The study of the physiology of form-development in a pure species has
already yielded results and makes slow but sure progress. The physiology
of the possibility of the transformation of one species into another is
based, as yet, rather on pious hope than on accomplished fact. From
the first it appeared to be hopeless to investigate physiologically
the origin of Linnean species and at the same time that of the natural
system, an aim which Darwin had before him in his enduring work. The
historical sequence of events, of which an organism is the expression,
can only be treated hypothetically with the help of facts supplied
by comparative morphology, the history of development, geographical
distribution, and palaeontology. (See Lotsy, "Vorlesungen" (Jena, I.
1906, II. 1908), for summary of the facts.) A glance at the controversy
which is going on today in regard to different hypotheses shows that
the same material may lead different investigators to form entirely
different opinions. Our ultimate aim is to find a solution of the
problem as to the cause of the origin of species. Indeed such
attempts are now being made: they are justified by the fact that under
cultivation new and permanent strains are produced; the fundamental
importance of this was first grasped by Darwin. New points of view in
regard to these lines of inquiry have been adopted by H. de Vries
who has succeeded in obtaining from Oenothera Lamarckiana a number of
constant "elementary" species. Even if it is demonstrated that he was
simply dealing with the complex splitting up of a hybrid (Bateson,
"Reports to the Evolution Committee of the Royal Society", London, 1902;
cf. also Lotsy, "Vorlesungen", Vol. I. page 234.), the facts adduced in
no sense lose their very great value.
We must look at the problem in its simplest form; we find it in every
case where a new race differs essentially from the original type in a
single character only; for example, in the colour of the flowers or in
the petalody of the stamens (doubling of flowers). In this connection
we must keep in view the fact that every visible character in a plant is
the resultant of the cooperation of specific structure, with its various
potentialities, and the influence of the environment. We know, that in
a pure species all characters vary, that a blue-flowering Campanula or
a red Sempervivum can be converted by experiment into white-flowering
forms, that a transformation of stamens into petals may be caused by
fungi or by the influence of changed conditions of nutrition, or
that plants in dry and poor soil become dwarfed. But so far as the
experiments justify a conclusion, it would appear that such alterations
are not inherited by the offspring. Like all other variations they
appear only so long as special conditions prevail in the surroundings.
It has been shown that the case is quite different as regards the
white-flowering, double or dwarf races, because these retain their
characters when cultivated under practically identical conditions, and
side by side with the blue, single-flowering or tall races. The problem
may therefore be stated thus: how can a character, which appears in the
one case only under the strictly limited conditions of the experiment,
in other cases become apparent under the very much wider conditions of
ordinary cultivation? If a character appears, in these circumstances,
in the case of all individuals, we then speak of constant races. In such
simple cases the essential point is not the creation of a new character
but rather an ALTERATION OF THIS CHARACTER IN ACCORDANCE WITH THE
ENVIRONMENT. In the examples mentioned the modified character in the
simple varieties (or a number of characters in elementary species)
appears more or less suddenly and is constant in the above sense. The
result is what de Vries has termed a Mutation. In this connection we
must bear in mind the fact that no difference, recognisable externally,
need exist between individual variation and mutation. Even the most
minute quantitative difference between two plants may be of specific
value if it is preserved under similar external conditions during many
successive generations. We do not know how this happens. We may state
the problem in other terms; by saying that the specific structure must
be altered. It is possible, to some extent, to explain this sudden
alteration, if we regard it as a chemical alteration of structure either
in the specific qualities of the proteids or of the unknown carriers
of life. In the case of many organic compounds their morphological
characters (the physical condition, crystalline form, etc.) are at once
changed by alteration of atomic relations or by incorporation of new
radicals. (For instance ethylchloride (C2H5Cl) is a gas at 21 deg
C., ethylenechloride (C2H4Cl2) a fluid boiling at 84 deg C., beta
trichlorethane (C2H3Cl3) a fluid boiling at 113 deg C.,
perchlorethane (C2Cl6) a crystalline substance. Klebs, ("Willkurliche
Entwickelungsanderungen" page 158.) Much more important, however, would
be an answer to the question, whether an individual variation can be
converted experimentally into an inherited character--a mutation in de
Vries's sense.
In all circumstances we may recognise as a guiding principle the
assumption adopted by Lamarck, Darwin, and many others, that the
inheritance of any one character, or in more general terms, the
transformation of one species into another, is, in the last instance,
to be referred to a change in the environment. From a causal-mechanical
point of view it is not a priori conceivable that one species can
ever become changed into another so long as external conditions remain
constant. The inner structure of a species must be essentially altered
by external influences. Two methods of experimental research may be
adopted, the effect of crossing distinct species and, secondly, the
effect of definite factors of the environment.
