SEX AS AN ADAPTATION
In what sense may the separation of all the individuals of a species
into two kinds of individuals, male and female, be called an adaptation?
Does any advantage result to the species that would not come from a
non-sexual method of reproduction? Many attempts have been made to
answer these questions, but with what success I shall now try to show.
There are four principal questions that must be considered:—
I. The different kinds of sexual individuals in the animal and plant
kingdoms.
II. The historical question as to the evolution of separate sexes.
III. The factors that determine the sex in each individual developing
from an egg.
IV. The question as to whether any advantage is gained by having each
new individual produced by the union of two germ-cells, or by having the
germ-cells carried by two kinds of individuals.
While our main problem is concerned with the last of these topics, yet
there would be little hope of giving a complete answer to it unless we
could get some answer to the first three questions.
The Different Kinds of Sexual Individuals
Amongst the unicellular animals and plants the fusion of two (or more)
individuals into a single one is generally regarded as the simplest, and
possibly also the most primitive, method of sexual reproduction. Two
amœbas, or amœba-like bodies, thus flow together, as it were, to produce
a new individual.
In the more highly specialized unicellular animals, the processes are
different. Thus in vorticella, a small, active individual unites with a
larger fixed individual. The protoplasm fuses into a common mass, and a
very complicated series of changes is passed through by the nucleus. In
paramœcium, a free-swimming form very much like vorticella, two
individuals that are alike unite only temporarily, and after an
interchange of nuclear material they separate.
In the lower plants, and more especially in some of the simple
aggregates or colonial forms, there are found a number of stages between
species in which the uniting individuals are alike, and those in which
they are different. There are several species whose individuals appear
to be exactly alike; and other species in which the only apparent
difference between the individuals that fuse together is one of size;
and still other species in which there are larger resting or passive
individuals, and smaller active individuals that unite with the larger
ones. In several of the higher groups, including the green algæ and
seaweeds, we find similar series, which give evidence of having arisen
independently of each other. If we are really justified in arranging the
members of these groups in series, beginning with the simpler cases and
ending with those showing a complete differentiation into two kinds of
germ-cells, we seem to get some light as to the way in which the change
has come about. It should not be forgotten, however, that it does not
follow because we can arrange such a series without any large gaps in
its continuity, that the more complex conditions have been gradually
formed in exactly this way from the simplest conditions.
So far we have spoken mainly of those cases in which the forms are
unicellular, or of many-celled species in which all the cells of the
individual resolve themselves into one or the other kind of germ-cells.
This occurs, however, only in the lowest forms. A step higher we find
that only a part of the cells of the colony are set aside for purposes
of reproduction. The cells surrounding these germ-cells may form
distinct organs, which may show certain differences according to whether
they contain male or female germ-cells. When these two kinds of cells
are produced by two separate individuals, the individuals themselves may
be different in other parts of the body, as well as in the reproductive
organs.
When this condition is reached, we have individuals that we call males
and females, because, although they do not themselves unite to form new
individuals, they produce one or the other kind of germ-cell. It is the
germ-cells alone that now combine to form the new individual.
Amongst living groups of animals we find no such complete series of
forms as exist in plants, and the transition from the one-celled to the
many-celled forms is also more abrupt. On the other hand, we find an
astonishing variety of ways in which the reproduction is accomplished,
and several ways in which the germ-cells are carried by the sexual
individuals. Let us examine some of the more typical conditions under
the following headings: (1) sexes separate; (2) sexes united in the same
individual; (3) parthenogenetic forms; (4) exceptional methods of
propagation.
1. _Sexes Separate; Unisexual Forms._[34]—Although the animals with
which we are more familiar have the sexes separate, this is far from
being universal amongst animals and plants; and, in fact, can scarcely
be said to be even the rule. When the sexes are separate they may be
externally alike, and this is especially true for those species that do
not unite, but set free their eggs and spermatozoa in the water, as
fish, frogs, corals, starfish, jellyfish, and many other forms. In other
animals there are sometimes other secondary differences in the sexes
besides those connected with the organs of reproduction. Such
differences are found, as we have seen, in insects, in some spiders,
crustaceans, and in many birds and mammals. In a few cases the
difference between the sexes is very great, especially when the female
is parasitic and the male free, as in some of the crustaceans. In some
other cases the male is parasitic on the female. Thus in Bonellia the
male is microscopic in size, being in length only one-hundredth part of
the female. In _Hydatina senta_ the male is only about a third as large
as the female. It has no digestive tract, and lives only a few days. In
another rotifer the males are mere sacs enclosing the male reproductive
organs.
Footnote 34:
Geddes and Thompson’s “The Evolution of Sex” has been freely used in
the preparation of this part of this chapter.
2. _Hermaphroditic Forms._—There are many species of animals and plants
in which each individual contains both the male and the female organs of
reproduction, and there are whole groups in which only these
hermaphroditic forms occur. Thus in the ctenophors the eggs develop
along one side of each radial canal and spermatozoa along the other. The
group of flatworms is almost exclusively hermaphroditic. The earthworms
and the leeches have only these bisexual forms, and in the mollusks,
while a few groups have separate sexes, yet certain groups of
gasteropods and of bivalve forms are entirely hermaphroditic.
In the common garden snail, although there are two sets of sexual ducts
closely united, yet from the same reproductive sac both eggs and sperm
are produced. The barnacles and the ascidians are for the most part
hermaphroditic forms. Many other examples might be cited, but these will
suffice to show that it is by no means unusual in the animal kingdom for
the same individual to produce both male and female germ-cells. However,
one of the most striking facts in this connection is that
self-fertilization seldom takes place, so that the result is the same in
certain respects as though separate sexes existed. This point will come
up later for further consideration.
3. _Parthenogenetic Reproduction._—It has long been known that, in some
cases, eggs that are not fertilized will begin to develop and may even
produce new individuals. Tichomiroff showed that by rubbing with a brush
the unfertilized eggs of the silkworm moth, a larger percentage would
produce caterpillars than if they were not rubbed. During the last few
years it has been shown that the development of a non-fertilized egg may
be started in a number of ways. Such, for example, as by certain
solutions of salt or of sugar, by subjecting the eggs to cold, or by
simply shaking them.
There are certain groups of animals in which the males appear only at
regular (in others at irregular) intervals. In their absence the females
produce eggs that develop without being fertilized, _i.e._
parthenogenetically. The following examples will serve to show some of
the principal ways in which this “virgin reproduction” takes place. In
the group of rotifers the males are generally smaller than the females
and are usually also degenerate. In some species, although degenerate
males are present, they are unnecessary, since parthenogenesis is the
rule. In still other species no males exist and the eggs develop,
therefore, without being fertilized. In some of the lower crustaceans
parthenogenesis occurs in varying degrees. In Apus males may be entirely
absent at times in certain localities, and at other times a few, or even
very many, males may appear. Some species of ostracod crustaceans seem
to be purely parthenogenetic; others reproduce by means of fertilized
eggs; and others by an alternation of the two processes. The crustaceans
of the genus _Daphnia_ produce two kinds of eggs. The summer eggs are
small, and have a thin shell. These eggs develop without being
fertilized, but in the autumn both male and female individuals develop
from these unfertilized eggs, and the eggs of the female, the so-called
winter eggs, are fertilized. These are also larger than the summer eggs,
have thicker shells, and are much more resistant to unfavorable
conditions. They give rise in the following spring to females only, and
these are the parthenogenetic individuals that continue to produce
during the summer new parthenogenetic eggs.
