have been unable to meet with a single instance which could shake my
conviction. There were many instances in which hereditary transmission
was clearly established, but in none of them was there any reason to
suppose that the characters transmitted were really acquired. For
example, Fritz Müller has recently informed me of an instance in which
he believes that there can be no doubt of the transmission of acquired
characters. His observations are so interesting in several respects
that I will quote them here. He says in his letter, ‘Among the bastards
of two species of _Abutilon_, in which I had never observed hexamerous
flowers, there was a single plant with a few such blossoms. As these
flowers are sterile with the pollen of the same plant, I was obliged to
fertilize it with pollen from another plant bearing only pentamerous
flowers, in order to obtain seeds from the former. For three weeks I
examined all the flowers from a plant grown from such seed, finding 145
pentamerous, 103 hexamerous, and 13 heptamerous flowers. I examined
similarly the flowers of another plant produced from seed obtained from
pentamerous flowers from the same parent plants. There were 454
pentamerous and 6 hexamerous flowers, and hence only 1·3 per cent. of
the latter kind.’
It must certainly be admitted that the large proportion of abnormal
hexamerous flowers depends upon heredity in the instance first quoted;
but the hexamerous condition is not an acquired character; it is merely
the first appearance of a new innate character. It is not due to the
reaction of the vegetable organism under some external stimulus, for it
appeared in a plant exposed to conditions similar to those which acted
upon the other plant which only produced the normal pentamerous
flowers. It must therefore have resulted from the tendencies which were
present in the germ from which the plant itself developed, either as a
spontaneous change in the germ-plasm or through the combination of two
parental germ-plasms—a combination which may lead to the appearance or
the reality of a new character. We know that the germ-plasm of each
individual is not a simple substance, but possesses a very complex
composition, for it consists of a number of ancestral germ-plasms
represented in very different proportions. Now, although we cannot
learn anything directly about the processes of growth of the
germ-plasm, and its resulting ontogenetic stages, yet we do know,
chiefly from observations upon man, that the characters of ancestors
appear in the offspring in very different combinations and in very
different degrees of strength. This may, perhaps, be explained by
assuming that in the union of parental germ-plasms which takes place at
fertilization, the contained ancestral germ-plasms unite in different
ways, and thus come to grow with different strengths. Certain ancestral
germ-plasms will meet and together produce a double effect: other
opposed germ-plasms will neutralize each other; and between these two
extremes all intermediate conditions will occur. And these combinations
will not only take place at fertilization, but also at every stage of
the whole ontogenetic history, for each stage is represented by its
idioplasm, which is itself composed of ancestral idioplasms.
We do not yet know enough to be able to prove in detail the manner in
which new characters may arise from such a combination of different
kinds of germ-plasm. And yet it appears to me that such a view, e.g. in
the case of the variation of buds, is by far the most natural. There is
indeed a single example in which we can, to some extent, understand how
it is that a new character may arise by these means. Certain
canary-birds have a tuft of feathers on the head, but if two such birds
are paired, their descendants are generally bare-headed, instead of
having larger tufts[226]. The formation of a tuft depends upon the fact
that the feathers are scanty and in fact absent from part of the skin
of the head. Now when the scanty plumage of both parents is combined in
the offspring the latter is bare-headed. Hence by the combination of
ancestral characters a new character (bare-headedness) is produced, and
one which is hardly likely to have ever occurred in the ancestors of
existing canaries.
We do not know the causes which have been in operation when a flower
possesses one petal more than the usual number, any more than we can
explain why it is that one star-fish has five and another six rays. We
cannot unravel the details of the mysterious relationship between two
parent germ-plasms, each of which is composed of a countless number of
ancestral germ-plasms from the first and second back to the _n_th
degree. But we can nevertheless maintain in a general way that such
irregularities are the result of this complex struggle between the
germ-plasms in the ovum and the idioplasms in the subsequent stages of
the developing organism, and that they are not the result of external
influences.
If, however, acquired characters are brought forward in connexion with
the question of the transformation of species, the term ‘acquired’ must
only be applied to those characters which do not arise from within the
organism, but which arise as the reaction of the organism under some
external stimulus, most commonly as the consequence of the increased or
diminished use of an organ or part. We have then to learn whether the
altered conditions of life, by forcing an organism to adopt new habits,
can by such means lead directly, and not indirectly through natural
selection, to the transformation of the species; or whether the effects
of increased or diminished use of certain parts, implied by the new
habits, are restricted to the individual itself, and therefore
powerless to effect any direct modification of the species.
