Many authors assume that the origin of species is directed by unknown
causes. These causes are assumed to work in each single case for the
improvement of the animals and plants, changing them in a manner
corresponding in a useful way to the changes that take place in their
environment. It is not easy to imagine the nature of these influences
nor how they would bring about the desired effect.
This difficulty was strongly felt by Darwin, and one of the chief
purposes of his selection theory may be said to have been the attempt
[571] to surmount it. Darwin tried to replace the unknown cause by
natural agencies, which lie under our immediate observation. On this
point Darwin was superior to his predecessors, and it is chiefly due to
the clear conception of this point that his theory has gained its
deserved general acceptance. According to Darwin, changes occur in all
directions, quite independently of the prevailing circumstances. Some
may be favorable, others detrimental, many of them without significance,
neither useful nor injurious. Some of them will sooner or later be
destroyed, while others will survive, but which of them will survive, is
obviously dependent upon whether their particular changes agree with the
existing environic conditions or not. This is what Darwin has called the
struggle for life. It is a large sieve, and it only acts as such. Some
fall through and are annihilated, others remain above and are selected,
as the phrase goes. Many are selected, but more are destroyed; daily
observation does not leave any doubt upon this point.
How the differences originate is quite another question. It has nothing
to do with the theory of natural selection nor with the struggle for
life. These have an active part only in the accumulation of useful
qualities, and only in so [572] far as they protect the bearers of such
characters against being crowded out by their more poorly constituted
competitors.
However, the differentiating characteristics of elementary species are
only very small. How widely distant they are from the beautiful
adaptative organizations of orchids, of insectivorous plants and of so
many others! Here the difference lies in the accumulation of numerous
elementary characters, which all contribute to the same end. Chance must
have produced them, and this would seem absolutely improbable, even
impossible, were it not for Darwin's ingenious theory. Chance there is,
but no more than anywhere else. It is not by mere chance that the
variations move in the required direction. They do go, according to
Darwin's view, in all directions, or at least in many. If these include
the useful ones, and if this is repeated a number of times, cumulation
is possible; if not, there is simply no progression, and the type
remains stable through the ages. Natural selection is continually acting
as a sieve, throwing out the useless changes and retaining the real
improvements. Hence the accumulation in apparently predisposed
directions, and hence the increasing adaptations to the more specialized
conditions of life. It must be obvious to any one who can free himself
from the current ideas, [573] that this theory of natural selection
leaves the question as to how the changes themselves are brought about,
quite undecided. There are two possibilities, and both have been
propounded by Darwin. One is the accumulation of the slight deviations
of fluctuating variability, the other consists of successive sports or
leaps taking place in the same direction.
In further lectures a critical comparison of the two views will be
given. Today I have only to show that the mutations of the
evening-primroses, though sudden, comply with the demands made by Darwin
as to the form of variability which is to be accepted as the cause of
evolution and as the origin of species.
Some of my new types are stouter and others weaker than their parents,
as shown by _gigas_ and _albida_. Some have broader leaves and some
narrower, _lata_ and _oblonga_. Some have larger flowers (_gigas_) or
deeper yellow ones (_rubrinervis_), or smaller blossoms (_scintillans_),
or of a paler hue (_albida_). In some the capsules are longer
(_rubrinervis_), or thicker (_gigas_), or more rounded (_lata_), or
small (_oblonga_), and nearly destitute of seeds (_brevistylis_). The
unevenness of the surface of the leaves may increase as in _lata_, or
decrease as in _laevifolia_. The tendency to become annual prevails in
_rubrinervis_, but _gigas_ tends to become [574] biennial. Some are rich
in pollen, while _scintillans_ is poor. Some have large seeds, others
small. _Lata_ has become pistillate, while _brevistylis_ has nearly lost
the faculty to produce seeds. Some undescribed forms were quite sterile,
and some I observed which produced no flowers at all. From this
statement it may be seen that nearly all qualities vary in opposite
directions and that our group of mutants affords wide material for the
sifting process of natural selection. On the original field the
_laevifolia_ and _brevistylis_ have held their own during sixteen years
and probably more, without, however, being able to increase their
numbers to any noticeable extent. Others perish as soon as they make
their appearance, or a few individuals are allowed to bloom, but
probably leave no progeny.
But perhaps the circumstances may change, or the whole strain may be
dispersed and spread to new localities with different conditions. Some
of the latter might be found to be favorable to the robust _gigas_, or
to _rubrinervis_, which requires a drier air, with rainfall in the
springtime and sunshine during the summer. It would be worth while to
see whether the climate of California, where neither _O. lamarckiana_
nor _O. biennis_ are found wild, would not exactly [575] suit the
requirements of the new species _rubrinervis_ and _gigas_.
NOTE. _Oenothera_s are native to America and all of the species growing
in Europe have escaped from gardens directly, or may have arisen by
mutation, or by hybridization of introduced species. A fixed hybrid
between _O. cruciata_ and _O. biennis_ constituting a species has been
in cultivation for many years. The form known as _O. biennis_ in Europe,
and used by de Vries in all of the experiments described in these
lectures, has not yet been found growing wild in America and is not
identical with the species bearing that name among American botanists.
Concerning this matter Professor de Vries writes under date of Sept. 12,
1905: "The '_biennis_' which I collected in America has proved to be a
motley collection of forms, which at that time I had no means of
distinguishing. No one of them, so far as they are now growing in my
garden is identical with our _biennis_ of the sand dunes." The same
appears to be the case with _O. muricata_. Plants from the Northeastern
American seaboard, identifiable with the species do not entirely agree
with those raised from seed received from Holland.
_O. lamarckiana_ has not been found growing wild in America in recent
years although the evidence at hand seems to favor the conclusion that
it was seen and collected in the southern states in the last century.
(See MacDougal, Vail, Shull, and Small: Mutants and Hybrids of the
_Oenotheras_. Publication 24. Carnegie Institution. Washington, D.C.,
1905.) EDITOR.
[576]
LECTURE XX