It is within the group of insects that some of the most remarkable cases
of parthenogenesis that we know are found. In the moth, _Psyche helix_,
only females are present, as a rule, but rarely males have been found.
In another moth, _Solenobia trinquetrella_, the female reproduces by
parthenogenesis, but at times males appear and may then be even more
numerous than the females. In the gall-wasps parthenogenetic generations
may alternate with a sexual generation, and it is interesting to note
that the sexual and the parthenogenetic generations are so different
that they were supposed to belong to separate species, until it was
found that they were only alternate generations of the same species.
The aphids or plant-lice reproduce during the summer by parthenogenesis,
but in the autumn winged males and females appear, and fertilized winter
eggs are produced. From these eggs there develop, in the following
spring, the wingless parthenogenetic summer forms, which produce the
successive generations of the wingless forms. As many as fourteen summer
broods may be produced. By keeping the aphids in a warm temperature and
supplying them with plenty of moist food, it has been possible to
continue the parthenogenetic reproduction of the wingless forms for
years. As many as fifty successive broods have been produced in this
way. It has not been entirely determined whether it is the temperature
or a change in the amount, or kind, of food that causes the appearance
of the winged males and females, although it seems fairly certain that
diminution in the food, or in the amount of water contained in it, is
the chief cause of the change.
In the honey-bee the remarkable fact has been well established that
fertilized eggs give rise only to females (queens and workers), while
unfertilized eggs develop into males. Whether a fertilized egg becomes a
queen or a worker (sterile female) depends solely on the kind of food
that is given to the young larva, and this is determined, in a sense,
entirely by the bees themselves.
In plants also there are many cases of parthenogenesis known. Some
species of Chara when kept under certain conditions produce only female
organs, and seem to produce new plants parthenogenetically. In this case
it appears that the same conditions that caused the plants to produce
only female organs may also lead to the development of the egg-cells
without fertilization. In fact it is only by a combination of this kind
that parthenogenesis could arise. The result is similar when the eggs of
insects produce only females whose eggs are capable of parthenogenetic
development. If a case should arise in which only females appeared whose
eggs did not possess the power of parthenogenetic development, the
species would die out.
In the green alga, Spirogyra, it has been found that if conjugation of
two cells is prevented, a single cell may become a parthenogenetic cell.
In a number of parasitic fungi the male organs appear to be degenerate,
and from the female organs parthenogenetic development takes place. A
small number of flowering plants are also capable of parthenogenetic
reproduction.
There is a peculiarity in the development of the parthenogenetic eggs of
animals that will be more fully discussed later, but may be mentioned
here. Ordinarily an egg that becomes fertilized gives off two polar
bodies, but in a number of cases in which parthenogenetic development
occurs it has been found that only one polar body is given off. It is
supposed that in such cases one polar body is retained, and that it
plays the same part as the entrance of the spermatozoon of the male.
4. _Exceptional Cases._—Occasionally in a species that is unisexual an
individual is found that is bisexual. The male of the toad, _Pelobates
fuscus_, has frequently a rudimentary ovary in front of the testis. The
same thing has been found in several species of fish. In Serranus, a
testis is present in the wall of the ovary, and the eggs are said to be
fertilized by the spermatozoa of the same individual. In frogs it has
been occasionally found that ovary and testis may be associated in the
same individual, or a testis may be present on one side, and a testis
with an anterior ovarian portion on the other. Cases like these lead up
to those in which the body itself may also show a mosaic of
sex-characters, and it is noticeable that when this occurs there is
nearly always a change in the reproductive organs also. Thus butterflies
have been found with the wings and the body of one side colored like the
male and the other side like the female. Similar cases have also been
found in bees and ants. Bees have been found with the anterior part of
the body of one sex and posterior part of another!
The preceding cases illustrate, in different ways, the fact that in the
same individual both kinds of reproductive organs may suddenly appear,
although it is the rule in such species that only one set develops.
Conversely, there are cases known, especially amongst plants, in which
individuals, that usually produce male and female organs (or more
strictly spores of two kinds from which these organs develop), produce
under special conditions only one or the other kind. Facts like these
have led to the belief that each individual is potentially bisexual, but
in all unisexual forms one sex predominates, and the other remains
latent. This idea has been the starting-point for nearly all modern
theories of sex.
An excellent illustration of this theory is found in those cases in
which the same individual may be male at one time and female at another.
For instance, it is said that in one of the species of starfish
(_Asterina gibbosa_) the individuals at Roscoff are males for one or two
years, and then become females. At Banyuls they are males for the first
two or three years, and then become females; while at Naples some are
always males, others females, some hermaphrodites, others transitional
as in the cases just given. In one of the isopod crustaceans,
_Angiostomum_, the young individuals are males and the older females. In
_Myzostomum glabrum_ the young animal is at first hermaphroditic, then
there is a functional male condition, followed by a hermaphroditic
condition, and finally a functional female phase, during which the male
reproductive organs disappear.
The flowers of most of the flowering plants have both stamens and
pistils, which contain the two kinds of spores out of which the male and
female germ-cells are formed. The stamens become mature before the
pistils, as a rule, but in some cases the reverse is the case. This
difference in the time of ripening of the two organs is often spoken of
as an adaptation which prevents self-fertilization. The latter is
supposed to be less advantageous than cross-fertilization. This question
will be more fully considered later.
Before we come to an examination of the question of the adaptations
involved in the cases in which the sexes are separate, and the different
times at which the sex-cells are ripened, it will be profitable first to
examine the question as to what determines in the egg or young whether a
male or a female or a hermaphroditic form shall arise.
The Determination of Sex
A large number of views have been advanced as to what determines whether
an egg will give rise to a male or to a female individual. The central
question is whether the fertilized egg has its sex already determined,
or whether it is indifferent; and if the latter, what external factor or
factors determine the sex of the embryo. Let us first examine the view
that some external factor determines the sex of the individual, and then
the evidence pointing in the opposite direction. Among the different
causes suggested as determining the sex of the embryo, that of the
condition of the egg itself at the time of fertilization has been
imagined to be an important factor in the result. Another similar view
holds that the condition of the spermatozoon plays the same rôle. For
instance, it has been suggested that if the egg is fertilized soon after
it leaves the ovary, it produces a female, but if the fertilization is
delayed, a male is produced. It has also been suggested that the
relative age of the male and the female parents produces an effect in
determining the sex of the young. There is no satisfactory evidence,
however, showing that this is really the case.
