and at the same time vital, organization is able to quicken, to
strengthen the metabolism, to bring about 'physiological regeneration,'
or whatever we may choose to call it, we do not yet understand. It has
been said that in plastogamy an exchange of 'substances' takes place;
that each gives to the other the substances which it possesses and
the other lacks, and that this causes an increase of vital energy.
But it is unlikely that we have here to do merely with chemical
substances, although these, of course, as the material basis of all
vital processes, are indispensable; it seems to me more probable that
the vital units (biophors) themselves in their specific individuality
must play the chief part. But even this is saying very little, for we
have not yet reached an understanding of these processes, and if we
were not forced by the fact of plastogamy to the conclusion that this
union must have some use, no one would have been likely to postulate it
as useful, still less as necessary. It has, of course, been frequently
suggested that multiplication by fission, if long-continued, results
in 'exhaustion,' and that this is corrected by amphimixis, but who
can tell why this 'exhaustion' might not be remedied, and even more
effectually remedied, by a fresh supply of fuel, that is, of food?
One might have thought that the vital processes would be thus more
readily recuperated than by the co-operative combination of two already
'exhausted' cells. Two exhausted horses may perhaps be able to pull the
load that one of them was no longer equal to, but in the case we are
considering it is the combined burdens of two units that have to be
borne, although each was no longer equal to its own share! That is more
than we can understand.
Zehnder has recently defined the effect of amphimixis as a
'strengthening of the power of adaptation,' and he infers that the
'digestive fistellæ' (Biophors) of two individuals, which have
somewhat different powers of digestion, are, when they combine, able
to assimilate more kinds of food than either was able to assimilate
by itself. But I confess that I do not see how an advantage for the
whole would be gained through this alone, since half of the digestive
biophors would have to work for the nutrition of the mass of the
individual _A_, the other half of the differently constituted biophors
for that of the individual _B_, and the nutritive capacity would thus
remain exactly what it was before conjugation. Nevertheless I believe
that Zehnder was right in his supposition that conjugation is concerned
with strengthening the power of adaptation, and I have long maintained
and defended this interpretation with regard to true amphimixis in
nucleated organisms. In these cases it is quite obvious that the
communication of fresh ids to the germ-plasm implies an augmentation
of the variational tendencies, and thus an increase of the power
of adaptation. Under certain circumstances this may be of _direct_
advantage to the individual which results from the amphimixis, but
in most cases the advantage will be only an indirect one, which may
not necessarily be apparent in the lifetime of this one individual,
but may become so only in the course of generations and with the aid
of selection. For amphimixis must bring together favourable as well
as unfavourable variations, and the advantage it has for the species
lies simply in the fact that the latter are weeded out in the struggle
for existence, and that by repetition of the process the unfavourable
variational tendencies are gradually eliminated more and more
completely from the germ-plasm of the species.
But this cannot have been the efficient cause in the introduction of
amphimixis into the series of vital phenomena; the reason for this
must be found in some _direct_ advantage, such as that it improved and
increased the assimilating power, the growth, and the multiplication
of the particular individual, so that it gained an advantage over
individuals which had not entered into conjugation. This advantage must
exist, at least in the lower forms of conjugation, in pure plastogamy,
i. e. in the mere coalescence of the protoplasmic bodies. But, as it
seems to me, we have not yet clearly recognized what the advantage
precisely is; we do not yet see how such a mingling or combination
of two plasms should every time be of advantage for the combined
conjugate. If we assume with Zehnder that two kinds of 'nutritive'
biophors are brought together which differ slightly from each other
in digestive capacity, three cases may occur. Either the food _a_,
adequate for the animal _A_, is just as abundant as the food _b_,
suitable for the animal _B_, and then half the conjugated animal will
be nourished by means of the biophors _a_, the other half by means
of the biophors _b_, and the state of matters is the same as it was
before conjugation; or the food _b_ is more abundant than the food
_a_, or conversely, and then the biophors _b_ will have to take the
larger share in the nourishment of the conjugate _A_ + _B_, and they
will therefore multiply more rapidly and the biophors _a_ will decrease
relatively in number. Nutrition and growth will then go on more slowly
for a time, but will soon attain to their former intensity. The
combined individual _A_ + _B_ has then certainly gained an advantage
over the isolated animal _A_, and the living substance of _A_ which,
if left to itself would probably have perished, can continue to live
in combination with _B_. But in that case it is not obvious where
the advantage in the union can lie, as far as _B_ is concerned. An
advantage to _B_ only results if there be a combination not of _one_
kind of biophor only, but of several or many kinds of biophors. If for
instance _A_, whose digestive biophors were weak, brought with it into
the partnership 'secretory' or nervous biophors stronger than those of
_B_, then there would be an advantage for both in the combination, and
it is thus that, in the meantime, I interpret the direct benefit which
results from pure plastogamy. This benefit must be the more important
and far-reaching the longer multiplication by fission continues without
the occurrence of conjugation.
