locomotion of the colony, but also for nutrition, for the secretion
of the jelly, and for the excretion of waste products; in short, they
perform all the functions necessary to the maintenance of life, but
not that of reproduction. They can, indeed, multiply by dividing when
the colony is young, like the cells of _Pandorina_, but they cannot
reproduce the whole colony but only cells like themselves, that is,
other somatic cells. In _Volvox_ the maintenance of the species, the
production of a daughter-colony, is the function of the second and
larger kind of cells, the reproductive cells, which are contained in
the interior (filled with a watery fluid) of the gelatinous sphere.
They possess no flagella (_kz_), and so take no share in the swimming
movements of the somatic cells. For the present we need not allude to
the fact that there are several kinds of these cells, and need only
state that the simplest among them, the so-called 'Parthenogonidia,'
after they have reached a considerable size, begin a process of
division which results in the formation of a daughter-colony. Usually
there are several of these large reproductive cells in a _Volvox_
colony, and as soon as these have developed into a similar number of
daughter-colonies they burst out through a rupture in the now flaccid
jelly of the maternal sphere and begin to lead an independent life. The
mother-sphere, which now consists only of somatic cells, is unable to
produce new reproductive cells; it gradually loses its spherical form,
sinks to the ground, and dies.
In _Volvox_ we have, for the first time, a cell-colony in which a
distinction has been established between body or somatic cells and
reproductive or germ-cells. In contrast to _Pandorina_, a large number,
indeed the majority of the cells of the colony, have lost the power
of reproducing the whole by division, and only the few reproductive
cells possess this, while they, in turn, have lost other functions,
notably that of locomotion. Their power of reproducing the whole, that
is to say, their hereditary capacity, gives them a greater theoretical
interest than the cells of _Pandorina_, for the latter require only to
produce others like themselves, because there is only one kind of cell
in the colony, while in _Volvox_ the reproductive cell can not only
produce others like itself, by division, but can produce the body-cells
as well. The problem is quite analogous to the one which we have had
to face in regard to the unicellular animals of complex structure, the
Infusorians. The question, How can the part of the trumpet-animalcule
which is mouthless develop from itself a new mouth and ciliated
apparatus? here transforms itself into the question, How can a cell
by division give rise not only to others like itself, but also to the
body-cells, which are of quite different structure? This is, in its
simplest form, the fundamental problem of all reproduction through
germ-cells, to which we must now pass on. But first a short digression.
We have already noted that unicellular organisms multiply by division,
and originally, as well as in the great majority of cases to-day, by
division into two. It follows, therefore, that there is no _natural_
death among them, for, if there were, the species would die out as
the individuals grew old; but this does not happen. The two daughter
organisms which arise from the binary fission of an Infusorian are
in no way different in regard to their power of life; each of them
possesses an equal power of doubling itself again by division, and
so it goes on, as far as we can see, for an unlimited time. Thus the
unicellular organisms are not subject to natural death; their body is
indeed used up in the course of ordinary life so that the formation of
new cilia and so on is necessary, but it is not worn away in the same
sense in which our body is and that of all Metazoa and Metaphytes,
where, through functioning, the organs are gradually worn away until
they become incapable of function. Our body grows old, and can at last
no longer continue to live; but among unicellular organisms there is
no growing old, and no death in the normal course of the development
of the individual. The unicellulars are, as we may say, immortal; that
is, while individuals may be annihilated, by external agencies, boiling
heat, poisons, being crushed, or eaten, and so on, at every period
some individuals escape such a fate, and perpetuate themselves through
succeeding ages. For, strictly speaking, the daughter-individual
is only a continuation of the mother-individual; it contains not
only half of the substance, but also the organization, and life is
continued directly from mother to daughter. The daughter is simply
half of the mother, which is subsequently regenerated; and the other
half of the mother lives on in the other daughter, so that nothing
dies in this multiplication. It may be said that the daughter has to
develop the other half of its body anew, and that therefore it is a
new individuality, and not merely a continuation of the old, and that
therefore the unicellular animals are not immortal. The 'immortality'
of the Protozoa may be scoffed at; the idea may seem absurd that the
'immortal' Protozoa are still the same individuals which lived upon
the earth millions of years ago, but all such objections mean no more
than doctrinaire quibbling with the concepts of 'individual' and
'immortality,' which do not exist in nature at all, but are mere human
abstractions, and therefore only of relative value. My thesis as to the
potential immortality of the Unicellulars aims at nothing more than
impressing on Science the fact that the occurrence of physiological,
that is, natural, death is causally associated with the transition
from single-celled to many-celled organisms; and this is a truth which
will not be overthrown by any sophisms. It is the Volvocineæ which
show us, so to speak, the exact point at which natural death set in,
at which it was introduced into the world of life. In _Pandorina_ the
state of things is still the same as in single-celled organisms, for
each cell is still all in all, each can bring forth the whole, none
dies from physiological causes involved in the course of development,
and they are therefore 'immortal' in the sense stated. But in _Volvox_
the 'individual' dies when it has given off its reproductive cells,
because here the contrast between germ-cells and body has developed.