Another view suggested is that the sex is determined by the more
vigorous parent; but again there is no proof that this is the case, and
it would be a difficult point to establish, since as Geddes and Thompson
point out, what is meant by greater vigor is capable of many
interpretations. Somewhat similar is the idea that if the conditions are
favorable, the embryo develops further, as it were, and becomes a male;
but there are several facts indicating that this view is untenable.
Düsing maintains that several of these factors may play a part in
determining the sex of the embryo, and if this be true, the problem
becomes a very complex one. He also suggests that there are
self-regulative influences of such a kind that, when one sex becomes
less numerous, the conditions imposed in consequence on the other sex
are such as to bring the number back to the normal condition; but this
idea is far from being established. The fact that in some species there
are generally more individuals of one sex than of the other shows that
this balance is not equally adjusted in such forms.
Of far greater value than these speculations as to the origin of sex are
the experiments that appear to show that nutrition is an important
factor in determining sex. Some of the earlier experiments in this
direction are those of Born and of Yung. By feeding one set of tadpoles
with beef, Yung found the percentage of females that developed to be
greatly increased, and a similar increase was observed when the tadpoles
were fed on the flesh of fish. An even greater effect was produced by
using the flesh of frogs, the percentage rising to 92 females in every
hundred. These results have been given a different interpretation by
Pflüger and by others, and, as will be pointed out later, there is a
possible source of error that may invalidate them.
Somewhat similar results have been obtained by Nussbaum for one of the
rotifers. He found that if the rotifer is abundantly fed in early life,
it produces _female eggs_, that is, larger eggs that become females;
while if sparingly fed, it produces only small eggs, from which males
develop. It has been claimed also in mammals, and even in man, that sex
is to some extent determined by the nourishment of the individual.
Some experiments made by Mrs. Treat with caterpillars seemed to show
that if the caterpillars were well nourished more female moths were
produced, and if starved before pupation more males emerged. But Riley
has pointed out that since the larger female caterpillars require more
food they will starve sooner than the males, and, in consequence, it may
appear that proportionately more male butterflies are born when the
caterpillars are subjected to a starvation diet. This point of view is
important in putting us on our guard against hastily supposing that food
may directly determine sex. Unless the entire number of individuals
present at the beginning of the experiment is taken into account, the
results may be misleading, because the conditions may be more fatal to
one sex than to the other.
In some of the hymenopterous insects, the bees for example, it has been
discovered that the sex of the embryo is determined by the entrance, or
lack of entrance, of the spermatozoon. In the honey-bee all the
fertilized eggs produce females and the unfertilized eggs males. The
same relation is probably true also in the case of ants and of wasps. In
the saw-flies, the conditions are very remarkable. Sharp gives the
following account of some of these forms:[35]—“It is a rule in this
family that males are very much less numerous than females, and there
are some species in which no males have been discovered. This would not
be of itself evidence of the occurrence of parthenogenesis, but this has
been placed beyond doubt by taking females bred in confinement,
obtaining unfertilized eggs from them, and rearing the larvæ produced
from the eggs. This has been done by numerous observers with curious
results. In many cases the parthenogenetic progeny, or a portion of it,
dies without attaining full maturity. This may or may not be due to
constitutional weakness, arising from the parthenogenetic state.
Cameron, who has made extensive observations on this subject, thinks
that the parthenogenesis does involve constitutional weakness, fewer of
the parthenogenetic young reaching maturity. This, he suggests, may be
compensated for—when the parthenogenetic progeny is all of the female
sex—by the fact that all those that grow up are producers of eggs. In
many cases the parthenogenetic young of Tenthredinidæ are of the male
sex, and sometimes the abnormal progeny is of both sexes. In the case of
one species—the common currant-fly, _Nematus ribesii_—the
parthenogenetic progeny is nearly, but not quite always, entirely of the
male sex; this has been ascertained again and again, and it is
impossible to suggest in these cases any advantage to the species to
compensate for constitutional parthenogenetic weakness. On the whole, it
appears most probable that the parthenogenesis, and the special sex
produced by it, whether male or female, are due to physiological
conditions of which we know little, and that the species continues in
spite of the parthenogenesis rather than profits by it. It is worthy of
remark that one of the species in which parthenogenesis with the
production of males occurs—_Nematus ribesii_—is perhaps the most
abundant of saw-flies.”
Footnote 35:
“The Cambridge Natural History,” Vol. V, “Insects,” by David Sharp.
It has been pointed out that in a number of species of animals and
plants only parthenogenetic females are present at certain times. In a
sense this means a preponderance of one sex, but since the eggs are
adapted only to this kind of development, it may be claimed that the
conditions in such cases are somewhat different from those in which eggs
that would be normally fertilized may develop in the absence of
fertilization. Nevertheless, it is generally supposed that the actual
state of affairs is about the same. It is usually assumed, and no doubt
with much probability, that these parthenogenetic forms have evolved
from a group which originally had both male and female forms. One of the
most striking facts in this connection is that in the groups to which
these parthenogenetic species belong there are, as a rule, other species
with occasional parthenogenesis, and in some of these the males are also
fewer in number than the females.
In the aphids, the parthenogenetic eggs give rise during the summer to
parthenogenetic females, but in the autumn the parthenogenetic eggs give
rise without fertilization both to males and to females. It appears,
therefore, that we can form no general rule as to a relation between
fertilization and the determination of sex. While in certain cases, as
in the bees, there appears to be a direct connection between these two,
in other cases, as in that of the aphids just mentioned, there is no
such relation apparent.
Geddes and Thompson have advocated a view in regard to sex which at best
can only serve as a sort of analogy under which the two forms of sex may
be considered, rather than as a legitimate explanation of the phenomenon
of sex. They rest their view on the idea that living material is
continually breaking down and building up. An animal in which there is
an excess of the breaking-down process is a male, and one that is more
constructive is a female. Furthermore, whichever process is in the
excess during development determines the sex of the individual. Thus, if
conditions are very favorable, there will be more females produced; but
if, on the other hand, there is an excess of the breaking-down process,
males are produced. So far, the process is conceived as a purely
physiological one, but to this the authors then apply the selection
hypothesis, which, they suppose, acts as a sort of break or regulation
of the physiological processes, or in other words as a directive agent.
They state: “Yet the sexual dimorphism, in the main, and in detail, has
an adaptive significance, also securing the advantages of
cross-fertilization and the like, and is, therefore, to some extent the
result of the continual action of natural selection, though this may, of
course, check variation in one form as well as favor it in another.”
Disregarding this last addition, with which Geddes and Thompson think it
necessary to burden their theory, let us return to the physiological
side of the hypothesis. Their idea appears to me a sort of symbolism
rather than a scientific attempt to explain sex. If their view had a
real value, it ought to be possible to determine the sex of the
developing organism with precision by regulating the conditions of its
growth, and yet we cannot do this, nor do the authors make any claim of
being able to do so. The hypothesis lacks the only support that can give
it scientific standing, the proof of experiment.
