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Species and Varieties, Their Origin by Mutation

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




My second line of inquiry was an experimental repetition of the
phenomena which were only partly discerned at the native locality. It
was not my aim to intrude into the process, nor to try to bring out new
features. My only object was to submit to the precepts just given
concerning pure treatment, individual seed gathering, exclusion of
crosses and accurate recording of all the facts. The result has been a
pedigree which now permits of stating the relation between all the
descendants of my original introduced plant. This pedigree at once
exhibits the laws followed by the mutating species. The main fact is,
that it does not change itself gradually, but remains unaffected during
all succeeding generations. It only throws off new forms, which are
sharply contrasted with the parent, and which are from the very
beginning as perfect and as constant, as narrowly [29] defined and as
pure of type as might be expected of any species.

These new species are not produced once or in single individuals, but
yearly and in large numbers. The whole phenomenon conveys the idea of a
close group of mutations, all belonging to one single condition of
mutability. Of course this mutable state must have had a beginning, as
it must sometime come to an end. It is to be considered as a period
within the life-time of the species and probably it is only a small part
of it.

The detailed description of this experiment, however, I must delay to a
subsequent lecture, but I may be allowed to state, that the discovery of
this period of mutability is of a definite theoretical importance. One
of the greatest objections to the Darwinian theory of descent arose from
the length of time it would require, if all evolution was to be
explained on the theory of slow and nearly invisible changes. This
difficulty is at once met and fully surmounted by the hypothesis of
periodical but sudden and quite noticeable steps. This assumption
requires only a limited number of mutative periods, which might well
occur within the time allowed by physicists and geologists for the
existence of animal and vegetable life on the earth.

[30] Summing up the main points of these introductory remarks, I propose
to deal with the subjects mentioned above at some length, devoting to
each of them, if possible at least an entire lecture. The decisive facts
and discussions upon which the conclusions are based will be given in
every case. Likewise I hope to point out the weak places and the lacunae
in our present knowledge, and to show the way in which each of you may
try to contribute his part towards the advancement of science in this
subject. Lastly I shall try to prove that sudden mutation is the normal
way in which nature produces new species and new varieties. These
mutations are more readily accessible to observation and experiment than
the slow and gradual changes surmised by Wallace and his followers,
which are entirely beyond our present and future experience.

The theory of mutations is a starting-point for direct investigation,
while the general belief in slow changes has held back science from such
investigations during half a century.

Coming now to the subdivisions and headings under which my material is
to be presented, I propose describing first the real nature of the
elementary species and retrograde varieties, both in normal form and in
hybridizations. A discussion of other types of varieties, including [31]
monstrosities will complete the general plan. The second subdivision
will deal with the origin of species and varieties as taught by
experiment and observation, treating separately the sudden variations
which to my mind do produce new forms, and subsequently the fluctuations
which I hold to be not adequate to this purpose.


[32]
B. ELEMENTARY SPECIES

LECTURE II

ELEMENTARY SPECIES IN NATURE

What are species? Species are considered as the true units of nature by
the vast majority of biologists. They have gained this high rank in our
estimation principally through the influence of Linnaeus. They have
supplanted the genera which were the accepted units before Linnaeus.
They are now to be replaced in their turn, by smaller types, for reasons
which do not rest upon comparative studies but upon direct experimental
evidence.

Biological studies and practical interests alike make new demands upon
systematic botany. Species are not only the subject-material of herbaria
and collections, but they are living entities, and their life-history
and life-conditions command a gradually increasing interest. One phase
of the question is to determine the easiest manner to deal with the
collected forms of a country, and another feature is the problem [33] as
to what groups are real units and will remain constant and unchanged
through all the years of our observations.

Before Linnaeus, the genera were the real units of the system. De
Candolle pointed out that the old common names of plants, such as roses
and clover, poplars and oaks, nearly all refer to genera. The type of
the clovers is rich in color, and the shape of the flower-heads and the
single flowers escape ordinary observation; but notwithstanding this,
clovers are easily recognized, even if new types come to hand. White and
red clovers and many other species are distinguished simply by
adjectives, the generic name remaining the same for all.