Fritz Müller’s observation is also interesting in another respect: it
appears to controvert my views upon heredity as expressed in the theory
of the continuity of the germ-plasm. If a single flower can transmit to
its descendants special peculiarities which were not possessed by its
ancestors, we seem to be driven to the conclusion that the ancestral
germ-plasm has not passed into the flower in question, but that new
germ-plasm has been formed, inasmuch as the new characters are derived
from the flower itself, and not from any of its ancestors. I think,
however, that the observation admits of another interpretation: a
specimen of _Abutilon_ with many hundred flowers is not a single
individual, but a colony consisting of numerous individuals which have
arisen by budding from the first individual developed from the seed.
I have not hitherto considered budding in relation to my theories, but
it is obvious that it is to be explained from my point of view, by
supposing that the germ-plasm which passes on into a budding individual
consists not only of the unchanged idioplasm of the first ontogenetic
stage (germ-plasm), but of this substance altered, so far as to
correspond with the altered structure of the individual which arises
from it—viz. the rootless shoot which springs from the stem or
branches. The alteration must be very slight, and perhaps quite
insignificant, for it is possible that the differences between the
secondary shoots and the primary plant may chiefly depend upon the
changed conditions of development, which takes place beneath the earth
in the latter case, and in the tissues of the plant in the former. Thus
we may imagine that the idioplasm, when it developes into a flowering
shoot, produces at the same time the germ-cells which are found in the
latter. We thus approach an understanding of Fritz Müller’s
observation; for if the whole shoot which produces the flower arises
from the same idioplasm which also forms its germ-cells, we can readily
understand why the latter should contain the same hereditary tendencies
which were previously expressed in the flower which produced them. The
fact that variations may occur in a single shoot depends upon the
changes explained above, which occur in the idioplasm during the course
of its growth, as a result of the varying proportions in which the
ancestral idioplasms may be contained in it.
Fritz Müller’s observation affords a beautiful confirmation of this
view, for if the flower itself transmitted the hexamerous condition to
its germ-cells, we could not understand why some of the extremely rare
hexamerous flowers were produced by the crossing of two pentamerous
flowers, in the control experiment. An explanation of this fact can
only be found in the assumption that the germ-plasm contained in the
mother plant, during its growth and consequent distribution through all
the branches of the colony, became arranged into a combination of
idioplasms, which, whenever it predominated (as it did at certain
places), necessarily led to the formation of hexamerous flowers. I will
not consider here the question as to whether this combination is to be
looked upon as an instance of reversion, or whether it represents
something new. Such a question is of no importance for our present
purpose; but the hexamerous flowers of the control experiment prove, in
my opinion, that germ-plasm containing the requisite combination was
distributed in the mother plant and also existed, but in insufficient
amount, in shoots which did not produce any hexamerous flowers.
Appendix V. On the Origin of Parthenogenesis[227].
The transformation of heterogeny into pure parthenogenesis has
obviously been produced by other causes as well as by those mentioned
in the main part of this paper. Other and quite different circumstances
have also had a share in its production. Pure parthenogenesis may be
produced without the intermediate condition of heterogeny. Thus, for
example, the pure and exclusive parthenogenesis with which the large
Phyllopod crustacean, _Apus_, is reproduced at most of its habitats,
has not arisen from the loss of previously existent sexual generations,
but simply from the non-appearance of males, accompanied by the
simultaneous acquisition of the power, on the part of the females, of
producing eggs which do not require fertilization. This is proved by
the fact that males occur in certain scattered colonies of this
species, and sometimes they are even present in considerable numbers.
But even if we were not aware of these facts, the same conclusions
might nevertheless have been drawn from the fact that _Apus_ produces
eggs of only one form—viz. resting eggs with hard shells. In every case
in which parthenogenesis has been first introduced in alternation with
sexual reproduction, the resting eggs are produced by the latter
generations, while the parthogenetic generations produce eggs with thin
shells, in which the embryo developes and hatches very rapidly. In this
way parthenogenesis leads to a rapid increase of the colony. In _Apus_
such increase in the number of individuals is gained in an entirely
different manner, viz. by the fact that all the animals become females,
which produce eggs at a very early age, and continue producing them in
increasing fertility for the whole of their life. In this manner an
enormous number of eggs collects at the bottom of the pool inhabited by
the colony, so that after it has dried up, in spite of loss from
various destructive agencies, there will still remain a sufficiency of
eggs to reproduce a numerous colony, as soon as the pool has filled
again.
This form of parthenogenetic reproduction is especially well suited to
the needs of species inhabiting small pools which entirely depend upon
rain-fall, and which may disappear at any time. In these cases the time
during which the colony can live is often too short to permit the
production of several generations even from rapidly developing
summer-eggs. Under these circumstances the pool would often suddenly
dry up before the series of parthenogenetic generations had been run
through, and hence before the appearance of the sexual generation and
resting eggs. In all such cases the colony would be exterminated.