There have been made, from time to time, a number of attempts to show
that the sex of the embryo is predetermined in the egg, and is not
determined later by external circumstances. In recent years this view
has come more to the front, despite the apparent experimental evidence
which seemed in one or two cases to point to the opposite view. One of
the most complete analyses of the question is that of Cuénot, who has
attempted to show that the sex of the embryo is determined in the egg,
before or at the time of fertilization. He has also examined critically
the evidence that appeared to show that external conditions, acting on
the embryo, may determine the sex, and has pointed out some possible
sources of error that had been overlooked. The best-known case is that
of the tadpole of the frog, but Cuénot shows not only that there are
chances of error in this experiment as carried out, but also, by his own
experiments and observations, that the facts themselves are not above
suspicion. He points out that at the age at which some of the tadpoles
were when the examination was made, it was not always possible to tell
definitely the sex of the individual, and least of all by means of the
size alone of the reproductive organs, as was supposed, in one case at
least, to be sufficient. In his own experiments he did not find an
excess of one sex over the other as a result of feeding.
Cuénot points out that Brocadello found that the larger eggs laid by the
silkworm give rise to from 88 to 95 per cent of females, and the small
eggs to from 88 to 92 per cent of males. Joseph has confirmed this for
_Ocneria dispar_, and Cuénot himself also reached this conclusion.
Korschelt found that the large eggs of Dinophilus produced females and
the small ones males. Cuénot experimented with three species of flies,
and found that when the maggots were well nourished the number of the
individuals of the two sexes was about equal, and when poorly nourished
there were a few more females in two cases, and in another about the
same number of males and females.
It has been claimed that the condition of nourishment of the mother may
determine the number of eggs of a particular sex, but Cuénot found, in
three species of flies which he raised, that there was a slight response
in the opposite direction. He concludes that the condition of the mother
is not a factor in the determination of sex.
The first egg of the two laid in each set by the pigeon is said, as a
rule, to produce a male, and the second a female. Both Flourens and
Cuénot found this to be the case in the few instances that they
examined, but Cuénot has shown that this does not always happen. Even
when this occurs, it has not been determined whether the result depends
on something in the egg itself, that causes a male egg to be set free
first, or on some external condition that determines that the first egg
shall become a male. It has been claimed that the age of the
spermatozoon might in this and in other cases determine the result; but
Gerbe has shown that if the domestic hen is isolated for fifteen days
after union with the male, she will continue to produce fertile eggs
from which both sexes are produced, without showing any relation between
the time the eggs are laid and the particular sex that develops.
Cuénot does not discuss whether sex is determined by the nucleus or by
the protoplasm, but if, as he thinks probable, the size of the egg is a
determining factor, it would appear that the protoplasm must be the
chief agent. Even if this were the case it would still be possible that
the size of the egg itself might be connected with some action on the
part of the nucleus. If, as seems probable, identical twins come from
halves of the same egg, then, since they are of the same sex, the
absolute amount of protoplasm cannot be a factor in sex determination.
[Illustration:
Fig. 6.—Diagram showing the maturation of the egg.]
As a basis for the discussion that follows, certain processes that take
place during the maturation divisions of the egg and of the spermatozoon
must be briefly noticed. After the egg leaves the ovary it extrudes a
minute body called the first polar body (Fig. 6 B, C, D). This process
of extrusion is really a cell division accompanied by the regular
mitotic division of the nucleus; but since one of the products of the
division, the polar body, is extremely small, the meaning of the process
was not at first understood. The half of the nucleus, that remains in
the egg, divides again, and one of its halves is thrown out into a
second polar body (Fig. 6 E, F, G)). Meanwhile, the first polar body has
divided into two equal parts, so that we find now three polar bodies and
the egg (Fig. 6 G)). A strictly analogous process takes place in the
formation of the spermatozoa (Fig. 7 B-F). The mother-cell of the
spermatozoon divides into two parts, which are equal in this case (Fig.
7 B-D). Each of these then divides again (Fig. 7 E, F), producing four
cells that are comparable to the three polar bodies and the mature egg.
Each of the four becomes a functional spermatozoon (Fig. 7 G, H). Thus
while in the maturation of the egg only the egg itself is capable of
development, in the case of the male cells all four products of the two
maturation divisions are functional.
[Illustration:
Fig. 7.—Diagram showing the maturation of the spermatozoon.]
Now, in certain cases of parthenogenesis, it has been found that one of
the polar bodies may not be given off, but, remaining in the egg, its
nucleus reunites with the egg nucleus, and thus takes the place of the
spermatozoon, which does exactly the same thing when it fertilizes the
egg, _i.e._ the nucleus of the spermatozoon unites with the nucleus of
the egg. This fact in regard to the action of the polar body in
fertilization is not as surprising as appears at first sight, for if
each of the polar bodies is equivalent to a spermatozoon, the
fertilization of the egg by one of its own polar bodies conforms to
theory.
There is a considerable body of evidence showing that in many eggs at
one of the two maturation divisions the chromatin rods derived from the
nucleus are divided crosswise (Fig. 6 B, C). The same thing occurs at
one of the two divisions in the formation of the spermatozoon (Fig. 7 B,
C). At the other division to form the other polar body (or the other
sperm-cell) the chromatin rods appear to be split lengthwise, as in
ordinary cell division (Fig. 6 E, F, G). In recent years the
_cross-division_ of the chromatin rods has attracted a great deal of
notice, and Weismann in particular drew attention to the possible
importance of this kind of division.
There is another fact that gives this division especial significance. It
has been discovered that the number of chromosomes that appears in each
dividing cell of the organism is a constant number, but it has also been
discovered that the egg, before extruding its polar bodies, and the
mother-cell of the spermatozoon (Figs. 6, 7 B), contain exactly half of
the number of chromosomes that are characteristic of the body-cells of
the same animal (Figs. 6, 7 A). Now there is good evidence to show that
the reduction in number is due to the chromosomes uniting sometimes end
to end in pairs, as shown in Figures A and B. Furthermore, it has been
suggested that at one of the maturation divisions, when the chromosomes
divide crosswise, the united chromosomes are separated (Figs. 6, 7 B,
C), so that one remains in the egg and the other goes out into the polar
body. The same thing is supposed to occur at one of the maturation
divisions of the sperm mother-cell. A further consideration of capital
importance in this connection has been advocated by Montgomery and by
Sutton, namely, that, when the chromosomes unite in pairs, a chromosome
from one parent unites with one from the other parent. Consequently at
one of the two reduction divisions maternal and paternal chromosomes may
separate again, some to go to one cell, some to the other.