The subject of hybridisation is dealt with in another part of this
essay. It is enough to refer here to the most important fact, that as
the result of combinations of characters of different species new and
constant forms are produced. Further, Tschermack, Bateson and others
have demonstrated the possibility that hitherto unknown inheritable
characters may be produced by hybridisation.
The other method of producing constant races by the influence of special
external conditions has often been employed. The sporeless races of
Bacteria and Yeasts (Cf. Detto, "Die Theorie der direkten Anpassung... ",
pages 98 et seq., Jena, 1904; see also Lotsy, "Vorlesungen", II. pages
636 et seq., where other similar cases are described.) are well known,
in which an internal alteration of the cells is induced by the influence
of poison or higher temperature, so that the power of producing spores
even under normal conditions appears to be lost. A similar state of
things is found in some races which under certain definite conditions
lose their colour or their virulence. Among the phanerogams the
investigations of Schubler on cereals afford parallel cases, in which
the influence of a northern climate produces individuals which ripen
their seeds early; these seeds produce plants which seed early in
southern countries. Analogous results were obtained by Cieslar in his
experiments; seeds of conifers from the Alps when planted in the plains
produced plants of slow growth and small diameter.
All these observations are of considerable interest theoretically; they
show that the action of environment certainly induces such internal
changes, and that these are transmitted to the next generation. But as
regards the main question, whether constant races may be obtained by
this means, the experiments cannot as yet supply a definite answer. In
phanerogams, the influence very soon dies out in succeeding generations;
in the case of bacteria, in which it is only a question of the loss of
a character it is relatively easy for this to reappear. It is not
impossible, that in all such cases there is a material hanging-on of
certain internal conditions, in consequence of which the modification
of the character persists for a time in the descendants, although the
original external conditions are no longer present.
Thus a slow dying-out of the effect of a stimulus was seen in my
experiments on Veronica chamaedrys. (Klebs, "Kunstliche Metamorphosen",
Stuttgart, 1906, page 132.) During the cultivation of an artificially
modified inflorescence I obtained a race showing modifications in
different directions, among which twisting was especially conspicuous.
This plant, however, does not behave as the twisted race of Dipsacus
isolated by de Vries (de Vries, "Mutationstheorie", Vol. II. Leipzig,
1903, page 573.), which produced each year a definite percentage of
twisted individuals. In the vegetative reproduction of this Veronica the
torsion appeared in the first, also in the second and third year, but
with diminishing intensity. In spite of good cultivation this character
has apparently now disappeared; it disappeared still more quickly in
seedlings. In another character of the same Veronica chamaedrys the
influence of the environment was stronger. The transformation of the
inflorescences to foliage-shoots formed the starting-point; it occurred
only under narrowly defined conditions, namely on cultivation as a
cutting in moist air and on removal of all other leaf-buds. In the
majority (7/10) of the plants obtained from the transformed shoots, the
modification appeared in the following year without any interference.
Of the three plants which were under observation several years the first
lost the character in a short time, while the two others still retain
it, after vegetative propagation, in varying degrees. The same character
occurs also in some of the seedlings; but anything approaching a
constant race has not been produced.
Another means of producing new races has been attempted by Blaringhem.
(Blaringhem, "Mutation et Traumatisme", Paris, 1907.) On removing at
an early stage the main shoots of different plants he observed various
abnormalities in the newly formed basal shoots. From the seeds of such
plants he obtained races, a large percentage of which exhibited these
abnormalities. Starting from a male Maize plant with a fasciated
inflorescence, on which a proportion of the flowers had become male,
a new race was bred in which hermaphrodite flowers were frequently
produced. In the same way Blaringhem obtained, among other similar
results, a race of barley with branched ears. These races, however,
behaved in essentials like those which have been demonstrated by de
Vries to be inconstant, e.g. Trifolium pratense quinquefolium and
others. The abnormality appears in a proportion of the individuals
and only under very special conditions. It must be remembered too that
Blaringhem worked with old cultivated plants, which from the first had
been disposed to split into a great variety of races. It is possible,
but difficult to prove, that injury contributed to this result.
A third method has been adopted by MacDougal (MacDougal, "Heredity and
Origin of species", "Monist", 1906; "Report of department of botanical
research", "Fifth Year-book of the Carnegie Institution of Washington",
page 119, 1907.) who injected strong (10 percent) sugar solution or weak
solutions of calcium nitrate and zinc sulphate into young carpels of
different plants. From the seeds of a plant of Raimannia odorata the
carpels of which had been thus treated he obtained several plants
distinguished from the parent-forms by the absence of hairs and by
distinct forms of leaves. Further examination showed that he had here to
do with a new elementary species. MacDougal also obtained a more or less
distinct mutant of Oenothera biennis. We cannot as yet form an opinion
as to how far the effect is due to the wound or to the injection of
fluid as such, or to its chemical properties. This, however, is not so
essential as to decide whether the mutant stands in any relation to the
influence of external factors. It is at any rate very important that
this kind of investigation should be carried further.