Tournefort, who lived in the second half of the 17th century
(1656-1708), is generally considered as the author of genera in
systematic botany. He adopted, what was at that time the general
conception and applied it throughout the vegetable kingdom. He grouped
the new and the rare and the previously overlooked forms in the same
manner in which the more conspicuous plants were already arranged by
universal consent. Species were distinguished by minor marks and often
indicated by short descriptions, but they were considered of secondary
importance.

Based on the idea of a direct creation of all [34] living beings, the
genera were then accepted as the created forms. They were therefore
regarded as the real existing types, and it was generally surmised that
species and varieties owed their origin to subsequent changes under the
influence of external conditions. Even Linnaeus agreed with this view in
his first treatises and in his "Philosophical Botany" he still kept to
the idea that all genera had been created at once with the beginning of
life.

Afterwards Linnaeus changed his opinion on this important point, and
adopted species as the units of the system. He declared them to be the
created forms, and by this decree, at once reduced the genera to the
rank of artificial groups. Linnaeus was well aware that this conception
was wholly arbitrary, and that even the species are not real indivisible
entities. But he simply forbade the study of lesser subdivisions. At his
time he was quite justified in doing so, because the first task of the
systematic botanists was the clearing up of the chaos of forms and the
bringing of them into connection with their real allies.

Linnaeus himself designated the subdivisions of the species as
varieties, but in doing so he followed two clearly distinct principles.
In some cases his species were real plants, and the varieties seemed to
be derived from them by [35] some simple changes. They were subordinated
to the parent-species. In other cases his species were groups of lesser
forms of equal value, and it was not possible to discern which was the
primary and which were the derivatives.

These two methods of subdivision seem in the main, and notwithstanding
their relatively imperfect application in many single examples, to
correspond with two really distinct cases. The derivative varieties are
distinguished from the parent-species by some single, but striking mark,
and often this attribute manifests itself as the loss of some apparent
quality. The loss of spines and of hairs and the loss of blue and red
flower-colors are the most notorious, but in rarer cases many single
peculiarities may disappear, thereby constituting a variety. This
relation of varieties to the parent-species is gradually increasing in
importance in the estimation of botanists, sharply contrasting with
those cases, in which such dependency is not to be met with.

If among the subdivisions of a species, no single one can be pointed out
as playing a primary part, and the others can not be traced back to it,
the relation between these lesser units is of course of another
character. They are to be considered of equal importance. They are
distinguished from each other by more than [36] one character, often by
slight differences in nearly all their organs and qualities. Such forms
have come to be designated as "elementary species." They are only
varieties in a broad and vague systematic significance of the word, not
in the sense accorded to this term in horticultural usage, nor in a
sharper and more scientific conception.

Genera and species are, at the present time, for a large part
artificial, or stated more correctly, conventional groups. Every
systematist is free to delimit them in a wider or in a narrower sense,
according to his judgment. The greater authorities have as a rule
preferred larger genera, others of late have elevated innumerable
subgenera to the rank of genera. This would work no real harm, if
unfortunately, the names of the plants had not to be changed each time,
according to current ideas concerning genera. Quite the same inconstancy
is observed with species. In the Handbook of the British Flora, Bentham
and Hooker describe the forms of brambles under 5 species, while
Babington in his Manual of British Botany makes 45 species out of the
same material. So also in other cases. For instance, the willows which
have 13 species in one and 31 species in the other of these manuals, and
the hawkweeds for which the figures are 7 and 32 [37] respectively.
Other authors have made still greater numbers of species in the same
groups.

It is very difficult to estimate systematic differences on the ground of
comparative studies alone. All sorts of variability occur, and no
individual or small group of specimens can really be considered as a
reliable representative of the supposed type. Many original diagnoses of
new species have been founded on divergent specimens and of course, the
type can afterwards neither be derived from this individual, nor from
the diagnosis given.