We thus reach what is perhaps a not wholly unsatisfactory conception of
amphimixis, in so far at least that we do not require to assume that
there has been a fundamental change in its significance between its
expression in the lowest organisms and in the higher and even highest
forms. Everywhere it is the same advantage: an increase in the power of
adaptation; but it sometimes finds expression directly in the product
of conjugation, sometimes only indirectly, sooner or later, among the
descendants of the product.
How far below the Myxomycetes pure plastogamy reaches we do not know;
whether it also occurs among non-nucleated organisms (Haeckel's
Monera) we cannot tell from experience, since these assumed organisms
have not yet been observed with certainty. Perhaps they all lie
below the limits of visibility, and then we could never do more than
_suppose_ that plastogamic processes occur among them. Logically and
purely theoretically we may _suppose_ that amphimixis occurred first
between the plasmic bodies of non-nucleated Monera, then between the
cell-bodies of true cells, and finally between the nuclei of cells.
Let us hold fast to what we have found to be probable, namely, that
the fusion of individually different simple organisms must or may
bring about a direct advantage--a stimulation of the metabolism, and
at the same time an improvement of the constitution in different
directions, and let us go on to the consideration of cell-fusion
combined with nuclear fusion, or complete amphimixis. In this something
is added which we can recognize as an important advantage, namely,
the combination of two hereditary substances, and thus the union of
two variation-complexes which, according to our view, is necessary if
transformation of species is to take place. In mere plastogamy such a
union of two hereditary masses could only take place in Monera, not in
nucleated organisms. If then there are really unicellular organisms
which exhibit plastogamy without karyogamy (certain Foraminifera),
we have a further proof that these processes of plasmic fusion imply
direct advantage, which is distinct from the indirect advantage lying
in the mingling of two different hereditary contributions, since
in these cases of plastogamy there is no demonstrable mingling of
hereditary bodies, no karyogamy.
But as soon as karyogamy or nuclear fusion was associated with mere
plastogamy, complete amphimixis could never be lost again, because it
alone made it possible that there should be harmonious transformation
and adaptation in organisms which were becoming ever more complex; the
primary effect of the mingling would be more and more transcended,
since, without amphimixis, transmutation with harmonious adaptation
in all directions would be less and less possible as organisms became
more complex in structure. I have already referred to the manifold
details in the structure and development of the lowest organisms which
make this conclusion appear luminous to us, but we can also infer the
necessity for an unceasingly active selection, from a quite different
set of facts, namely, from what we know of rudimentary organs in Man.
We may regard Mankind as a species which has its local races and
sub-races, but which is fixed in its essential characters, and only
fluctuates hither and thither in individual variation in each sub-race,
just like any other modern mammal, such as the marmot or the hare.
Nevertheless we know that Man, as regards certain fairly numerous
parts, is continually and persistently varying in a definite direction.
