forests, has often been described in its position of rest as almost
exactly resembling a withered leaf; the resemblance in colour being
aided by the markings which imitate the venation of a leaf. These
markings are composed of two parts, the upper of which is on the
fore-wings, while the lower one is on the hind wings. The butterfly
when at rest must therefore keep the wings in such a position that the
two parts of each marking exactly correspond, for otherwise the
character would be valueless; and as a matter of fact the wings are
held in the appropriate position, although the butterfly is of course
unconscious of what it is doing. Hence a mechanism must exist in the
insect’s brain which compels it to assume this attitude, and it is
clear that the mechanism cannot have been developed before the peculiar
manner of holding the wings became advantageous to the butterfly, viz.
before the similarity to a leaf had made its first appearance.
Conversely, this latter resemblance could not develope before the
butterfly had gained the habit of holding its wings in the appropriate
position. Both characters must therefore have come into existence
simultaneously, and must have undergone increase side by side: the
marking progressing from an imperfect to a very close similarity, while
the position of the wings gradually approached the attitude which was
exactly appropriate. The development of certain minute structural
elements of the central nervous system, and the appropriate
distribution of colouring matter on the wings, must have taken place
simultaneously, and only those individuals have been selected to
continue the species which possessed the favourable variations in both
these directions.
It is, however, obvious that sexual reproduction will readily afford
such combinations of required characters, for by its means the most
diverse features are continually united in the same individual, and
this seems to me to be one of its most important results.
I do not know what meaning can be attributed to sexual reproduction
other than the creation of hereditary individual characters to form the
material upon which natural selection may work. Sexual reproduction is
so universal in all classes of multicellular organisms, and nature
deviates so rarely from it, that it must necessarily be of pre-eminent
importance. If it be true that new species are produced by processes of
selection, it follows that the development of the whole organic world
depends on these processes, and the part that amphigony has to play in
nature, by rendering selection possible among multicellular organisms,
is not only important, but of the very highest imaginable importance.
But when I maintain that the meaning of sexual reproduction is to
render possible the transformation of the higher organisms by means of
natural selection, such a statement is not equivalent to the assertion
that sexual reproduction originally came into existence in order to
achieve this end. The effects which are now produced by sexual
reproduction did not constitute the causes which led to its first
appearance. Sexual reproduction came into existence before it could
lead to hereditary individual variability. Its first appearance must
therefore have had some other cause; but the nature of this cause can
hardly be determined with any degree of certainty or precision from the
facts with which we are at present acquainted. The general solution of
the problem will, however, be found to lie in the conjugation of
unicellular organisms, which forms the precursor of true sexual
reproduction. The coalescence of two unicellular individuals which
represents the simplest and therefore probably the most primitive form
of conjugation, must have some directly beneficial effect upon the
species in which it occurs.
Various assumptions may be made as to the nature of these beneficial
effects, and it will be useful to consider in detail some of those
suggestions which have been brought forward. Eminent biologists, such
as Victor Hensen[185] and Edouard van Beneden[186], believe that
conjugation, and indeed sexual reproduction generally, must be
considered as ‘a rejuvenescence of life.’ Bütschli also accepts this
view, at any rate as regards conjugation. These authorities imagine
that the wonderful phenomena of life, of which the underlying cause is
still an unsolved problem, cannot be continued indefinitely by the
action of forces arising from within itself, that the clock-work would
be stopped after a longer or shorter time, that the reproduction of
purely asexual organisms would cease, just as the life of the
individual finally comes to an end, or as a spinning wheel comes to
rest in consequence of friction, and requires a renewed impetus if its
motion is to continue. In order that reproduction may continue without
interruption, these writers believe that a rejuvenescence of the living
substance is necessary, that the clock-work of reproduction must be
wound up afresh; and they recognize such a rejuvenescence in sexual
reproduction and in conjugation, or in other words in the fusion of two
cells, whether in the form of germ-cells or of two unicellular
organisms.
Edouard van Beneden expresses this idea in the following words:—‘Il
semble que la faculté que possèdent les cellules, de se multiplier par
division soit limitée: il arrive un moment où elles ne sont plus
capables de se diviser ultérieurement, à moins qu’elles ne subissent le
phénomène du rajeunissement par le fait de la fécondation. Chez les
animaux et les plantes les seules cellules capables d’être rajeunies
sont les œufs; les seules capables de rajeunir sont les spermatocytes.