This chaotic state of things has brought some botanists to the
conviction that even in systematic studies only direct experimental
evidence can be relied upon. This conception has induced them to test
the constancy of species and varieties, and to admit as real units only
such groups of individuals as prove to be uniform and constant
throughout succeeding generations. The late Alexis Jordan, of Lyons in
France, made extensive cultures in this direction. In doing so, he
discovered that systematic species, as a rule, comprise some lesser
forms, which often cannot easily be distinguished when grown in
different regions, or by comparing dried material. This fact was, of
course, most distasteful to the systematists of his time and even for a
long period afterwards [38] they attempted to discredit it. Milde and
many others have opposed these new ideas with some temporary success.
Only of late has the school of Jordan received due recognition, after
Thuret, de Bary, Rosen and others tested its practices and openly
pronounced for them. Of late Wittrock of Sweden has joined them, making
extensive experimental studies concerning the real units of some of the
larger species of his country.

From the evidence given by these eminent authorities, we may conclude
that systematic species, as they are accepted nowadays, are as a rule
compound groups. Sometimes they consist of two or three, or a few
elementary types, but in other cases they comprise twenty, or fifty, or
even hundreds of constant and well differentiated forms.

The inner constitution of these groups is however, not at all the same
in all cases. This will be seen by the description of some of the more
interesting of them. The European heartsease, from which our
garden-pansies have been chiefly derived, will serve as an example. The
garden-pansies are a hybrid race, won by crossing the _Viola tricolor_
with the large flowered and bright yellow _V. lutea_. They combine, as
everyone knows, in their wide range of [39] varieties, the attributes of
the latter with the peculiarities of the former species.

Besides the _lutea_, there are some other species, nearly allied to
tricolor, as for instance, _cornuta_, _calcarata_, and _altaica_, which
are combined with it under the head of _Melanium_ as a subgenus, and
which together constitute a systematic unity of undoubted value, but
ranging between the common conceptions of genus and species. These forms
are so nearly allied to the heartsease that they have of late been made
use of in crosses, in order to widen the range of variability of
garden-pansies.

_Viola tricolor_ is a common European weed. It is widely dispersed and
very abundant, growing in many localities in large numbers. It is an
annual and ripens its seeds freely, and if opportunity is afforded, it
multiplies rapidly.

_Viola tricolor_ has three subspecies, which have been elevated to the
rank of species by some authors, and which may here be called, for
brevity's sake, by their binary names. One is the typical _V. tricolor_,
with broad flowers, variously colored and veined with yellow, purple and
white. It occurs in waste places on sandy soil. The second is called _V.
arvensis_ or the field-pansy; it has small inconspicuous flowers, with
pale-yellowish petals which are shorter than the sepals. It pollinates
itself without the [40] aid of insects, and is widely dispersed in
cultivated fields. The third form, _V. alpestris_, grows in the Alps,
but is of lesser importance for our present discussion.

Anywhere throughout the central part of Europe _V. tricolor_ and _V.
arvensis_ may be seen, each occupying its own locality. They may be
considered as ranging among the most common native plants of the
particular regions they inhabit. They vary in the color of the flowers,
branching of the stems, in the foliage and other parts, but not to such
an extent as to constitute distinct strains. They have been brought into
cultivation by Jordan, Wittrock and others, but throughout Europe each
of them constitutes a single type.

These types must be very old and constant, fluctuating always within the
same distinct and narrow limits. No slow, gradual changes can have taken
place. In different countries their various habitats are as old as the
historical records, and probably many centuries older. They are quite
independent of one another, the distance being in numerous cases far too
great for the exchange of pollen or of seeds. If slow and gradual
changes were the rule, the types could not have remained so uniform
throughout the whole range of these two species. They would necessarily
have split up into thousands [41] and thousands of minor races, which
would show their peculiar characteristics if tested by cultures in
adjacent beds. This however, is not what happens. As a matter of fact
_V. tricolor_ and _V. arvensis_ are widely distributed but wholly
constant types.