Wiedersheim, in his book _On the Structure of Man_[23], enumerates a
long series of parts and organs of the human body, which are in process
of gradual degeneration, and of which it may be predicted that they
will disappear from the human structure since they have lost functional
significance. Among these dwindling structures are the two last ribs,
the eleventh and twelfth, while the thirteenth has already disappeared,
and only occurs exceptionally as a small vestige in the adult human
being of to-day. The series includes also the seventh cervical rib,
the _os centrale_ of the wrist, the wisdom teeth, and the vermiform
appendix of the intestine. The last is much larger in many mammals,
and represents an important part of the digestive apparatus, but in
Man it has dwindled to an unimportant appendage, which is a source of
danger when foreign bodies (cherry stones and such like) lodge in it
and set up inflammation. The variations in its length warrant us in
concluding that it is still in process of degeneration; its average
length is about 8½ cm., but it varies from 2 cm. to 23 cm. in length,
and in about 25 per cent. of cases a partial or entire closing up of
its opening into the intestine may be observed.
[23] _Ueber den Bau des Menschen_, 2nd ed., Freiburg-i.-Br., 1893.
Trans. London, 1896.
Wiedersheim enumerates nearly a hundred parts thus in process of
degeneration: this means that nearly a hundred structures in Man
are at the present time in process of variation, and this could not
be so unless amphimixis were continually mingling the hereditary
contributions anew from generation to generation, so that the
minus-variations of the parts in question, starting from the germ-plasm
in which they arose at one time as chance variations, and confirmed in
their direction by means of germinal selection, are gradually being
transmitted to all the germ-plasms of the species. We thus see that
even in a period of species-life, which we may fairly call a period of
constancy, variations of a phyletic kind are continually in process,
which could not become general without the co-operation of amphimixis.
Now, we have already seen that personal selection plays no part, or, at
least, no important part in such degenerations, because the variations
which are here concerned do not usually attain to selection value,
but it is just such variations proceeding with infinite slowness that
occur in functionally important organs likewise, and in the progressive
advance of which personal selection and mutual adaptation probably play
a part, so that in this way we can understand why the preservation of
amphigony by natural selection must be effected. It is impossible--for
obvious reasons--to name particular instances with certainty, as we
can do in the case of the rudimentary organs, but even on general
considerations we might expect that among the incipient variations
of the determinants of the germ-plasm there would be some which were
in an ascending direction, and that among these there would be some
which, advanced by germinal selection, would go on ascending until
they attained selection value. Wiedersheim reckons, for instance,
the gradually increasing differentiation of the cortical zone of the
human brain among the parts which are still in process of ascending
variation, and he is probably right in doing so.
But if variations, so slow as to be unnoticeable, are still of abundant
occurrence in Man, we have no reason to doubt that similar processes
are going on in other animals; among the higher Vertebrates at least
there is hardly a species which does not exhibit regressive variations
even now, and in many cases progressive variations also are occurring,
although we cannot give definite proofs of this.
The appearance of fixity which most species have is, therefore,
illusory; in reality they exhibit a slow flux, gradually setting aside
the superfluities they received from their ancestors, perfecting the
important parts to more precise adaptation and greater functional
capacity, and at the same time endeavouring to maintain all the parts
in constant harmony. We can understand that as long as this process of
gradual perfecting goes on, amphimixis will not readily be given up.
Those that retain it must always, in the long run, have the preference.
Moreover, as we have seen, _it cannot be given up_, when it has existed
through æons, because of the power of persistence which the germ-plasm
has gradually acquired in the course of such a long hereditary
succession. It could only be given up if an advantage decisive as to
survival were associated with its abandonment, such as can be actually
recognized in most cases of parthenogenesis, among animals at least.
In my opinion this indirect effect of amphimixis, that is, the
increasing of the possibilities of adaptation by new combinations of
individual variational tendencies, is the main one, while the direct
nutritive effect of the two germ-cells upon one another is quite
subsidiary. In this opinion I find myself in opposition to the views of
many if not most naturalists, who assume that amphimixis has a direct,
sometimes, indeed, only a direct effect, and believe that they can
prove it by facts.
