary species may both be seen to be produced by
sudden mutations. Varieties have often been observed to appear at once
and quite unexpectedly in horticulture and agriculture, and a survey of
these historical facts will be the subject of one of my lectures. In
some instances I have succeeded in repeating these observations in my
garden under the strict conditions of a scientific experiment, and these
instances teach us the real nature of the process of mutation in all its
visible features. New elementary [17] species are far more rare, but I
have discovered in the great evening-primrose, or _Oenothera
lamarckiana_ a strain which is producing them yearly in the wild state
as well as in my garden. These observations and pedigree-experiments
will be dealt with at due length in subsequent lectures.

Having proved the existence and importance of mutations, it remains to
inquire how far the improvements may go which are due only to
fluctuating variability. As the term indicates, this variability is
fluctuating to and fro, oscillating around an average type. It never
fails nor does it, under ordinary circumstances, depart far from the
fixed average.

But the deviation may be enlarged by a choice of extremes. In sowing
their seed, the average of the strain is seen to be changed, and in
repeating the experiment the change may be considerable. It is not
clear, whether theoretically by such an accumulation, deviations might
be reached which could not be attained at once in a single sowing. This
question is hardly susceptible of an experimental answer, as it would
require such an enormous amount of seed from a few mother plants as can
scarcely ever be produced.

The whole character of the fluctuations shows them to be of an opposite
nature, contrasting [18] manifestly with specific and varietal
characters. By this method they may be proved to be inadequate ever to
make a single step along the great lines of evolution, in regard to
progressive as well as to retrograde development.

First of all fluctuations are linear, amplifying or lessening the
existing qualities, but not really changing their nature. They are not
observed to produce anything quite new, and evolution of course, is not
restricted to the increase of the already existing peculiarities, but
depends chiefly on the continuous addition of new characters to the
stock. Fluctuations always oscillate around an average, and if removed
from this for some time, they show a tendency to return to it. This
tendency, called retrogression, has never been observed to fail, as it
should, in order to free the new strain from the links with the average,
while new species and new varieties are seen to be quite free from their
ancestors and not linked to them by intermediates.

The last few lectures will be devoted to questions concerning the great
problem of the analogy between natural and artificial selection. As
already stated, Darwin made this analogy the foundation stone of his
theory of descent, and he met with the severest objections and
criticisms precisely on this point. But I hope to [19] show that he was
quite right, and that the cause of the divergence of opinions is due
simply to the very incomplete state of knowledge concerning both
processes. If both are critically analyzed they may be seen to comprise
the same factors, and further discussion may be limited to the
appreciation of the part which each of them has played in nature and
among cultivated plants.

Both natural and artificial selection are partly specific, and partly
intra-specific or individual. Nature of course, and intelligent men
first chose the best elementary species from among the swarms. In
cultivation this is the process of variety-testing. In nature it is the
survival of the fittest species, or, as Morgan designates it, the
survival of species in the struggle for existence. The species are not
changed by this struggle, they are only weighed against each other, the
weak being thrown aside.

Within the chosen elementary species there is also a struggle. It is
obvious, that the fluctuating variability adapts some to the given
circumstances, while it lessens the chances of others. A choice results,
and this choice is what is often exclusively called selection, either
natural or artificial. In cultivation it produces the improved and the
local races; in nature little is known about improvement in this way,
but [19] local adaptations with slight changes of the average character
in separate localities, seem to be of quite normal occurrence.

A new method of individual selection has been used in recent years in
America, especially by W.M. Hays. It consists in judging the hereditary
worth of a plant by the average condition of its offspring, instead of
by its own visible characters. If this determination of the "centgener
power," as Hays calls it, should prove to be the true principle of
selection, then indeed the analogy between natural and artificial
selection would lose a large part of its importance. We will reserve
this question for the last lecture, as it pertains more to the future,
than to our present stock of knowledge.