Besides these, there occur distinct types in numerous localities. Some
of them evidently have had time and opportunity to spread more or less
widely and now occupy larger regions or even whole countries. Others are
narrowly limited, being restricted to a single locality. Wittrock
collected seeds or plants from as many localities as possible in
different parts of Sweden and neighboring states and sowed them in his
garden near Stockholm. He secured seeds from his plants, and grew from
them a second, and in many cases a third generation in order to estimate
the amount of variability. As a rule the forms introduced into his
garden proved constant, notwithstanding the new and abnormal conditions
under which they were propagated.

First of all we may mention three perennial forms called by him _Viola
tricolor ammotropha_, _V. tricolor coniophila_ and _V. stenochila_. The
typical _V. tricolor_ is an annual plant; sowing itself in summer and
germinating soon afterwards. The young plants thrive throughout [42] the
latter part of the summer and during the fall, reaching an advanced
stage of development of the branched stems before winter. Early in the
spring the flowers begin to open, but after the ripening of the seeds
the whole plant dies.

The three perennial species just mentioned develop in the same manner in
the first year. During their flowering period, however, and afterwards,
they produce new shoots from the lower parts of the stem. They prefer
dry and sandy soils, often becoming covered with the sand that is blown
on them by the winds. They are prepared for such seemingly adverse
circumstances by the accumulation of food in the older stems and by the
capacity of the new shoots to thrive on this food till they have become
long enough to reach the light. _V. tricolor ammotropha_ is native near
Ystad in Sweden, and the other two forms on Gotland. All three have
narrowly limited habitats.

The typical tricolored heartsease has remained annual in all its other
subspecies. It may be divided into two types in the first place, _V.
tricolor genuina_ and _V. tricolor versicolor_. Both of them have a wide
distribution and seem to be the prototypes from which the rarer forms
must have been derived. Among these latter Wittrock describes seven
local types, which [43] proved to be constant in his pedigree-cultures.
Some of them have produced other forms, related to them in the way of
varieties. They all have nearly the same general habit and do not
exhibit any marked differences in their growth, in the structure and
branching of the stems, or in the character of their foliage.
Differentiating points are to be found mainly in the colors and patterns
of the flowers. The veins, which radiate from the centre of the corolla
are branched in some and undivided in others; in one elementary species
they are wholly lacking. The purple color may be absent, leaving the
flowers of a pale or a deep yellow. Or the purple may be reddish or
bluish. Of the petals all five may have the purple hue on their tips, or
this attribute may be limited to the two upper ones. Contrasting with
this wide variability is the stability of the yellow spot in the centre,
which is always present and becomes inconspicuous only, when the whole
petals are of the same hue. It is a general conception that colors and
color-markings are liable to great variability and do not constitute
reliable standards. But the cultures of Wittrock have proved the
contrary, at least in the case of the violets. No pattern, however
quaint, appears changeable, if one elementary species only is
considered. Hundreds of plants from seeds [44] from one locality may be
grown, and all will exhibit exactly the same markings. Most of these
forms are of very local occurrence. The most beautiful of all, the
_ornatissima_, is found only in Jemtland, the _aurobadia_ only in
Sodermanland, the anopetala_ in other localities in the same country,
the _roseola_ near Stockholm, and the yellow _lutescens_ in Finmarken.

The researches of Wittrock included only a small number of elementary
species, but every one who has observed the violets in the central parts
of Europe must be convinced that many dozens of constant forms of the
typical _Viola tricolor_ might easily be found and isolated.

We now come to the field pansy, the _Viola arvensis_, a very common weed
in the grain-fields of central Europe. I have already mentioned its
small corolla, surpassed by the lobes of the calyx and its capacity of
self-fertilization. It has still other curious differentiating
characters; the pollen grains, which are square in _V. tricolor_, are
five-sided in _V. arvensis_. Some transgressive fluctuating variability
may occur in both cases through the admixture of pollen-grains. Even
three-angled pollen grains are seen sometimes. Other marks are observed
in the form of the anthers and the spur.