In support of this position it has been pointed out that allogamy, that
is, the mingling of individuals of different ancestry, occurs even
among lowly unicellulars, and then higher up among most organisms;
but the question has not been asked whether this mutual attraction of
the unlike really expresses a primary characteristic of organisms,
and may not possibly be a secondary acquisition adapted to ensure the
occurrence of amphimixis. If we examine the facts we find that even in
the lowly Algæ, such as _Pandorina_ and _Ulothrix_, only the migratory
cells or swarm-spores of different cell-colonies conjugate with one
another, but not those of the same lineage, and this phenomenon may be
observed in many unicellular plants and animals. We are justified in
concluding from this that a fairly large degree of difference between
the conjugating gametes secures the best results, whether this result
is to be looked for in a 'rejuvenation' or in an increased adaptive
capacity; but it is erroneous to regard the stronger attraction
between individuals of different descent as a direct outcome of this.
To me, at least, it seems to be an adaptive arrangement. The whole
of the long and complex phylogenetic history of the sex-cells, the
gametes, shows clearly that we have here to deal with a succession of
adaptations, and that the degree of attraction which obtains between
gametes has gradually been increased and specialized in the course of
the phylogeny. I need only briefly recall what we have discussed in a
former lecture, that at first the copulating cells were exactly alike
in appearance and size, that then one kind of cell became rather larger
than the other, and that only gametes which were thus different in size
were mutually attractive--the micro-gametes and the macro-gametes,
or male and female germ-cells; we have seen that these differences
between the two became more and more accentuated, that the female
cell continued to grow larger than the male, and to accumulate more
and more nutritive material for the building up of the young organism
which arises from its union with the male cell, and that the male cells
became smaller, but more numerous, as was essential if their chances of
finding the often remote female cell were not to disappear altogether.
And besides, there are all the innumerable adaptations of the egg-cell
to the countless special circumstances which obtain in the different
groups, and the innumerable varieties in the form of the sperm-cell,
with all its delicate and complicated adaptations to the special
conditions under which the egg-cell can be reached and fertilized in
this or that group of organisms. Of a truth, he is past helping who
does not regard with wonder and admiration the adaptations which have
been worked out in this connexion in the course of evolution! But if
all these details are adaptations, so is the beginning of the whole
process of differentiation; allogamy, the attraction of conjugating
cells of different lineage, is not a primary outcome of individual
diversity; gametes of different descent did not strongly attract each
other of themselves, but they were equipped with a strong power of
mutual attraction, because the union of very different individualities
was the more advantageous.
This is an important distinction, for the adaptation to allogamy is
widely distributed, and its latest manifestations have frequently
been misunderstood in the same way as its beginnings. The widespread
occurrence of allogamy has been interpreted as evidence in favour of
the rejuvenation theory, and the endeavour on Nature's part to secure
the union of the unlike has been associated with the hypothetical
'rejuvenating' power of amphimixis, and regarded as a direct and
inevitable outcome of this. That this view is erroneous we shall see
even more clearly from what follows.
As among unicellular Algæ it is frequently only gametes of different
lineage which conjugate, so among animals and plants there are numerous
cases in which the union of nearly related gametes is more or less
strictly excluded, both by the prevention of self-fertilization
(autogamy) in hermaphrodites, or by the prevention of inbreeding, that
is, the continued pairing of near relatives. Now all the preventive
measures which effect this are of a secondary nature; they are
adaptations which result from the advantage involved in the mingling of
unrelated germ-plasms, even though it sometimes seems as if they were
an outcome of the primary nature of the germ-cells.
The primary result of the mutual chemical influence of the two
germ-cells upon one another is--apart from the impulse to development
which the centrosphere of the sperm-cell supplies--as far as I see,
only the more favourable or the more unfavourable mingling of the
biophor- or determinant-variants, and the resulting increase or
decrease in adaptive capacity, which leads to the better thriving of
the offspring, or conversely to its degeneration. Everything else
is secondary and depends upon adaptation, effected in very diverse
ways, to secure the most favourable mingling of the germ-plasms for
the particular species concerned. Undoubtedly the parental ids united
through amphimixis have an effect upon each other, since throughout the
building up of the organism of the child the homologous determinants
struggle with one another for food, but they do not affect each other
in the way that many prominent physiological and medical writers
suppose, namely, that the union of the parental germ-plasms sets up a
'formative stimulus' which 'advances' or even 'greatly advances' the
process of development in the egg.