When the spermatozoon enters the egg it brings into the egg as many new
chromosomes as the egg itself possesses at this time, and the two
nuclei, uniting into a single one, furnish the total number of
chromosomes characteristic of the animal that develops from the egg. At
first the chromosomes that are brought in by the spermatozoon lie at one
side of the fused nucleus, and those from the egg itself at the other
side. This arrangement appears, however, in some cases at least, to be
lost later. At every division of the nucleus, each chromosome divides
and sends a half to each of the daughter-nuclei. Thus every cell in the
body contains as many paternal as maternal chromosomes. This statement
also applies to the first cells that go into the reproductive organs,
some of which become the mother-cells of the germ-cells. Later, however,
in the history of the germ-cells,—just before the maturation
divisions,—these chromosomes are supposed to unite in pairs, end to end,
as explained above, to give the reduced number. Later there follows the
separation of these paired chromosomes at one of the two maturation
divisions. If at this time all the paternal chromosomes should pass to
one pole, and all the maternal to the other, the germ-cell ceases to be
mixed, and becomes purely paternal or maternal. If this ever occurs, the
problem of heredity may become simplified, and even the question of sex
may be indirectly involved; but it has not been established that, when
the reduced number of chromosomes is formed, there is a strict union
between the paternal and maternal chromosomes, and if not, the
subsequent separation is probably not along these lines. If, however,
the chromosomes contain different qualities, as Boveri believes, there
may be two kinds of eggs, and two kinds of spermatozoa in regard to
_each particular character_. It is this last assumption only that is
made in Mendel’s theory of the purity of the germ-cells.
Several attempts have been made at different times to connect the facts
in regard to the extrusion of the polar bodies with those involved in
the determination of sex. Minot suggested, in 1877, that the egg ejects
by means of the polar bodies its male elements, which are again received
in the fertilization of the egg by the spermatozoon. The same idea has
also been expressed by others. It has been objected to this view that
one polar body ought to suffice, and that no similar throwing out of
part of its substance is found in the process of formation of the
spermatozoon, which should, on the hypothesis, throw out its female
elements. It would seem, on first thought, that this view might find
support in the idea expressed above, namely, that in one of the polar
bodies half of the chromosomes pass out, so that there is conceivably a
separation of the maternal from the paternal. If this were the case also
in the spermatozoa, then two of each four would be paternal and two
maternal. This is, however, a very different thing from supposing them
to be male and female, for it by no means follows, because the
chromosomes correspond to those of the father or of the mother in the
sum of their characters, that they are, therefore, also male or female
in regard to sex.
It has been pointed out already, that in most parthenogenetic eggs only
one polar body is extruded. There are, it is true, a few apparent
exceptions to this rule, but in most cases it is certain that only one
is extruded. In several cases the beginning of the formation of the
second maturation division of the nucleus takes place, but after the
chromosomes have divided they come together again in the nucleus. If
each polar body be interpreted as equivalent to a spermatozoon, then
this result is rather a process of self-fertilization than true
parthenogenesis. It is, nevertheless, true that in some cases
development seems to go on after both polar bodies have been extruded.
Moreover, it has been found possible to cause the eggs of the sea-urchin
to begin their development by artificial solutions after they have
extruded both polar bodies. A single spermatozoon may also produce an
embryo if it enters a piece of egg-protoplasm without a nucleus. The
last instance is a case of male parthenogenesis, and if the theory of
the equivalency of spermatozoon and egg be correct, this is what should
occur.
Quite recently, Cuénot, Beard, Castle, and Lenhossek have contended that
the differentiation of sex is the outcome of internal factors. They
think that the view that sex is determined by external agents is
fundamentally erroneous. The fallacies that have given rise to this
conception, Castle points out, are, first, that in animals that
reproduce sometimes by parthenogenesis and sometimes by fertilized eggs,
the former process is favored by good nutrition and the latter by poor
nutrition. This only means, in reality, Castle thinks, that
parthenogenetic reproduction is favored by external conditions, and this
kind of reproduction, he thinks, is a thing _sui generis_, and not to be
compared to the formation of more females in the sexual forms of
reproduction. There is no proof, however, that this is anything more
than a superficial distinction, and it ignores the fact that in ordinary
cases the females sometimes lay parthenogenetic eggs which differ, as
far as we can see, from eggs that are destined to be fertilized in no
important respect. More significant, it seems to me, is the fact that
only parthenogenetic females develop the following spring from the
fertilized eggs of the last generation of the autumn series, whose
origin is described to be due to lack of food. We find, in the case of
aphids, that unfertilized parthenogenetic eggs and also fertilized eggs
give rise to females only, while a change in the amount of food causes
the parthenogenetic eggs to give rise both to males and to females. This
point is not, I think, fully met by Castle, for even if the change in
food does not, as he claims, cause only one sex to appear, yet lack of
food does seem to account for the appearance of the males at least.
The other fallacy, mentioned by Cuénot, is that the excess of males that
has been observed when the food supply is limited is due to the early
death of a larger percentage of females, which require more food, but
this still fails to account for the excess of females when more food is
given, provided Yung’s experiments on tadpoles are correct. It may be,
however, in the light of Pflüger’s results, that there has been some
mistake in the experiments themselves.
We may now proceed to examine Castle’s argument, attempting to show in
what way sex is predetermined in the embryo. His hypothesis rests on the
three following premises: “(1) the idea of Darwin, that in animals and
plants of either sex the characters of the opposite sex are latent; (2)
the idea of Mendel, that in the formation of the gametes [germ-cells] of
hybrids a segregation of the parental characters takes place, and when
in fertilization different segregated characters meet, one will
dominate, the other become latent or recessive; (3) the idea of
Weismann, that in the maturation of egg and spermatozoon a segregation
is attended by a visible reduction in the number of chromosomes in the
germinal nuclei.”
Expressed in a somewhat more general way, Castle suggests that each egg
and each spermatozoon is either a male or a female germ-cell (and not a
mixture of the two), and when a female egg is fertilized by a male
spermatozoon, or _vice versa_, the individual is a sexual hybrid with
one sex dominating and the other latent. The assumption that there are
two kinds of eggs, male and female, and two kinds of spermatozoa, male
and female, is not supported by any direct or experimental evidence.
Moreover, in order to carry out the hypothesis, it is necessary to make
the further assumption that a female egg can only be fertilized by a
male spermatozoon, and a male egg by a female spermatozoon. While such a
view is contrary to all our previous ideas, yet it must be admitted that
there are no facts which disprove directly that such a selection on the
part of the germ-cells takes place. If these two suppositions be
granted, then Castle’s hypothesis is as follows:—
In order that half of the individuals shall become males and half
females it is necessary to assume that in some individuals the male
element dominates and in others the female, and since each fertilized
egg contains both male and female elements, it is necessary to assume
that either the egg or the spermatozoon contains the dominating element.
Castle supposes that in hermaphroditic organisms the two characters
“exist in the balanced relationship in which they were received from the
parents,” but, as has just been stated, in unisexual forms one or the
other sex dominates, except of course in those rare cases, as in the
bees and ants, where half of the body may bear the characters of one
sex, and the other half that of the other sex.
