sow in question be the result of a formative stimulus, exercised by
the sperm-cells of the unrelated boar upon the ova of the female
animal? If the progeny of the sow had been more fertile than herself,
then we should have been at least logically justified in concluding
that this was the case, but it is not intelligible that the egg-cells
of this mother sow should be increased twice or three times because
they were fertilized by a new kind of sperm as they glided from the
ovary. The number of ova which are liberated from the ovary depends
in the first instance upon the number of mature ova contained in
it; and unless we are to make the highly improbable assumption that
the crossing with the strange boar had as an immediate result the
maturing of a large number of ova, we must look elsewhere than in the
ovary of the animal for the cause of this sudden fertility, possibly
in chance circumstances which we are unaware of and which make the
ovary occasionally more productive, possibly however in the fact that
inbreeding may have brought about various slight structural variations
in the animal, and among these some which made the fertilization of
the abundantly produced ova by the sperm of the related boar less
easy, and caused it to fail more frequently. As will be readily
understood, I cannot say anything definite on this point, but we know
that very slight variations in the sperm-cell or the ovum may make
fertilization difficult, or may even prevent it. I need only remind
you of the interesting experiments in hybridization which Pflüger
and Born made with Batrachians nearly thirty years ago, which showed
that in two nearly related species of frog the ova of the species A
were frequently fertilized by the sperms of the species B, but not
conversely, the ova of the species B by the sperms of A. This is the
case, for instance, with the green edible frog (_Rana esculenta_),
and the brown grass-frog (_Rana fusca_), and the reason of this
dissimilarity in the effectiveness of the sperm lies simply in 'rough
mechanical conditions,' in the width of the micropyle of the ovum,
and the thickness of the head of the spermatozoon. If each species
possesses a micropyle which is exactly wide enough to admit of the
passage of the spermatozoon of its own species, another species will
only be able to fertilize these eggs if the head of its spermatozoon
be not larger than that of the first species. Thus, as experiment has
proved, the spermatozoa of _Rana fusca_ fertilize the ova of almost
all other related species, for they have the thinnest head and it is
at the same time very pointed. In this case, therefore, it depends
upon the microscopic structure of the ovum whether fertilization can
take place or not, and we can imagine that similar or perhaps other
minute variations had taken place in the ova in the case of Nathusius's
sow, and that these made it difficult for the sperms of boars of the
same family to effect fertilization. These variations may have arisen
as a result of the continued inbreeding, because the same ids were
constantly being brought together in the fertilized ova, and thus any
unfavourable directions of variations which existed were strengthened.
It seems to me that in this way alone can the injurious effect of
inbreeding be made intelligible. From both parents identical ids meet
in the fertilized ovum, in greater numbers the longer inbreeding
continues, for at the maturation of every germ-cell the number of
different ids is diminished by a few, and their number must therefore
gradually decrease, and it is conceivable that ultimately it may
sink to one kind of id, that is, that the germ-plasm may then consist
entirely of identical ids. If chance variations of certain determinants
in unfavourable directions occur in some of the ids composing the
germ-plasm, these are brought together in the offspring from both the
maternal and the paternal side, and will occur in an increasing number
of ids the longer the inbreeding has gone on, that is, the smaller the
number of different ids has become. The unfavourable variation-tendency
is therefore persisted in, and its influence upon the development of a
new descendant will be the greater the larger the number of identical
ids with these unfavourable variations. It is obvious that the crossing
of an animal, which is thus, so to speak, degenerating slightly, with
a member of an unrelated family must immediately have a good effect
upon the descendants, for in this way quite different ids with other
variations of their determinants are introduced into the inbred
germ-plasm which had become too monotonous.
From this theoretical interpretation of the injurious consequences of
inbreeding we may at once infer that not every inbreeding necessarily
implies degeneration, for the occurrence of unfavourable variational
tendencies in the germ-plasm is presupposed as the starting-point of
degeneration, and if these do not exist there can be no degeneration.