This consideration might lead us to think that Crustacea, such as the
_Daphnidae_, which develope by means of heterogeny, would hardly be
able to exist in small pools filled by the rain; but here also nature
has met the difficulty by another adaptation. As I have shown in a
previous paper[228], the heterogeny of the species of _Daphnidae_ which
inhabit such pools is modified in such a manner, that only the first
generation produced from the resting eggs consists of purely
parthenogenetic females, while the second includes many sexual animals,
so that resting eggs are produced and laid, and the continuance of the
colony is secured a few days after it has been first founded; viz.
after the appearance of the first generation.
But it is also certain that in the _Daphnidae_, heterogeny may pass
into pure parthenogenesis by the non-appearance of the sexual
generations. This seems to have taken place in certain species of
_Bosmina_ and _Chydorus_, although perhaps only in those colonies of
which the continuance is secured for the whole year; viz. those which
inhabit lakes, water-pipes, or wells in which the water cannot freeze.
In certain insects also (e. g. _Rhodites rosae_) pure parthenogenesis
seems to be produced in a similar manner, by the non-appearance of
males.
But the utility which we may look upon as the cause of parthenogenesis
is by no means so clear in all cases. Sometimes, especially in certain
species of Ostracoda, its appearance seems almost like a mere caprice
of nature. In this group of the Crustacea, one species may be purely
parthenogenetic, while a second reproduces itself by the sexual method,
and a third by an alternation of the two methods: and yet all these
species may be very closely allied and may frequently live in the same
locality and apparently with the same habit of life. But it must not be
forgotten that it is only with the greatest difficulty that we can
acquire knowledge about the details of the life of these minute forms,
and that where we can only recognize the appearance of identical
conditions, there may be highly important differences in nutrition,
habits, enemies and the means by which they are resisted, and in the
mode by which the prey is captured—circumstances which may place two
species living in the same locality upon an entirely different basis of
existence. It is not merely probable that this is the case; for the
fact that certain species have modified their modes of reproduction is
in itself a sufficient proof of the validity of the conclusions which
have just been advanced.
The fact that different methods of reproduction may obtain in different
colonies of the same species, although with thoroughly identical
habits, may depend upon differences in the external conditions (as in
_Bosmina_ and _Chydorus_ mentioned above), or upon the fact that the
transition from sexual to parthenogenetic reproduction is not effected
with the same ease and rapidity in all the colonies of the same
species. As long as males continue to make their appearance in a colony
of _Apus_, sexual reproduction cannot wholly disappear. Although we are
unable to appreciate, with any degree of certainty, the causes by which
sex is determined, we may nevertheless confidently maintain that such
determining influences may be different in two widely separated
colonies. As soon, however, as parthenogenesis becomes advantageous to
the species, securing its existence more efficiently than sexual
reproduction, it will not only be the case that the colonies which
produce the fewest males will gain advantage, but within the limits of
the colony itself, those females will gain an advantage which produce
eggs that can develope without fertilization. When the males are only
present in small numbers, it must be very uncertain whether any given
female will be fertilized: if therefore the eggs of such a female
required fertilization in order to develope, it is clear that there
would be great danger of entire failure in this necessary condition. In
other words:—as soon as any females begin to produce eggs which are
capable of development without fertilization, from that very time a
tendency towards the loss of sexual reproduction springs into
existence. It seems, however, that the power of producing eggs which
can develope without fertilization is very widely distributed among the
Arthropoda.
Appendix VI. W. K. Brooks’ Theory of Heredity[229].
The only theory of heredity which, at any rate in one point, agrees
with my own, was brought forward two years ago by W. K. Brooks of
Baltimore[230]. The point of agreement lies in the fact that Brooks
also looks upon sexual reproduction as the means employed by nature in
order to produce variation. The manner in which he supposes that the
variability arises is, however, very different from that suggested in
my theory, and our fundamental conceptions are also widely divergent.
While I look upon the continuity of the germ-plasm as the foundation of
my theory of heredity, and therefore believe that permanent hereditary
variability can only have arisen through some direct change in the
germ-plasm effected by external influences, or following from the
varied combinations which are due to the mixture of two individually
distinct germ-plasms at each act of fertilization, Brooks, on the other
hand, bases his theory upon the transmission of acquired characters,
and upon the idea which I have previously called ‘the cyclical
development of the germ-plasm.’
Brooks’ theory of heredity is a modification of Darwin’s pangenesis,
for Brooks also assumes that minute gemmules are thrown off by each
cell in the body of the higher organisms; but such gemmules are not
emitted always, and under all circumstances, but only when the cell is
subjected to unaccustomed conditions. During the persistence of the
ordinary conditions to which it is adapted, the cell continues to
perform its special functions as part of the body, but as soon as the
conditions of life become unfavourable and its functions are disturbed,
the cell ‘throws off minute particles which are its germs or gemmules.’
These gemmules may then pass into any part of the organism; they may
penetrate the ova in the ovary, or may enter into a bud, but the male
germ-cells possess a special power of attracting them and of storing
them up within themselves.