Toutes les autres parties de l’individu sont vouées à la mort. La
fécondation est la condition de la continuité de la vie. Par elle le
générateur échappe à la mort’ (l. c., p. 405). Victor Hensen thinks it
possible that the germ and its products are prevented from dying by
means of normal fertilization: he says that the law which states that
every egg must be fertilized, was formulated before the discovery of
parthenogenesis and cannot now be maintained, but that we are
nevertheless compelled to assume that even the most completely
parthenogenetic species requires fertilization after many generations
(l. c., p. 236).
If the theory of rejuvenescence be thoroughly examined, it will be
found to be nothing more than the expression of the fact that sexual
reproduction persists without any ascertainable limit. From the fact of
its general occurrence, the conclusion is, however, drawn that asexual
reproduction could not persist indefinitely as the only mode of
reproduction in any species of animal. But proofs in support of this
opinion are wanting, and it is very probable that it would never have
been advanced if it had been possible to explain the general occurrence
of sexual reproduction in any other way,—if we had been able to ascribe
any other significance to this pre-eminently important process.
But quite apart from the fact that it is impossible to bring forward
any proofs, the theory of rejuvenescence seems to me to be
unsatisfactory in other ways. The whole conception of rejuvenescence,
although very ingenious, has something uncertain about it, and can
hardly be brought into accordance with the usual conception of life as
based upon physical and mechanical forces. How can any one imagine that
an Infusorian, which by continued division had lost its power of
reproduction, could regain this power by forming a new individual,
after fusion with another Infusorian, which had similarly become
incapable of division? Twice nothing cannot make one. If indeed we
could assume that each animal contained half the power necessary for
reproduction, then both together would certainly form an efficient
whole; but it is hardly possible to apply the term rejuvenescence to a
process which is simply an addition, such as would be attained under
other circumstances by mere growth; neglecting, for the present, that
factor which, in my opinion, is of the utmost importance in
conjugation,—the fusion of two hereditary tendencies. If rejuvenescence
possesses any significance at all, it must be this,—that by its means a
force, which did not previously exist in the conjugating individuals,
is called into activity. Such a force would, however, owe its existence
to latent energy stored up in each single animal during the period of
asexual reproduction, and such latent forces would necessarily be of
different natures, and of such a constitution that their union at the
moment of conjugation would give rise to the active force of
reproduction.
The process might perhaps be compared to the flight of two rockets,
which by the combustion of some explosive substance (such as
nitro-glycerine) stored up within themselves are impelled in such a
direction that they would meet at the end of their course, when all the
nitro-glycerine had been completely exhausted. The movement would then
come to an end, unless the explosive material could have been meanwhile
renewed. Now suppose that such a renewal were achieved by the formation
of nitric acid in one of the rockets and glycerine in the other, so
that when they came into contact nitro-glycerine would be formed afresh
equal in quantity and in distribution on both the rockets to that which
was originally present. In this way the movement would be renewed again
and again with the same velocity, and might continue for ever.
Rejuvenescence can be rendered intelligible in theory by some such
metaphor, but considerable difficulties are encountered in the rigid
application of the metaphor to the facts of the case. In the first
place, how is it possible that the motive force can be exhausted by
continual division, while one of its components is being formed afresh
in the same body and during the same time? When thoroughly examined the
loss of the power of division is seen to follow from the loss of the
powers of assimilation, nutrition, and growth. How is it possible that
such a power can be weakened and finally entirely lost while one of its
components is accumulated?
I believe that, instead of accepting such daring assumptions, it is
better to be satisfied with the simple conception of living matter
possessing as attributes the powers of unlimited assimilation and
capacity for reproduction. With such a theory the mere form of
reproduction, whether sexual or asexual, will have no influence upon
the duration of the capacity: for force and matter undergo simultaneous
increase, and are inseparably connected in this as in all other
instances. This theory does not, however, exclude the possible
occurrence of circumstances under which such an association is no
longer necessary.
I could only consent to adopt the hypothesis of rejuvenescence, if it
were rendered absolutely certain that reproduction by division could
never under any circumstances persist indefinitely. But this cannot be
proved with any greater certainty than the converse proposition, and
hence, as far as direct proof is concerned, the facts are equally
uncertain on both sides. The hypothesis of rejuvenescence is, however,
opposed by the fact of parthenogenesis; for if fertilization possesses
in any way the meaning of rejuvenescence, and depends upon the union of
two different forms of force and of matter, which thus produce the
power of reproduction, it follows that we cannot understand how it
happens that the same power of reproduction may be sometimes produced
from one form of matter, alone and unaided. Logically speaking,
parthenogenesis should be as impossible as that either nitric acid or
glycerine should separately produce the effect of nitro-glycerine. The
supposition has indeed been made that in the case of parthenogenesis,
one fertilization is sufficient for a whole series of generations, but
this supposition is not only incapable of proof, but it is contradicted
by the fact that certain eggs which may develope parthenogenetically
are also capable of fertilization. If, in this case, the power of
reproduction were sufficient for development, how is it that the egg is
also capable of fertilization; and if the power were insufficient, how
is it that the egg can develope parthenogenetically? And yet one and
the same egg (in the bee) can develope into a new individual, with or
without fertilization. We cannot escape this dilemma by making the
further assumption, which is also incapable of proof, that a smaller
amount of reproductive force is required for the development of a male
individual than for the development of a female. It is true that the
unfertilized eggs of the bee produce male individuals, while the
fertilized ones develope into females, but in certain other species the
converse association holds good, while in others, again, fertilization
bears no relation to the sex of the offspring.