This harmonizes with the fact that the evil effects of inbreeding are
observed _to vary greatly in amount, and may not occur at all_. But
they are greatest in breeds artificially selected by man, which have
long been under unnatural, directly influential conditions, and are
also removed from the purifying influence of natural selection. In such
cases, therefore, there is every probability that diverse unfavourable
variational tendencies in the determinants will occur.
But how are we to understand the fact that pure parthenogenesis
may last through innumerable generations, and yet no degeneration
sets in? I believe very simply. In this case too, the same ids
which were peculiar to the mother of the race are contained in the
descendants, but they do not diminish in number, for in pure and
normal parthenogenesis, such as that of _Cypris reptans_, the second
maturation-division of the ovum does not take place, and this is
precisely the nuclear division which effects reduction. In addition,
the introduction of identical ids, which must take place in the case of
inbreeding at every amphimixis, does not occur, and, what is certainly
of great importance, all these cases are old species, living under
natural conditions--the same conditions under which they lived as
amphigonous species, and not newly formed breeds under artificial
conditions, as has probably always been the case in the experiments in
inbreeding.
It is true that even in old species, living in a state of nature,
unfavourable variations may arise in the germ-plasm, and may go on
increasing during purely parthenogenetic multiplication, for the ids
with unfavourably varying determinants will no longer be set aside
by means of reducing division. But those individuals in which the
unfavourable variational tendency increases until it has attained
selection-value will be subject to selection and will be gradually
eliminated; indeed, the weeding out of the inferior individuals will be
more drastic here than where amphigony obtains, because in this case
all the offspring of one mother are nearly alike, so that the whole
progeny is exterminated if the mother varies unfavourably.
On the other hand, a transformation in a favourable direction, an
adaptation to new conditions of life, as far at least as that implies
the simultaneous variation and harmonious co-adaptation of many parts,
cannot, as far as I can see, be effected in the course of purely
parthenogenetic reproduction, nor can a degeneration of complicated
parts which have become superfluous. For both these changes, in my
opinion, require that the ids of the germ-plasm should be frequently
mingled afresh, since apart from this there cannot be a harmonious
readjustment of complicated structures, nor can a uniform degeneration
affecting all parts set in. As an example of this last case we may take
that organ which became functionless in the purely parthenogenetic
species of Ostracods when amphigonous reproduction was given up--the
sperm-pocket or receptaculum of the female. All these species still
possess an unaltered receptaculum seminis, a large pear-shaped bladder
with a long, narrow, spirally coiled entrance-duct, very well adapted
for allowing the enormous spermatozoa of the males to make their way
in singly, and to arrange themselves within the receptacle side by
side in the most beautiful order, like a long ribbon, and finally to
migrate out again singly to fertilize the liberated ova. In _Cypris
reptans_ and several other species, however, no males have been found
in any of the places which have been carefully searched, and the
receptaculum of the female is always found to be empty. Nevertheless
it shows no hint of degeneration. It is possible enough that, as in
_Apus cancriformis_, which is of similar habit, the males have become
extinct in most colonies of these species, but that nevertheless they
do occur here and there from time to time in the area inhabited by the
species, and if this should prove to be the case, it would confirm the
conclusion, which is very probable on other grounds, that the pure
parthenogenesis of these species has not existed in most of their
habitats for a long time, speaking phylogenetically. For this reason we
must not over-estimate the significance of the complete persistence of
the receptaculum even with exclusively parthenogenetic reproduction.
It proves, however, that degeneration of a superfluous organ does
not necessarily set in even after hundreds of generations, and in
this fact there is certainly a corroboration of the view that it is
'chance' germinal variations which give the impulse to degeneration.
These first induce a downgrade variation through germinal selection,
and this, if it concerns an organ of no importance to the survival of
the species, is not hindered in its progress by personal selection.