Parthenogenetic development goes on just as rapidly, sometimes even
more rapidly than that of the fertilized ova of the same species! How
can the supposed 'formative stimulus' be so entirely dispensed with in
this case?
Of course I am well aware that the two kinds of germ-cells have a
strong attraction for each other, and that the protoplasm of the ovum
actually exhibits tremulous movement when the spermatozoon penetrates
through the micropyle. I myself observed this in the case of the
lamprey (_Petromyzon_) when Calberla instituted his investigations on
the fertilization of that animal, but has that anything to do with
a formative stimulus? Is it anything more than the result of the
chemotactic stimulus exerted by the substance of the ovum upon that
of the spermatozoon and conversely? And have we any ground for seeing
anything more in this than an adaptation of the sex-cells to the
necessity of mutually finding each other out and thereafter combining?
Two quite different things are often confused with one another in this
connexion: the mutual attraction of the two kinds of sex-cells which
tends to secure their union, and the results of this union. A more
exact distinction is necessary between the effects and the advantages
which allogamy brings in its train and the means by which it is secured
in the different species.
If amphimixis really set up a 'formative' stimulus, and if the amount
of this was regulated by the differences between the two parental
germ-plasms, then parthenogenesis, which implies the entire absence
of the mingling of two parental cells, would necessarily be even less
advantageous than amphimixis between near relatives; but this is not
the case. Continued inbreeding leads in many cases to the degeneration
of the descendants, and particularly to lessened fertility and even
to complete sterility. Thus in my prolonged breeding experiments with
white mice, which were later carried on by G. von Guaita, strict
inbreeding, effected throughout twenty-nine generations, resulted in
a gradually diminishing fertility, and similar observations have been
made by Ritzema Bos and others. But why does not the same thing happen
in pure parthenogenesis? My experiments in breeding parthenogenetic
Ostracods (_Cypris reptans_) shows that these crustaceans, in the
course of the eighty generations which I have observed till now[24],
have lost nothing of their prolific fertility and vital power; and
the same is true in free nature of the rose-gall wasp (_Rhodites
rosæ_), which enjoys the greatest fertility notwithstanding its purely
parthenogenetic reproduction, the females not infrequently laying
a hundred eggs in a single bud. How does it happen that 'the mutual
influence of two different hereditary substances which so powerfully
promotes individual development' can be here altogether dispensed with?
Only because it does not really exist, except in the imagination of my
opponents, still influenced by the old dynamic theory of fertilization.
[24] The cultures were begun in 1884 and are still continued (March 6,
1902), still multiplying as abundantly as at the outset. I reckon that
there are on an average five generations in a year, which means about
eighty generations in sixteen years.
But it may be asked, whence come the injurious results of inbreeding,
if not from the union of two nearly related germ-plasms? They certainly
do arise from that cause, but it is not through a 'formative stimulus,'
_too slight_ in this case, exercising a direct formative chemical
effect upon the two hereditary substances, but through the indirect
influences exerted by these _too similar_ hereditary contributions
during the development of the new individual. Lest it be imagined that
I am tilting against windmills, I will refer to one of the numerous
examples of the evil effects of inbreeding which have been submitted
to me as specially corroborative of the conception of amphimixis as a
'formative stimulus' whose strength depends upon the difference between
the germ-substances. The renowned breeder, Nathusius, allowed the
progeny of a sow of the large Yorkshire breed, imported from England
when with young, to reproduce by inbreeding for three generations. The
result was unfavourable, for the young were weakly in constitution
and were not prolific. One of the last female animals, for instance,
when paired with its own uncle--known to be fertile with sows of a
different breed--produced a litter of six, and a second litter of five
weakly piglings. But when Nathusius paired the same sow with a boar of
a small black breed, which boar had begotten seven to nine young when
paired with sows of his own breed, the sow of the large Yorkshire breed
produced in the first litter twenty-one and in the second eighteen
piglings.