Although the mere fact that parthenogenesis occurs at all is, in my
opinion, sufficient to disprove the theory of rejuvenescence, it is
well to remember that parthenogenesis is now the only method of
reproduction in many species (although we do not know the period of
time over which these conditions have extended), and is nevertheless
unattended by any perceptible decrease in fertility.
From all these considerations we may draw the conclusion that the
process of rejuvenescence, as described above, cannot be accepted
either as the existing or the original meaning of conjugation, and the
question naturally arises as to what other significance this latter
process can have possessed at its first beginning.
Rolph[187] has expressed the opinion that conjugation is a form of
nutrition, so that the two conjugating individuals, as it were, devour
each other. Cienkowsky[188] also regards conjugation as merely
‘accelerated’ assimilation. There is, however, not only an essential
difference but a direct contrast between the processes of conjugation
and nutrition. With regard to Cienkowsky’s view, Hensen[189] has well
said that ‘coalescence in itself cannot be an accelerated nutrition,
because even if we admit that both individuals are in want of
nourishment, it is impossible that the need can be supplied by this
process, unless one of them perishes and is really devoured.’ In order
that an animal may serve as the food of another, it must perish and
must be brought into a fluid form, and finally it must be assimilated.
In the case before us, however, two protoplasmic bodies are placed side
by side and coalesce, without either of them passing into the liquid
state. Two idioplasms unite, together with all the hereditary
tendencies contained in them; but although it is certain that nutrition
in the proper sense of the word cannot take place, because neither of
the animals receives an addition of liquid food by the coalescence, yet
the consequence of this process must be in one respect similar to that
of nutrition and growth:—the mass of the body and the quantity of the
forces contained in it undergo simultaneous increase. It is not
inconceivable that effects are by this means rendered possible, which
under the peculiar circumstances leading to conjugation, could not have
been otherwise produced.
I believe that this is at any rate the direction in which we shall have
to seek for the first meaning of conjugation and for its phyletic
origin. This first result and meaning of conjugation may be
provisionally expressed in the following formula:—conjugation
originally signified a strengthening of the organism in relation to
reproduction, which happened when from some external cause, such as
want of oxygen, warmth, or food, the growth of the individual to the
extent necessary for reproduction could not take place.
This explanation must not be regarded as equivalent to that afforded by
the theory of rejuvenescence; for the latter process is said to be
necessary for the continuance of reproduction, and ought therefore to
occur periodically quite independently of external circumstances; while
according to my theory, conjugation at first only occurred under
unfavourable conditions, and assisted the species to overcome such
difficulties.
But whatever the original meaning of conjugation may have been, it
seems to have become already subordinated in the higher Protozoa, as is
indicated by the changes in the course taken by this process. The
higher Protozoa when conjugating do not as a rule coalesce completely
and permanently[190] in the manner followed by the lower Protozoa, and
it seems to me possible, or even probable, that in the former the
process has already gained the full significance of sexual
reproduction, and is to be looked upon as a source of variability.
Whether this be so or not, I believe it is certain that sexual
reproduction could not have been entirely abandoned at any period since
the time when the Metazoa and Metaphyta first arose; for they derived
this form of reproduction from their unicellular ancestors.
We know that organs and characters which have persisted through a long
series of generations are transmitted with extreme tenacity, even when
they have ceased to be of any direct use to their immediate possessors.