Whether degeneration of the receptaculum would have occurred in
these parthenogenetic species if they had retained even a periodic
sexual reproduction, as is actually the case in the generations of
the alternately parthenogenetic and sexually reproducing Aphides,
we cannot decide, since we know nothing in either case as to the
length of time that parthenogenesis has prevailed among them, nor
have we any method of computing the number of generations that must
elapse before a superfluous organ begins to vacillate. We only know
that the parthenogenetic generations of Aphides no longer possess
a receptaculum, while other forms with alternating bi-sexual and
parthenogenetic modes of reproduction, which are in this respect
possibly more modern, e. g. some of the gall-wasps, possess one similar
to that of the Ostracods.
[Illustration: FIG. 79 (repeated). The two maturation divisions of the
'drone eggs' (unfertilized eggs) of the bee, after Petrunkewitsch.
_Rsp_ 1, the first directive spindle. _K_ 1 and _K_ 2, the two
daughter-nuclei of the same. _Rsp_ 2, the second directive spindle. _K_
3 and _K_ 4, the two daughter-nuclei. In the next stage _K_ 2 and _K_ 3
unite to form the primitive sex-nucleus. Highly magnified.]
I must refer to one other case of parthenogenesis, since it has been
hitherto regarded as a formidable puzzle for the germ-plasm theory,
and has only recently found its solution, I mean the facultative
parthenogenesis of the queen-bee. As the 'male' eggs of the bee remain
unfertilized, and yet undergo two reducing divisions, which must
diminish the number of ids in the ovum-nucleus by a half, the number of
ids in the germ-plasm of the bee must be steadily decreasing, and this
state of things has therefore been regarded by some English biologists
as convincing evidence of the untenability of the conception of ids and
of the whole germ-plasm theory. Apparently, indeed, it is contradictory
to the theory, and we must inquire whether the contradiction is merely
an _apparent_ one, disappearing when the facts are more precisely
known. It was mainly on this ground that I instituted the researches
carried out by Dr. Petrunkewitsch, the results of which I have already
in part communicated in a former lecture. These results confirmed
the previous conclusions that the 'male' eggs of the queen-bee
remain unfertilized, that two reducing divisions occur, and that in
consequence the ovum-nucleus only contains half the normal number of
chromosomes. That these increase again by division to the normal
number does not save the theory, for only _identical_ ids can arise in
this way, while the significance of the multiplicity of the ids lies
mainly in their difference. The halving of the number of ids in each
'male' ovum would necessarily lead, if not to a permanent diminution
in the number of ids, at least to a monotony of the germ-plasm, since
the number of _different_ ids would be steadily decreasing and the
number of _identical_ ids as steadily increasing. This too would be a
contradiction of the theory. But Dr. Petrunkewitsch's investigations
have shown that, of the four nuclei which are formed by the two
reducing divisions, the two middle ones (Fig. 79, _K_ 2 and _K_ 3)
recombine with one another, and fuse into a single nucleus, and that
from this copulation-nucleus in the course of development the primitive
germ-cells of the embryo arise. Now all the ids which were originally
present in the nucleus of the immature ovum may be reunited in this
'polar copulation-nucleus' if the two nuclei _K_ 2 and _K_ 3 turned
towards each other in Fig. 79 contain different ids. That this is the
case cannot of course be seen from the ids themselves, but it seems to
me extremely probable, since it is dissimilar poles of the two nuclear
spindles which here unite, namely, the lower pole (daughter-nucleus) of
the upper spindle and the upper pole of the lower spindle. In the first
directive or polar spindle there lay thirty-two chromosomes, which had
increased by duplication from sixteen, and of these sixteen passed over
into the first polar nucleus, while sixteen formed the basis of the
second directive spindle. These two sets of sixteen chromosomes must
have been quite similar, since the two sets arose by division of the
sixteen mother-chromosomes. Let us call the chromosomes _a_, _b_, _c_,
_d-q_, then similar sets of chromosomes must have been contained in the
two nuclear spindle figures depicted in Fig. 79 at the beginning of the
division, and eight of these went to each daughter-nucleus. Now, if
_a-k_ migrated to the upper pole of the spindle and _l-q_ to the lower
pole, then the union of _K_ 2 with _K_ 3 would bring together again
all the ids that had before been present. In consideration of this I
predicted to Dr. Petrunkewitsch that this copulation-product might be
the basis of the formation of the germ-cells in the drone-bee, and his
painstaking and difficult researches have confirmed this prediction,
strange though it may seem, that the male germ-cells have a different
origin from the female germ-cells. But this discovery gives a strong
support to the germ-plasm theory. It may, of course, be objected that
the assumed regular distribution of the ids in the two daughter-nuclei
cannot be proved, but we know already that this dividing apparatus
does _very exact_ work, and we are at liberty to assume it in an even
higher degree. Moreover, what other interpretation of the unexpected
development of the germ-cells discovered by Petrunkewitsch could be
given if this had to be rejected? A clearer proof of the individual
differences of ids and of their essential importance could not be
desired, than lies in the fact that in the 'male' eggs of the queen-bee
a different and novel mode of germ-cell formation is instituted, after
half the ids have been irrecoverably withdrawn from the ovum-nucleus.