In parthenogenetic species the female character is supposed to be
uniformly stronger, so that it dominates in every contest, “for the
fertilized egg in such species develops invariably into a female.” Under
certain circumstances, as Castle points out, the parthenogenetic female
produces both males and females, and this is also true in the occasional
development of the unfertilized egg of the silkworm moth, and of the
gypsy moth, in which both male and female individuals are produced by
parthenogenesis. These facts show that even in unfertilized eggs both
sexes are potentially present; but this might be interpreted to mean
that some eggs are male and some female, rather than that each egg has
the possibility of both kinds of development. If, however, one polar
body is retained in these parthenogenetic eggs, then _ex hypothese_ each
egg would contain the potentialities of both sexes (if the polar body
were of the opposite sex character). It seems necessary to make this
assumption because in some parthenogenetic forms males and females may
be produced later by each individual, as in the aphids, and this could
not occur if we assume that some parthenogenetic eggs are purely male
and some female.
Castle assumes, in fact, that in animals like daphnids and rotifers one
polar body only is extruded, and the other (the second) is retained in
the egg, and hence the potentiality of producing males is present. In
the honey-bee, on the contrary, Castle assumes that both polar bodies
are extruded in the unfertilized egg (and there are some observations
that support this idea), and since only males are produced from these,
he believes it is the female element that has been sent out into the
second polar body. This hypothesis is necessary, because Castle assumes
that when both elements are present in the bee’s eggs, the female
element dominates. “Hence, if the egg which has formed two polar cells
develops without fertilization, it must develop into a male. But if such
an egg is fertilized, it invariably forms a parthenogenetic female ♀
(♂), that is, an individual in which the male character is recessive.
Accordingly the _functional_ spermatozoon must in such cases invariably
bear the _female character_, and this is invariably dominant over the
male character when the two meet in fertilization.”
If it should prove generally true that the size of the egg is one of the
factors determining the sex, we have still the further question to
consider as to whether some eggs are bigger because they are already
female, or whether all eggs that go beyond a certain size are females,
and all those that fail to reach this are males. If this is the case, an
animal might produce more females if the external conditions were
favorable to the growth of the eggs, and if in some cases these large
eggs were capable of developing, parthenogenetic races might become
established. Should, however, the conditions for nutrition become less
favorable, some of the eggs might fall below the former size and produce
males. It is not apparent, however, why all the fertilized autumn eggs
of the aphids should give rise to females, for although these eggs are
known to be larger than the summer eggs, yet they are produced under
unfavorable conditions.
The preceding discussion will show how far we still are from knowing
what factors determine sex. Castle’s argument well illustrates how many
assumptions must be made in order to make possible the view that sex is
a predetermined quality of each germ-cell. Even if these assumptions
were admissible, we still return to the old idea that the _fertilized
egg_ has both possibilities, and something determines which shall
dominate. Until we have ascertained definitely by experimental work
whether the sex in some forms can be determined by external conditions,
it is almost worthless to speculate further. Whatever decision is
reached, the conclusion will have an immediate bearing on the question
to be next discussed. Meanwhile, we can at least examine some of the
theories that have been advanced as to what advantage, if any, has been
gained by having the individuals of many classes divided into two kinds,
male and female.
Sex as a Phenomenon of Adaptation
Of what advantage is it to have the individuals of many species
separated into males and females? It is obviously a disadvantage from
the point of view of propagation to have half of the individuals
incapable of producing young, and the other half also incapable of doing
so, as a rule, unless the eggs are fertilized by the other sex. Is there
any compensation gained because each new individual arises from two
parents instead of from one? Many answers have been attempted to these
questions.
At the outset it should be recognized that we are by no means forced to
assume, as is so often done, that because there is this separation of
the sexes it must have arisen on account of its advantage to the
species. Whether the result may be of some benefit regardless of how it
arose, may be an entirely different question. It would be extremely
difficult to weigh the relative advantages (if there are any) and
disadvantages (that are obvious as pointed out above), nor is it
probable that in this way we can hope to get a final answer to our
problem. We may begin by examining some of the modern hypotheses that
have been advanced in this connection.
Darwin has brought together a large number of facts which appear to show
the beneficial effects of the union of germ-cells from two different
individuals. Conversely, it is very generally believed, both by breeders
and by some experimenters, that self-fertilization in the case of
hermaphroditic forms leads, in many cases, though apparently not in all,
to the production of less vigorous offspring. Darwin’s general position
is that it is an advantage to the offspring to have been derived from
two parents rather than to have come from the union of the germ-cells of
the same individual, and he sees, in the manifold contrivances in
hermaphroditic animals and plants to insure cross-fertilization, an
adaptation for this purpose.
This question of whether self-fertilization is less advantageous than
cross-fertilization is, however, a different question from that of
whether _non-sexual_ methods of reproduction are less advantageous than
sexual ones. Since some plants, like the banana, have been propagated
for a very long time solely by non-sexual methods without any obvious
detriment to them, it is at first sight not easy to see what other
advantage could be gained by the sexual method. The case of the banana
shows that some forms do not require a sexual method of propagation.
Other forms, however, are so constituted, as we find them, that they
cannot reproduce at the present time except by the sexual method. In
other words, the latter are now adapted, as it were, to the sexual
method, and there is no longer any choice between the two methods. The
question of whether a non-sexual form might do better if it had another
method of propagation is not, perhaps, a profitable question to discuss.
What we really need to know is whether or not the sexual method was once
acquired, because it was an advantage to a particular organism, or to
the species to reproduce in this way. It is assumed by many writers that
this was the case, but whether they have sufficient ground for forming
such an opinion is our chief concern here. On the other hand, it is
conceivable, at least, that if the sexual method once became
established, it might continue without respect to any superiority it
gave over other methods, and might finally become a necessary condition
for the propagation of particular species. Thus the method would become
essential to propagation without respect to whether the species lost
more than it gained. Whichever way the balance should turn, it might
make little difference, so long as the species was still able to
propagate itself.
Brooks made the interesting and ingenious suggestion that the separation
of the sexes has been brought about as a sort of specialization of the
individuals in two directions. The male cells are supposed to accumulate
the newly acquired characters, and represent, therefore, the progressive
element in evolution. The female cells are the conservative element,
holding on to what has been gained in the past. It does not seem
probable, in the light of more recent work, that this is the function of
the two sexes, and it is unlikely that we could account for the origin
of the two sexes through the supposed advantage that such a
specialization might bring about. A number of writers, Galton, Van
Beneden, Bütschli, Maupas, and others, have looked at the process of
sexual reproduction as a sort of renewal of youth, or rejuvenescence of
the individuals. There is certainly a good deal in the process to
suggest that something of this sort takes place, although we must be on
our guard against assuming that the rejuvenescence is anything more than
the fulfilment of a necessary stage in the life history. Weismann has
ridiculed this suggestion on the ground that it is inconceivable that
two organisms, decrepit with old age, could renew their youth by
uniting. Two spent rockets, he says, cannot be imagined to form a new
one by combining. There is apparent soundness in this argument, if the
implication is taken in a narrow physical sense. If, on the other hand,
the egg is so constituted that at a certain stage in its development an
outside change is required to introduce a new phase, then the conception
of rejuvenescence does not appear in quite so absurd a light.