If it could be shown that new and inherited races were obtained by
MacDougal's method, it would be safe to conclude that the same end might
be gained by altering the conditions of the food-stuff conducted to the
sexual cells. New races or elementary species, however, arise without
wounding or injection. This at once raises the much discussed question,
how far garden-cultivation has led to the creation of new races?
Contrary to the opinion expressed by Darwin and others, de Vries
("Mutationstheorie", Vol. I. pages 412 et seq.) tried to show that
garden-races have been produced only from spontaneous types which occur
in a wild state or from sub-races, which the breeder has accidentally
discovered but not originated. In a small number of cases only has de
Vries adduced definite proof. On the other side we have the work of
Korschinsky (Korschinsky, "Heterogenesis und Evolution", "Flora", 1901.)
which shows that whole series of garden-races have made their appearance
only after years of cultivation. In the majority of races we are
entirely ignorant of their origin.
It is, however, a fact that if a plant is removed from natural
conditions into cultivation, a well-marked variation occurs. The
well-known plant-breeder L. de Vilmorin (L. de Vilmorin, "Notices sur
l'amelioration des plantes", Paris, 1886, page 36.), speaking from his
own experience, states that a plant is induced to "affoler," that is
to exhibit all possible variations from which the breeder may make a
further selection only after cultivation for several generations. The
effect of cultivation was particularly striking in Veronica chamaedrys
(Klebs, "Kunstliche Metamorphosen", Stuttgart, 1906, page 152.) which,
in spite of its wide distribution in nature, varies very little. After a
few years of cultivation this "good" and constant species becomes highly
variable. The specimens on which the experiments were made were three
modified inflorescence cuttings, the parent-plants of which certainly
exhibited no striking abnormalities. In a short time many hitherto
latent potentialities became apparent, so that characters, never
previously observed, or at least very rarely, were exhibited, such
as scattered leaf-arrangement, torsion, terminal or branched
inflorescences, the conversion of the inflorescence into foliage-shoots,
every conceivable alteration in the colour of flowers, the assumption of
a green colour by parts of the flowers, the proliferation of flowers.
All this points to some disturbance in the species resulting from
methods of cultivation. It has, however, not yet been possible to
produce constant races with any one of these modified characters. But
variations appeared among the seedlings, some of which, e.g. yellow
variegation, were not inheritable, while others have proved constant.
This holds good, so far as we know at present, for a small rose-coloured
form which is to be reckoned as a mutation. Thus the prospect of
producing new races by cultivation appears to be full of promise.
So long as the view is held that good nourishment, i.e. a plentiful
supply of water and salts, constitutes the essential characteristic of
garden-cultivation, we can hardly conceive that new mutations can be
thus produced. But perhaps the view here put forward in regard to the
production of form throws new light on this puzzling problem.
Good manuring is in the highest degree favourable to vegetative growth,
but is in no way equally favourable to the formation of flowers. The
constantly repeated expression, good or favourable nourishment, is not
only vague but misleading, because circumstances favourable to growth
differ from those which promote reproduction; for the production of
every form there are certain favourable conditions of nourishment, which
may be defined for each species. Experience shows that, within definite
and often very wide limits, it does not depend upon the ABSOLUTE AMOUNT
of the various food substances, but upon their respective degrees of
concentration. As we have already stated, the production of flowers
follows a relative increase in the amount of carbohydrates formed in
the presence of light, as compared with the inorganic salts on which
the formation of albuminous substances depends. (Klebs, "Kunstliche
Metamorphosen", page 117.) The various modifications of flowers are due
to the fact that a relatively too strong solution of salts is supplied
to the rudiments of these organs. As a general rule every plant
form depends upon a certain relation between the different chemical
substances in the cells and is modified by an alteration of that
relation.
During long cultivation under conditions which vary in very different
degrees, such as moisture, the amount of salts, light intensity,
temperature, oxygen, it is possible that sudden and special disturbances
in the relations of the cell substances have a directive influence on
the inner organisation of the sexual cells, so that not only inconstant
but also constant varieties will be formed.
Definite proof in support of this view has not yet been furnished, and
we must admit that the question as to the cause of heredity remains,
fundamentally, as far from solution as it was in Darwin's time. As the
result of the work of many investigators, particularly de Vries,
the problem is constantly becoming clearer and more definite. The
penetration into this most difficult and therefore most interesting
problem of life and the creation by experiment of new races or
elementary species are no longer beyond the region of possibility.
XIV. EXPERIMENTAL STUDY OF THE INFLUENCE OF ENVIRONMENT ON ANIMALS.
By Jacques Loeb, M.D. Professor of Physiology in the University of
California.