We see from this that for individual development a duplication of
individual ids may suffice, but that for the further development of the
species a retention of the diversity of the ids is important.
LECTURE XXX
INBREEDING, PARTHENOGENESIS, ASEXUAL REPRODUCTION, AND THEIR
CONSEQUENCES
The separation of the sexes exists even among the
Protozoa--Conditions determining the occurrence of
Hermaphroditism--Tape-worms, Cirrhipeds--Primordial males--Advantages
of parthenogenesis--Alternation with bi-sexual generations--In
Gall-wasps--In Aphides--Cross-fertilization secured in
plants--Self-fertilization is avoided whenever possible--The
mechanism of fertilization and the mingling of germ-plasms must
be clearly distinguished from one another--Cases of persistent
self-fertilization--The effects of inbreeding compared with those
of parthenogenesis--The effect of purely asexual reproduction--In
sea-wracks--In lichens and fungi--In cultivated plants--Degeneration
of the sex-organs--Summary.
We have seen that continued inbreeding must make the germ-plasm
monotonous, and therefore unplastic as regards the requirements of
adaptation. Accordingly, we found that the gametes of many unicellulars
are so constituted that they only possess a power of attraction for
gametes of a different lineage, not for those of their own stock. Among
multicellular organisms the most intense mode of inbreeding is to be
found in the uninterrupted self-fertilization of hermaphrodites: in
such cases the monotony of the germ-plasm must reach extreme expression
more readily than in the case of ordinary inbreeding. We can thus
understand why, in the scale of organisms, there is such an early
occurrence of gonochorism, the separation of the species into male and
female individuals. Even among unicellular plants or Protophytes this
occurs occasionally, as it does in the Vorticellids among Infusorians.
In the Metazoa and Metaphyta the separation of the sexes finds emphatic
expression; it is absent from no important group, and in many, such
as, for instance, among the Vertebrates, it has become the absolutely
normal condition, with hardly any exception. But in many divisions of
the animal and plant kingdoms hermaphroditism also plays an important
part, as, for instance, in terrestrial snails and in flowering plants.
Obviously the sexual adaptations of a species are definitely related
to the conditions of its life, and, though Nature's endeavour to
prevent inbreeding and to secure cross-fertilization is evidenced by
the occurrence of separate sexes in such a multitude of forms yet
in many cases gonochorism has been relinquished, and always where
this was necessitated by the conditions of life to which the group
concerned was subject. In such a case inbreeding is regulated as
far as possible, for instance, by an arrangement which ensures that
individuals shall be crossed at least from time to time. But cases of
exclusive and constant self-fertilization do also seem to occur, and
even these may be brought into harmony with our conception, according
to which cross-fertilization is an advantage, but only an advantage
which must be weighed against others, and which may eventually be given
up in favour of greater advantages. This occurrence of persistent
autogamy can no more be reconciled with the rejuvenation theory than
can continuous parthenogenesis, because, according to this theory,
the mingling of different individuals is a _sine qua non_, for the
continued life of the species.