This hypothesis of rejuvenescence is based mainly on certain processes
that take place in the life history of some of the unicellular animals.
Let us now see what this evidence is. The results of certain experiments
carried out by Maupas on some of the ciliate protozoans have been
fruitful in arousing discussion as to the ultimate meaning of the sexual
process. Maupas’ experiments consisted in isolating single individuals,
and in following the history of the descendants that were produced
non-sexually by division. He found that the descendants of an individual
kept on dividing, but showed no tendency to unite with each other. After
a large number of generations had been passed through (in _Stylonychia
pustulata_, between 128 and 175; in _Leucophys patula_, 300 to 450; and
in _Onychodromus grandis_, 140 to 230 generations), the division began
to slow down, and finally came to a standstill. Maupas found that if he
took one of these run-down individuals, and placed it with another in
the same condition from another culture, that had had a different
parentage, the two would unite and the so-called process of conjugation
take place. This process consists for the species used, in the temporary
union and partial fusion of the protoplasm of the two individuals, of an
interchange of micronuclei, and of a fusion, in each individual, of the
micronucleus received from the other individual with one of its own. The
individuals then separate, and a new nucleus (or nuclei) is formed out
of the fused pair.
The individuals in question, in which this interchange of micronuclei
has taken place, undergo a change, and behave differently from what they
did before. They feed, become larger and less vacuolated, and are more
active. They soon begin once more to divide. Maupas found that an
individual that has conjugated will run through a new cycle of
divisions, which will, however, after a time also slow down, unless
conjugation with another individual having a different history takes
place. If conjugation is prevented, the individual will die after a
time. These results seemed to show that the division phase of the life
history cannot go on indefinitely, and that through conjugation the
individual is again brought back to the starting-point.
Quite recently Calkins has carried out a somewhat similar series of
experiments, which have an important bearing on the interpretation of
Maupas’ results. The experiment of isolating an individual and tracing
the career of its descendants was repeated with the following results:
two series were started, the original forms coming from different
localities. Of their eight descendants four of each were isolated. The
remaining four of each set were kept together as stock material. The
rate of division was taken as the measure of vitality. The animals
divided more or less regularly from February to July. After each
division (or sometimes after two divisions) the individuals were
separated. About the 30th of July the paramœcia began to die “at an
alarming rate, indicating that a period of depression had apparently set
in, or degeneration in Maupas’ sense.” Up to this time the animals had
been living in hay infusion, renewed every few days, from which they
obtained the bacteria on which they feed. Calkins tried the effect of
putting the weakened paramœcia into a new environment. Infusion of
vegetables gave no good results, but meat infusions proved successful.
“The first experiment with the latter was with teased liver, which was
added to the usual hay infusion. The result was very gratifying, for the
organisms began immediately to grow and to divide, the rate of division
rising from five to nine divisions in successive ten-day periods.” This
beneficial effect was not lasting, however, and after ten days the
paramœcia began to die off faster than before, and the renewed
application of the liver extract failed to revive them. A number of
other extracts were then tried without effect. Finally they were
transferred to the clear extract of lean beef in tap water. The effect
of this medium was interesting, for, although it restored the weakened
vitality, there was no rapid increase in the rate of division, as when
first treated with the teased liver. The infusoria were, however, now
large and vigorous, and did not die unless transferred from the beef
medium to the usual hay infusion. “When this was attempted, they would
become abnormally active and would finally die. The division rate
gradually increased during the month of August until, in the last ten
days, they averaged six generations. Finally, in September, the attempts
to get them back on the old diet of hay infusion were successful, and
then the division rate went up at once to twelve times in ten days, and
a month later they were dividing at the rate of fifty times a month.”
“These cultures went on well until December, when the paramœcia began to
die again. They were saved once more with the beef extract, and when
returned later to the hay infusion continued through another cycle of
almost three months. Some of these were treated, once a week for
twenty-four hours, with the beef extract, and while the two sets ran a
parallel course at first, those kept continuously in the hay infusion
died after a time, but those that had been put once a week into the beef
extract (which had been stopped, however, in March) continued their high
rate of division throughout the period of decline of their sister cells,
and did not show signs of diminished vitality until the first period in
June.” At this time their rate of division increased rapidly. They were
put back into the beef extract, but it failed now to have a beneficial
effect, and the animals continued to die at a rapid rate. To judge from
the appearance of the organisms the new decline was due to a different
cause; for, while in the former periods the food vacuoles contained
undigested food, at this period the interior was free from food masses.
The protoplasm became granular and different from that of a healthy
individual. None of the former remedies were now of any avail. “When the
last of the _B_-series stock had died in the five hundred and seventieth
generation (June 16th), it looked as though the cultures were about to
come to an end.” Extract of the brain and of the pancreas were then
tried. To this a favorable response took place at once. The organisms
became normal in appearance and began to divide. After forty-eight
hours’ treatment they were returned to the usual hay infusion. Here they
continued to multiply and reached on June 28th the six hundred and
sixty-fifth generation.
There can be no doubt that the periods of depression that appear in
these infusoria kept in cultures can be successfully passed if the
animals are introduced into a new environment. Without a change of this
sort they will die. Calkins thinks that the effect is produced, not by
the new kind of food that is supplied, but by the presence of certain
chemical compounds. The beef extract “does not have a direct stimulating
effect upon the digestive process and upon division, for, while the
organisms are immersed in it, there is a very slow division rate; when
transferred again to the hay infusion, however, they divide more rapidly
than before.”
This brings us back to the idea of the “renewal of youth” through
conjugation. Maupas claimed that union of individuals having the same
immediate descent is profitless. Calkins suggests that this is due to
the similarity in the chemical composition of the protoplasm of the two
individuals. When in nature two individuals that have lived under
somewhat different conditions conjugate, the result should be
beneficial, since there takes place the commingling of different
protoplasms.
Calkins’s work has shown that by means of certain substances much the
same effect can be produced as that which is supposed to follow from the
conjugation of two unrelated individuals. The presumption, therefore, is
in favor of the view that the two results may be brought about in the
same way, although we should be careful against a too ready acceptation
of this plausible argument; for we have ample evidence to show that many
closely similar (if not identical) responses of organisms may be brought
about by very different agencies. The experiments seem to indicate that
paramœcium might go on indefinitely reproducing by division, provided
its environment is changed from time to time. If this is true, it is
conceivable that the same thing is accomplished through conjugation. In
the light of this possible interpretation much of the mystery connected
with the term _rejuvenescence_ is removed, for we see that there is
nothing in the process itself except that it brings the organism into a
new relation with other substances. Difficult as it assuredly is to
understand how this benefits the animal, the experimental fact shows,
nevertheless, that such a change is for its good. That there is really
nothing in the process of conjugation itself apart from the difference
in the constitution of the conjugating individuals is shown by the
result that the union of individuals having the same history and kept
under the same conditions is of no benefit.