Something should be said here concerning hybrids and hybridism. This
problem has of late reached such large proportions that it cannot be
dealt with adequately in a short survey of the phenomena of heredity in
general. It requires a separate treatment. For this reason I shall limit
myself to a single phase of the problem, which seems to be indispensable
for a true and at the same time easy distinction between elementary
species and retrograde varieties. According to accepted terminology,
some crosses are to be considered as unsymmetrical, while others are
symmetrical. The first are one-sided, [21] some peculiarity being found
in one of the parents and lacking in the other. The second are balanced,
as all the characters are present in both parents, but are found in a
different condition. Active in one of them, they are concealed or
inactive in the other. Hence pairs of contrasting units result, while in
unbalanced crosses no pairing of the particular character under
consideration is possible. This leads to the principal difference
between species and varieties, and to an experimental method of deciding
between them in difficult and doubtful cases.

Having thus indicated the general outlines of the subjects I shall deal
with, something now may be said as to methods of investigation.

There are two points in which scientific investigation differs from
ordinary pedigree-culture in practice. First the isolation of the
individuals and the study of individual inheritance, instead of
averages. Next comes the task of keeping records. Every individual must
be entered, its ancestry must be known as completely as possible, and
all its relations must be noted in such a form, that the most complete
reference is always possible. Mutations may come unexpectedly, and when
once arisen, their parents and grand-parents should be known. Records
must be available which will allow of a most complete knowledge of the
whole ancestral [22] line. This, and approximately this only, is the
essential difference between experimental and accidental observation.

Mutations are occurring from time to time in the wild state as well as
in horticulture and agriculture. A selection of the most interesting
instances will be given later. But in all such cases the experimental
proof is wanting. The observations as a rule, only began when the
mutation had made its appearance. A more or less vague remembrance about
the previous state of the plants in question might be available, though
even this is generally absent. But on doubtful points, concerning
possible crosses or possible introduction of foreign strains, mere
recollection is insufficient. The fact of the mutation may be very
probable, but the full proof is, of course, wanting. Such is the case
with the mutative origin of _Xanthium commune_ Wootoni from New Mexico
and of _Oenothera biennis cruciata_ from Holland. The same doubt exists
as to the origin of the _Capsella heegeri_ of Solms-Laubach, and of the
oldest recorded mutation, that of _Chelidonium laciniatum_ in Heidelberg
about 1600.

First, we have doubts about the fact itself. These, however, gradually
lose their importance in the increasing accumulation of evidence.
Secondly, the impossibility of a closer [23] inquiry into the real
nature of the change. For experimental purposes a single mutation does
not suffice; it must be studied repeatedly, and be produced more or less
arbitrarily, according to the nature of the problems to be solved. And
in order to do this, it is evidently not enough to have in hand the
mutated individual, but it is indispensable to have also the mutable
parents, or the mutable strain from which it sprang.

All conditions previous to the mutation are to be considered as of far
higher importance than all those subsequent to it.

Now mutations come unexpectedly, and if the ancestry of an accidental
mutation is to be known, it is of course necessary to keep accounts of
all the strains cultivated. It is evident that the required knowledge
concerning the ancestry of a supposed mutation, must necessarily nearly
all be acquired from the plants in the experimental garden.

Obviously this rule is as simple in theory, as it is difficult to carry
out in practice. First of all comes the book-keeping. The parents,
grandparents and previous ancestors must be known individually. Accounts
of them must be kept under two headings. A full description of their
individual character and peculiarities must always be available on the
one hand, and on the other, all facts concerning their hereditary [24]
qualities. These are to be deduced from the composition of the progeny,
and in order to obtain complete evidence on this point, two successive
generations are often required. The investigation must ascertain the
average condition of this offspring and the occurrence of any deviating
specimens, and for both purposes it is necessary to cultivate them in
relatively large numbers. It is obvious that, properly speaking, the
whole family of a mutated individual, including all its nearer and more
remote relatives, should be known and recorded.