The rudimentary organs in various animals, and not least in man, afford
very strong proofs of the soundness of this conclusion. Another example
has only recently been discovered in the sixth finger, which has been
shown to exist in the human embryo[191], a part which has only been
present in a rudimentary form ever since the origin of the
Amphibia[192]. Superfluous organs become rudimentary very slowly, and
enormous periods must elapse before they completely disappear, while
the older a character is, the more firmly it becomes rooted in the
organism. What I have above called the physical constitution of a
species is based upon these facts, and upon them depend the _tout
ensemble_ of inherited characters, which are adapted to one another and
woven together into a harmonious whole. It is this specific nature of
an organism which causes it to respond to external influences in a
manner different from that followed by any other organism, which
prevents it from changing in any way except along certain definite
lines of variation, although these may be very numerous. Furthermore
these facts ensure that characters cannot be taken at random from the
constitution of a species and others substituted for them. Such a
variation as a mammal wanting the firm axis of the backbone is an
impossibility, not only because the backbone is necessary as a support
to the body, but chiefly because this structure has been inherited from
times immemorial, and has become so impressed upon the mammalian
organization that any variation so great as to threaten its very
existence cannot now take place. The view here set forth of the origin
of hereditary variability by amphigonic reproduction, makes it clear
that an organism is in a state of continual oscillation only upon the
surface, so to speak, while the fundamental parts of its constitution,
which have been inherited from extremely remote periods, remain
unaffected.
Thus sexual reproduction itself did not cease after it had existed in
the form of conjugation through innumerable generations of the vast
numbers of species which have been included under the Protozoa; it did
not cease even when its original physiological significance had lost
its importance, either completely or in part. This process, however,
had come to possess a new significance which ensured its continuance,
in the enormous advantage conferred on a species by the power of
adapting itself to new conditions of life, a power which could only be
preserved by means of this method of reproduction. The formation of new
species which among the lower Protozoa could be achieved without
amphigony, could only be attained by means of this process in the
Metazoa and Metaphyta. It was only in this way that hereditary
individual differences could arise and persist. It was impossible for
amphigony to disappear, for each species in which it was preserved was
necessarily superior to those which had lost it, and must have replaced
them in the course of time; for the former alone could adapt itself to
the ever-changing conditions of life, and the longer sexual
reproduction endured, the more firmly was it necessarily impressed upon
the constitution of the species, and the more difficult its
disappearance became.
Sexual reproduction has nevertheless been lost in some cases, although
only at first in certain generations. Thus in the _Aphidae_ and in many
lower Crustacea, generations with parthenogenetic reproduction
alternate with others which reproduce themselves by the sexual method.
But in most cases it is clear that this partial loss of amphigony
conferred considerable advantages upon the species by giving increased
capabilities for the maintenance of existence. By means of partial
parthenogenesis a much more rapid increase in the number of individuals
could be attained in a given time, and this fact is of the highest
importance for the peculiar circumstances under which these species
exist. A species of Crustacean which inhabits rapidly drying pools, and
developes from winter-eggs which have remained dried up in the mud,
has, as a rule, only a very short time in which to secure the existence
of succeeding generations. The few sexual eggs which have escaped the
attacks of numerous enemies develope immediately after the first shower
of rain; the animals attain their full size in a few days and reproduce
themselves as virgin females. Their descendants are propagated in the
same manner, and thus in a short time almost incredible numbers of
individuals are formed, until finally the sexual eggs are again
produced. If now the pool dries up again, the existence of the colony
is secured, for the number of animals which produce sexual eggs is very
large, and the eggs themselves are of course far more numerous, so that
in spite of the destructive agencies to which they are subjected, there
will be every chance of the survival of a sufficient number to produce
a new generation at a later period. Here, therefore, sexual
reproduction has not been abandoned accidentally or from any internal
cause, but as an adaptation to certain definite necessities imposed
upon the organism by its surroundings.
It is, however, well known that there are certain instances in which
sexual reproduction has been altogether lost, and in which
parthenogenesis is the only form of propagation. In the animal kingdom,
such a condition chiefly occurs in species of which the closely-allied
forms exhibit the above-mentioned alternation between parthenogenesis
and amphigony, viz. in many _Cynipidae_ and _Aphidae_, and also in
certain freshwater and marine Crustacea. We may imagine that these
parthenogenetic species have arisen from forms with alternating methods
of reproduction, by the disappearance of the sexual phase.
In any particular case, it may be difficult to point out the motive by
which this change has been determined; but it is most probable that the
same conditions which originally caused the intercalation of a
parthenogenetic stage have been efficient in causing the gradual
disappearance of the sexual stage. If a species of Crustacean, with the
above-described alternating method of reproduction (heterogeny), were
killed off by its enemies on a larger scale than before, it is obvious
that the threatened extinction of the species could be checked by the
attainment of a correspondingly greater degree of fertility. Such
increased fertility might well be produced by pure parthenogenesis (see
Appendix V, p. 323), by means of which the number of egg-producing
individuals in all the previous sexual generations would be doubled.