According to Brooks, variability arises as a consequence of the fact
that each gemmule of the sperm-cell unites, during fertilization, with
that part of the ovum which, in the course of development, is destined
to become a cell corresponding to that from which the gemmule has been
derived.
Now, when this cell developes in the offspring, it must, as a hybrid,
have a tendency to vary. The ova themselves, as cells, are subject to
the same laws; and the cells of the organism will continue to vary
until one of the variations is made use of by natural selection. As
soon as this is the case, the organism becomes, _ipso facto_, adapted
to its conditions; and the production of gemmules ceases, and with it
the manifestation of variability itself, for the cells of the organism
then derive the whole of their qualities from the egg, and being no
longer hybrid, have no tendency to vary. For the same reason the ova
themselves will also cease to vary, and the favourable variation will
be transmitted from generation to generation in a stereotyped
succession, until unfavourable conditions arise, and again lead to a
fresh disposition to vary.
In this way Brooks[231] attempts to mediate between Darwin and Lamarck,
for he assumes, on the one hand, that external influences render the
body or one of its parts variable, while, on the other hand, the nature
of the successful variations is determined by natural selection. There
is, however, a difference between the views of Brooks and Darwin,
although not a fundamental difference. Darwin also holds that the
organism becomes variable by the operation of external influences, and
he further assumes that changes acquired in this way can be
communicated to the germ and transmitted to the offspring. But
according to his hypothesis, every part of the organism is continually
throwing off gemmules which may be collected in the germ-cells of the
animal, while, according to Brooks, this only takes place in those
parts which are placed under unfavourable conditions or the function of
which is in some way disturbed. In this manner the ingenious author
attempts to diminish the incredible number of gemmules which, according
to Darwin’s theory, must collect in the germ-cells. At the same time he
endeavours to show that those parts must always vary which are no
longer well adapted to the conditions of life.
I am afraid, however, that Brooks is confounding two things which are
in reality very different, and which ought necessarily to be treated
separately if we wish to arrive at correct conclusions: viz., the
adaptation of a part of the body to the body itself, and its adaptation
to external conditions. The first of these adaptations may exist
without the second. How can those parts become variable which are badly
adapted to the external conditions, but are nevertheless in complete
harmony with the other parts of the body? If the conditions of life of
the cells which constitute the part in question must become
unfavourable, in order that the gemmules which produce variation may be
thrown off, it is obvious that such a result would not occur in the
case mentioned above. Suppose, for example, that the spines of a
hedgehog are not sufficiently long or sharply pointed to afford
protection to the animal, how could such an unfavourable development
afford the occasion for the throwing off of gemmules, and a resulting
variability of the spines, inasmuch as the epidermic tissue in which
these structures arise, remains under completely normal and favourable
conditions, whatever length or sharpness the spines may attain? The
conditions of the epidermis are not unfavourably affected because, as
the result of short and blunt spines, the number of hedgehogs is
reduced to far below the average. Or consider the case of a brown
caterpillar which would gain great advantage by becoming green; what
reason is there for believing that the cells of the skin are placed in
unfavourable conditions, because, in consequence of the brown colour,
far more caterpillars are detected by their enemies, than would have
been the case if the colour were green? And the case is the same with
all adaptations. Harmony between the parts of the organism is an
essential condition for the existence of the individual. If it is
wanting, the individual is doomed; but such harmony between any one
part and all others, i. e. proper nutrition for each part, and adequate
performance of its proper function, can never be disturbed by the fact
that the part in question is insufficiently adapted to the outer
conditions of life. According to Darwin, all the cells of the body are
continually throwing off gemmules, and against such an assumption no
similar objection can be raised. It can only be objected that the
assumption has never been proved, and that it is extremely improbable.
A further essential difference between Darwin’s theory of pangenesis
and Brooks’ hypothesis lies in the fact that Brooks holds that the male
and female germ-cells play a different part, and that they tend to
become charged with gemmules in different degrees, the egg-cell
containing a far smaller number than the sperm-cell. According to
Brooks the egg-cell is the conservative principle which brings about
the permanent transmission of the true characters of the race or
species, while he believes that the sperm-cell is the progressive
principle which causes variation.
The transformation of species is therefore believed to take place, for
the most part, as follows:—those parts which are placed in unfavourable
conditions by the operation of external influences, and which have
varied, throw off gemmules which reach the sperm-cells, and the latter
by fertilization further propagate the variation. An increase of
variation is produced because the gemmules which reach the egg through
the sperm-cell may unite or conjugate with parts of the former which
are not the exact equivalents of the cells from which the gemmules
arose, but only very nearly related to them. Brooks calls this
‘hybridization,’ and he concludes that, just as hybrids are more
variable than pure species, so such hybridized cells are also more
variable than other cells.