Hence pedigree-book-keeping must become the general rule. Subordinate to
this are two further points, which should likewise be stated here. One
pertains to the pure or hybrid nature of the original strain, and the
other to the life-conditions and all other external influences. It is
manifest that a complete understanding of a mutation depends upon full
information upon these points.

All experiments must have a beginning. The starting-point may be a
single individual, or a small group of plants, or a lot of seeds. In
many cases the whole previous history is obscure, but sometimes a little
historical evidence is at hand. Often it is evident that the initial
material belongs to a pure species, but with respect to the question of
elementary species it is [25] not rarely open to doubt. Large numbers of
hybrid plants and hybrid races are in existence, concerning the origin
of which it is impossible to decide. It is impossible in many instances
to ascertain whether they are of hybrid or of pure origin. Often there
is only one way of determining the matter; it is to guess at the
probable parents in case of a cross and to repeat the cross. This is a
point which always requires great care in the interpretation of unusual
facts.

Three cases are to be distinguished as to heredity. Many plants are so
constituted as to be fertilized with their own pollen. In this case the
visits of insects have simply to be excluded, which may be done by
covering plants with iron gauze or with bags of prepared paper.
Sometimes they fertilize themselves without any aid, as for instance,
the common evening-primrose; in other cases the pollen has to be placed
on the stigma artificially, as with Lamarck's evening-primrose and its
derivatives. Other plants need cross-fertilization in order to produce a
normal yield of seeds. Here two individuals have always to be combined,
and the pedigree becomes a more complicated one. Such is the case with
the toad-flax, which is nearly sterile with its own pollen. But even in
these cases the visits of insects bringing pollen [26] from other
plants, must be carefully excluded. A special lecture will be devoted to
this very interesting source of impurity and of uncertainty in ordinary
cultures.

Of course, crosses may lie in the proposed line of work, and this is the
third point to be alluded to. They must be surrounded with the same
careful isolation and protection against bees, as any other
fertilizations. And not only the seed-parent, but also the pollen must
be kept pure from all possible foreign admixtures.

A pure and accurately recorded ancestry is thus to be considered as the
most important condition of success in experimental plant breeding. Next
to this comes the gathering of the seeds of each individual separately.
Fifty or sixty, and often more, bags of seeds are by no means uncommon
for a single experiment, and in ordinary years the harvest of my garden
is preserved in over a thousand separate lots.

Complying with these conditions, the origin of species may be seen as
easily as any other phenomenon. It is only necessary to have a plant in
a mutable condition. Not all species are in such a state at present, and
therefore I have begun by ascertaining which were stable and which were
not. These attempts, of course, had to be made in the experimental
garden, and large quantities of seed had to be procured and [27] sown.
Cultivated plants of course, had only a small chance to exhibit new
qualities, as they have been so strictly controlled during so many
years. Moreover their purity of origin is in many cases doubtful. Among
wild plants only those could be expected to reward the investigator
which were of easy cultivation. For this reason I have limited myself to
the trial of wild plants of Holland, and have had the good fortune to
find among them at least one species in a state of mutability. It was
not really a native plant, but one that had been introduced from America
and belongs to an American genus. I refer to the great evening-primrose
or the evening-primrose of Lamarck. A strain of this beautiful species
is growing in an abandoned field in the vicinity of Hilversum, at a
short distance from Amsterdam. Here it has escaped from a park and
multiplied. In doing so it has produced and is still producing quite a
number of new types, some of which may be considered as retrograde
varieties, while others evidently are of the nature of progressive
elementary species.

This interesting plant has afforded me the means of observing directly
how new species originate, and of studying the laws of these changes. My
researches have followed a double line of inquiry. On one side, I have
limited [28] myself to direct field observations, and to tests of seed,
collected from the wild plants in their native locality. Obviously the
mutations are decided within the seed, and the culture of young plants
from them had no other aim than that of ascertaining what had occurred
in the field. And then the many chances of destruction that threaten
young plants in a wild state, could be avoided in the garden, where
environmental factors can be controlled.

Copyright (c) 2007. topbookz.net. All rights reserved.