All phenomena of animal and plant life are governed by two sets of
causes; one of these is external, the other internal. As a rule the
internal causes determine the nature of a phenomenon--what an organism
can do and what it cannot do. The external causes, on the other hand,
decide when a certain variation will occur, and to what extent its
features may be developed.
As a very clear and wholly typical instance I cite the cocks-combs
(Celosia). This race is distinguished from allied forms by its faculty
of producing the well-known broad and much twisted combs. Every single
individual possesses this power, but all individuals do not exhibit
it in its most complete form. In some cases this faculty may not be
exhibited at the top of the main stem, although developed in lateral
branches: in others it begins too late for full development. Much
depends upon nourishment and cultivation, but almost always the
horticulturist has to single out the best individuals and to reject
those which do not come up to the standard.
The internal causes are of a historical nature. The external ones may be
defined as nourishment and environment. In some cases nutrition is
the main factor, as, for instance, in fluctuating variability, but in
natural selection environment usually plays the larger part.
The internal or historical causes are constant during the life-time of
a species, using the term species in its most limited sense, as
designating the so-called elementary species or the units out of which
the ordinary species are built up. These historical causes are simply
the specific characters, since in the origin of a species one or more of
these must have been changed, thus producing the characters of the new
type. These changes must, of course, also be due partly to internal and
partly to external causes.
In contrast to these changes of the internal causes, the ordinary
variability which is exhibited during the life-time of a species
is called fluctuating variability. The name mutations or mutating
variability is then given to the changes in the specific characters.
It is desirable to consider these two main divisions of variability
separately.
In the case of fluctuations the internal causes, as well as the external
ones, are often apparent. The specific characters may be designated as
the mean about which the observed forms vary. Almost every character may
be developed to a greater or a less degree, but the variations of the
single characters producing a small deviation from the mean are usually
the commonest. The limits of these fluctuations may be called wide or
narrow, according to the way we look at them, but in numerous cases the
extreme on the favoured side hardly surpasses double the value of that
on the other side. The degree of this development, for every individual
and for every organ, is dependent mainly on nutrition. Better
nourishment or an increased supply of food produces a higher
development; only it is not always easy to determine which direction
is the fuller and which is the poorer one. The differences among
individuals grown from different seeds are described as examples of
individual variability, but those which may be observed on the same
plant, or on cuttings, bulbs or roots derived from one individual
are referred to as cases of partial variability. Partial variability,
therefore, determines the differences among the flowers, fruits, leaves
or branches of one individual: in the main, it follows the same laws
as individual variability, but the position of a branch on a plant also
determines its strength, and the part it may take in the nourishment of
the whole. Composite flowers and umbels therefore have, as a rule,
fewer rays on weak branches than on the strong main ones. The number of
carpels in the fruits of poppies becomes very small on the weak lateral
branches, which are produced towards the autumn, as well as on crowded,
and therefore on weakened individuals. Double flowers follow the same
rule, and numerous other instances could easily be adduced.
Mutating variability occurs along three main lines. Either a character
may disappear, or, as we now say, become latent; or a latent character
may reappear, reproducing thereby a character which was once prominent
in more or less remote ancestors. The third and most interesting case
is that of the production of quite new characters which never existed in
the ancestors. Upon this progressive mutability the main development of
the animal and vegetable kingdom evidently depends. In contrast to this,
the two other cases are called retrogressive and degressive mutability.
In nature retrogressive mutability plays a large part; in agriculture
and in horticulture it gives rise to numerous varieties, which have
in the past been preserved, either on account of their usefulness or
beauty, or simply as fancy-types. In fact the possession of numbers
of varieties may be considered as the main character of domesticated
animals and cultivated plants.
In the case of retrogressive and degressive mutability the internal
cause is at once apparent, for it is this which causes the disappearance
or reappearance of some character. With progressive mutations the case
is not so simple, since the new character must first be produced and
then displayed. These two processes are theoretically different, but
they may occur together or after long intervals. The production of the
new character I call premutation, and the displaying mutation. Both of
course must have their external as well as their internal causes, as
I have repeatedly pointed out in my work on the Mutation Theory. ("Die
Mutationstheorie", 2 vols., Leipzig, 1901.)
It is probable that nutrition plays as important a part among the
external causes of mutability as it does among those of fluctuating
variability. Observations in support of this view, however, are too
scanty to allow of a definite judgment. Darwin assumed an accumulative
influence of external causes in the case of the production of new
varieties or species. The accumulation might be limited to the life-time
of a single individual, or embrace that of two or more generations.
In the end a degree of instability in the equilibrium of one or more
characters might be attained, great enough for a character to give
way under a small shock produced by changed conditions of life. The
character would then be thrown over from the old state of equilibrium
into a new one.
Characters which happen to be in this state of unstable equilibrium are
called mutable. They may be either latent or active, being in the
former case derived from old active ones or produced as new ones (by the
process, designated premutation). They may be inherited in this mutable
condition during a long series of generations. I have shown that in the
case of the evening primrose of Lamarck this state of mutability
must have existed for at least half a century, for this species was
introduced from Texas into England about the year 1860, and since then
all the strains derived from its first distribution over the several
countries of Europe show the same phenomena in producing new forms.
The production of the dwarf evening primrose, or Oenothera nanella,
is assumed to be due to one of the factors, which determines the tall
stature of the parent form, becoming latent; this would, therefore,
afford an example of retrogressive mutation. Most of the other types
of my new mutants, on the other hand, seem to be due to progressive
mutability.
The external causes of this curious period of mutability are as yet
wholly unknown and can hardly be guessed at, since the origin of the
Oenothera Lamarckiana is veiled in mystery. The seeds, introduced into
England about 1860, were said to have come from Texas, but whether from
wild or from cultivated plants we do not know. Nor has the species been
recorded as having been observed in the wild condition. This, however,
is nothing peculiar. The European types of Oenothera biennis and O.
muricata are in the same condition. The first is said to have been
introduced from Virginia, and the second from Canada, but both probably
from plants cultivated in the gardens of these countries. Whether the
same elementary species are still growing on those spots is unknown,
mainly because the different sub-species of the species mentioned have
not been systematically studied and distinguished.
The origin of new species, which is in part the effect of mutability,
is, however, due mainly to natural selection. Mutability provides the
new characters and new elementary species. Natural selection, on the
other hand, decides what is to live and what to die. Mutability seems to
be free, and not restricted to previously determined lines. Selection,
however, may take place along the same main lines in the course of long
geological epochs, thus directing the development of large branches of
the animal and vegetable kingdom. In natural selection it is evident
that nutrition and environment are the main factors. But it is probable
that, while nutrition may be one of the main causes of mutability,
environment may play the chief part in the decisions ascribed to natural
selection. Relations to neighbouring plants and to injurious or useful
animals, have been considered the most important determining factors
ever since the time when Darwin pointed out their prevailing influence.
From this discussion of the main causes of variability we may derive the
proposition that the study of every phenomenon in the field of heredity,
of variability, and of the origin of new species will have to be
considered from two standpoints; on one hand we have the internal
causes, on the other the external ones. Sometimes the first are more
easily detected, in other cases the latter are more accessible to
investigation. But the complete elucidation of any phenomenon of life
must always combine the study of the influence of internal with that of
external causes.