There seem to be very many local subspecies [45] of the field-pansy.
Jordan has described some from the vicinity of Lyons, and Wittrock
others from the northern parts of Europe. They diverge from their common
prototype in nearly all attributes, the flowers not showing the
essential differentiating characters as in the _V. tricolor_. Some have
their flower-stalks erect, and in others the flowers are held nearly at
right angles to the stem. _V. pallescens_ is a small, almost unbranched
species with small pale flowers. _V. segetalis_ is a stouter species
with two dark blue spots on the tips of the upper petals. _V. agrestis_
is a tall and branched, hairy form. _V. nemausensis_ attains a height of
only 10 cm., has rounded leaves and long flower-stalks. Even the seeds
afford characters which may be made use of in isolating the various
species.

The above-mentioned elementary forms belong to the flora of southern
France, and Wittrock has isolated and cultivated a number of others from
the fields of Sweden. A species from Stockholm is called _Viola patens_;
_V. arvensis curtisepala_ occurs in Gotland, and _V. arvensis striolata_
is a distinct form, which has appeared in his cultures without its true
origin being ascertained.

The alpine violets comprise a more widespread type with some local
elementary species [46] derived exactly in the same way as the
tricolored field pansies.

Summarizing the general result of this description we see that the
original species _Viola tricolor_ may be split up into larger and lesser
groups of separate forms. These last prove to be constant in
pedigree-cultures, and therefore are to be considered as really existent
units. They are very numerous, comprising many dozens in each of the two
larger subdivisions.

All systematic grouping of these forms, and their combination into
subspecies and species rests on the comparative study of their
characters. The result of such studies must necessarily depend on
principles which underlie them. According to the choice of these
principles, the construction of the groups will be found to be
different. Wittrock trusts in the first place to morphologic characters,
and considers the development as passing from the more simple to the
more complex types. On the other hand the geographic distribution may be
considered as an indication of the direction of evolution, the
wide-spread forms being regarded as the common parents of the minor
local species.

However, such considerations are only of secondary importance. It must
be borne in mind that an ordinary systematic species may include [47]
many dozens of elementary forms, each of which remains constant and
unchanged in successive generations, even if cultivated in the same
garden and under similar external conditions.

Leaving the violets, we may take the vernal whitlow-grass or _Draba
verna_ for a second illustration. This little annual cruciferous plant
is common in the fields of many parts of the United States, though
originally introduced from Europe. It has small basal rosettes which
develop during summer and winter, and produce numerous leafless
flowering stems early in the spring. It is a native of central Europe
and western Asia, and may be considered as one of the most common
plants, occurring anywhere in immense numbers on sandy soils. Jordan was
the first to point out that it is not the same throughout its entire
range. Although a hasty survey does not reveal differences, they show
themselves on closer inspection. De Bary, Thuret, Rosen and many others
confirmed this result, and repeated the pedigree-cultures of Jordan.
Every type is constant and remains unchanged in successive generations.
The anthers open in the flower-buds and pollinate the stigmas before the
expansion of the flowers, thus assuring self-fertilization. Moreover,
these inconspicuous little flowers are only sparingly visited by
insects. Dozens of subspecies [48] may be cultivated in the same garden
without any real danger of their intercrossing. They remain as pure as
under perfect isolation.

It is very interesting to observe the aspect of such types, when growing
near each other. Hundreds of rosettes exhibit one type, and are
undoubtedly similar. The alternative group is distinguishable at first
sight, though the differentiating marks are often so slight as to be
traceable with difficulty. Two elementary species occur in Holland, one
with narrow leaves in the western provinces and one with broader foliage
in the northern parts. I have cultivated them side by side, and was as
much struck with the uniformity within each group, as with the contrast
between the two sets.

Nearly all organs show differences. The most marked are those of the
leaves, which may be small or large, linear or elliptic or oblong and
even rhomboidal in shape, more or less hairy with simple or with
stellate branched hairs, and finally of a pure green or of a glaucous
color. The petals are as a rule obcordate, but this type may be combined
with others having more or less broad emarginations at the summit, and
with differences in breadth which vary from almost linear types to
others which touch along their margins. The pods are short and broad, or
long and narrow, or varying in sundry other [49] ways. All in all there
are constant differences which are so great that it has been possible to
distinguish and to describe large numbers of types.

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