Can we apply this same conception to the process of fertilization in the
higher animals and plants? Is the substance of which their bodies are
made of such a sort that it cannot go on living indefinitely under the
same conditions, but must at times be supplied with a new environment?
If this could be established, we could see the advantage of sexual
reproduction over the non-sexual method. It would be extremely rash at
present to make a generalization of this kind, for there are many forms
known in which the only method of propagation that exists is the
non-sexual one. In other words, there are no grounds for the assumption
that this is a necessary condition for all kinds of protoplasm, but only
for certain kinds.
In the insects, crustaceans, rotifers, and in some plants there are a
few species whose egg develops without fertilization. This makes it
appear probable that the particular kind of protoplasm of these animals
does not absolutely require union from time to time with the protoplasm
of another individual having a somewhat different constitution.
There is also an interesting parallel between the effects of solutions
on the protozoans in Calkins’s experiments and certain results that have
been obtained in artificial parthenogenesis. It has been stated, that by
brushing the unfertilized eggs of the silkworm moth a larger percentage
will develop parthenogenetically; and more recently it has been shown by
Matthews that by agitation of the water in which the unfertilized eggs
of the starfish have been placed many of them will begin their
development. It was first shown by Richard Hertwig that by putting the
unfertilized eggs of the sea-urchin in strychnine solutions, they will
begin to segment, and I obtained the same results much better by placing
the eggs in solutions of magnesium chloride. Loeb then succeeded in
carrying the development to a later stage by using a different strength
of the same solution, as well as by other solutions. Under the most
favorable circumstances some of the eggs may produce larvæ that seem
normal in all respects, but whether they can develop into adult
sea-urchins has not yet been shown.
These results indicate that one at least of the factors of fertilization
is the stimulus given to the egg. On the other hand, the lack of vigor
shown by many eggs that have been artificially fertilized indicates that
some other result is also accomplished by the normal method of
fertilization that is here absent. This may mean no more than that as
yet we have not found all the conditions necessary to supply the place
of the spermatozoon.
In our study of the phenomena of adaptation we have found that sometimes
the adaptation is for the benefit of the individual and at other times
for the benefit of the species. May it not be true also that the process
of sexual reproduction has more to do with a benefit conferred on the
race rather than on the individual? In fact, Weismann has elaborated a
view based on the conception that the process of sexual reproduction is
beneficial to the race rather than to the individual. His idea, however,
is not so much that the result is of direct benefit to a particular
species, as it is advantageous to the formation of new species from the
original one. In a sense this amounts, perhaps, to nearly the same
thing, but in another sense the idea involves a somewhat different point
of view.
According to his view “the deeper significance of conjugation” and of
sexual reproduction is concerned “with the mingling of the hereditary
tendencies of two individuals.” In this way, through the different
combinations that are formed, variations which he supposes are
indispensable for the action of natural selection originate. The purpose
of the sexual process is solely, according to Weismann, to supply the
variations for natural selection. If it be asked how this process has
been acquired for the purpose of supplying natural selection with the
material on which it can work, we find the following reply given by
Weismann. “But if amphimixis [by which he means the union of sex-cells
from different individuals] is not absolutely necessary, the rarity of
purely parthenogenetic reproduction shows that it must have a widespread
and deep significance. Its benefits are not to be sought in the single
individual; for organisms can arise by agamic methods, without thereby
suffering any loss of vital energy; amphimixis must rather be
advantageous for the maintenance and modification of species. As soon as
we admit that amphimixis confers some such benefits, it is clear that
the latter must be augmented, as the method appears more frequently in
the course of generations; hence we are led to inquire how nature can
best have undertaken to give this amphimixis the widest possible range
in the organic world.” Nature, Weismann says, could find no more
effectual means of bringing about the union of the sexual cells than by
rendering them incapable of developing alone. “The male germ-cells,
being specially adapted for seeking and entering the ovum, are, as a
rule, so ill provided with nutriment that their unaided development into
an individual would be impossible; but with the ovum it is otherwise,
and accordingly the ‘reduction division’ removes half the germ-plasm and
the power of developing is withdrawn.” It can scarcely be claimed, in
the light of more recent discoveries, that the reduction division takes
place in order to prevent the development of the ovum, for how then
could we explain the corresponding division of the male germ-cells?
Whatever means has been employed to bring about the process of sexual
reproduction, the guiding principle is supposed by Weismann to be
natural selection as stated in the following paragraph: “If we regard
amphimixis as an adaptation of the highest importance, the phenomenon
can be explained in a simple way. I only assume that amphimixis is of
advantage in the phyletic development of life, and furthermore that it
is beneficial in maintaining the level of adaptation, which has been
once attained, in every single organism; for this is as dependent upon
the continuous activity of natural selection as the coming of new
species. According to the frequency with which amphimixis recurs in the
life of a species, is the efficiency with which the species is
maintained; since so much the more easily will it adapt itself to new
conditions of life, and thus become modified.”
Thus we reach the somewhat startling conclusion that through natural
selection the germ-cells and their protozoan prototypes have been
rendered incapable, through natural selection, of reproducing by
non-sexual methods, in order that variations may be supplied for the
farther action of this same process of natural selection. The
speculation has the appearance of arguing in a circle, although if it
were worth the attempt an ingenious mind might perhaps succeed in
showing that such a thing is not logically inconceivable.
It seems strange that a claim of this sort should have been made, when
it is so apparent that the most immediate effect of intercrossing is to
swamp all variations that depart from the average. Even if it were true
that new combinations of characters would arise through the union of the
germ-cells of two different animals, it is certainly true that in the
case of fluctuating variations this new combination would be lost by
later crossing with average individuals. Moreover, it is well known that
variations occur amongst forms that are produced asexually. On the
whole, it does not seem to be a satisfactory solution of the problem to
assume that sexual reproduction has been acquired in order to supply
natural selection with material on which it may work.
Our examination of the suggestions that have been made and of the
speculation indulged in, as to what benefit the process of sexual
reproduction confers on the animals and plants that make use of this
method of propagation, has failed to show convincingly that any
advantage to the individual or to the species is the outcome. This may
mean, either that there is no advantage, or that we have as yet failed
to understand the meaning of the phenomenon. The only light that has
been thrown on the question is that a certain amount of renewed vigor is
a consequence of this process, but we cannot explain how this takes
place. There is also the suggestion that the union of different cells
produces the same beneficial effect as a change in the conditions of
life produces on the organism. The bad effects of close interbreeding
that seem sometimes to follow is explicable on this view. This, it seems
to me, is the most plausible solution of the question that has been
advanced; but, even if this should prove to be the correct view, we need
not assume that the process has been acquired on account of this
advantage, for there is nothing to show that it has been acquired in
this way.
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