It is impossible for me here to discuss in detail all the deviations
from pure gonochorism or bi-sexuality which occur in nature, but I must
at least attempt to take a general survey, and to arrange the chief
phenomena of these various modes of 'sexual reproduction' in an orderly
scheme. I must take a survey of both plants and animals, but I shall
give the precedence to animals, as being to me more familiar ground.
Where do we find, in the animal kingdom, that Nature has departed from
gonochorism, from the separation of the sexes, and for what reasons was
this departure necessary? And further, what means does Nature take to
compensate for this renunciation of the simplest method of securing the
continual cross-fertilization of individuals?
Let us glance over the animal kingdom with special reference to these
questions: we find that hermaphroditism prevails chiefly among species
which at maturity have lost their power of free locomotion, and have
become sedentary, such as oysters, barnacles among Crustaceans, the
Bryozoa, and the sea-squirts (Ascidians) which are fixed to the rocks
at the bottom of the sea. For forms such as these it must often have
been advantageous that each individual could function both as male
and as female, especially when it was capable of self-fertilization,
since individuals which settled down singly, or in very small numbers
together, would not be lost as regards the persistence of the species.
The continuance of the species is thus better secured than it would
be by separation of the sexes, because in the latter case it might
frequently have happened that the animals which had settled beside
each other by chance were of the same sex, and would therefore remain
unfertile. But many of these species do not fertilize themselves, but
fertilize each other mutually; and this, too, carries a great advantage
with it, because in sedentary animals the sperms will fertilize
twice as many individuals, if each contains eggs, than if half were
exclusively male. It is thus to some extent an economy of sperms, but
at the same time also of ova, which is effected by hermaphroditism:
the result is that these valuable products are wasted as little as
possible. On this account we find that not only sedentary, but also
sluggish, slow-moving animals are equipped with male and female organs
of reproduction, as, for instance, all our terrestrial snails. They
fertilize each other mutually: when two meet it is always as males and
females, and notwithstanding the sluggishness of movement, it is not
likely to happen that a snail does not attain to reproduction because
it has not found a mate. The same is true of the earthworms, which
are likewise not adapted for making long journeys in search of the
opposite sex; they, and the leeches also, function as male and female
simultaneously, while their nearest relatives, the marine Chætopods,
are of separate sexes, which may be associated with their much greater
power of free movement in the water.
In these cases self-fertilization is often absolutely excluded; it
may be physically impossible, and hermaphroditism therefore secures
cross-fertilization in such cases just as effectively as if the sexes
were separate. Similarly, in many hermaphrodite flowers, as we have
already seen, the pollen is so constituted and so placed within the
flower that it cannot of itself make its way to the stigma. In oysters,
for instance, the young animal is male, and liberates into the water an
enormous quantity of minute spermatozoa, and therewith fertilizes the
older individuals, functioning only as females, which have grown upon
the same bank. At a later stage of its development the oyster which was
male becomes female, and produces only ova. This state of affairs, of
which I shall shortly mention another case, has been called _temporary_
hermaphroditism. In this case not only is self-fertilization excluded,
but close inbreeding also, since it is always a young generation
functioning simultaneously as males that mingles with an older
generation which has become female.
It is quite otherwise with parasites which live singly within the body
of a host: for these it was indispensably necessary that they should
not only produce both kinds of germ-cells, but that they should unite
the two kinds in fertilization, and they therefore possess the power
of self-fertilization. Thus, in the urinary bladder of the frog, there
occurs a flat-worm (_Polystomum integerrimum_) which possesses special
organs for pairing with another individual, but which is also capable
of self-fertilization when, as frequently occurs, it has no companion
in its place of abode. But this self-fertilization is always liable to
be interrupted by cross-fertilization, for not infrequently there are
two, three, or even four such parasites within the bladder of a single
frog.