Only the body is mortal in the sense of being subject to natural death;
the germ-cells possess the potential immortality of the single-celled
animals, and it is necessary that they should possess it if the species
is to continue to exist.
From this alone it does not seem quite clear why the body or soma
should be subject to death, and when I first endeavoured to arrive at
clearness in regard to these matters I tried to find out why a natural
death of the body was necessitated by the course of evolution. I did
not at once discover the true explanation, but without delaying to
discuss my mistakes I shall proceed to expound what I believe to be the
true reason. It lies simply in the fact, which we shall inquire into
later on in more detail, that every function and every organ disappears
as soon as it becomes superfluous for the maintenance of the particular
form of life in question. The power of being able to live on without
limit is useless for the somatic cells, and thus also for the body,
since these cannot produce new reproductive cells after those that had
been present are liberated; and with this the individual ceases to be
of any value for the preservation of the species. What advantage would
it be to the species if the _Volvox_ balls were to continue living for
an unlimited time after the reproductive cells were developed and had
been liberated? Obviously their further fate can have no influence
whatever in determining or preserving the characters of the species,
and it is quite indifferent to the continuance of the species whether
and how long they go on living. Therefore the soma has lost the
capacity which conditions endless continuance of life and continued
renewal of body-cells.
In regard to these views it has been asked jeeringly, how
'immortality,' if it were really a property of the Unicellulars and of
undifferentiated cell-colonies, could be lost, as if the world, which
we believe to be everlasting, should give up its everlastingness.
But the jeer recoils on the superficial outlook which is unable to
distinguish between the immortality dreamed of by the poets, religious
and secular, and the real power that certain forms of life have to
resist being permanently exhausted by their own metabolism. That
we should call this 'immortality' does not seem to me to require
any apology, for the right has always been conceded to science to
transfer popular words and ideas in a restricted and somewhat altered
sense to scientific conceptions when it seems necessary. That the
word 'immortality' in this case expresses the state of matters more
precisely and better than any other cannot be doubted, any more than
we can doubt that there exists in regard to natural death a real
difference, which we must take account of, between the Unicellulars
and the higher organisms. What enables the species in the case of the
higher organisms, like ourselves for instance, to last through ages is
not the immortality of the individual, of the person, but only that
of the germ-cells; these alone, among the cells of the whole body,
have retained the primæval power. A small piece of the individual is
still immortal, but only a minute part, which cannot be considered as
equivalent to the whole, either morphologically or from the point of
view of the conception of individuality. Can anyone consider himself
identical with his children? If any one should imagine this, it would
still not be the case, for he himself would in the course of time
suffer natural death, and his children would continue to live on until
they too had brought forth children, and in their turn also came to
die. It is quite different with an Infusorian, which never lies down to
die, but simply splits itself afresh into two halves which continue to
live.
It is hardly credible that such a simple and clear truth should have
remained so long undiscovered, and it is even more incredible that
since it was enunciated it should have been until quite recently
laughed at as false, as a piece of pseudo-science, and as valueless.
But it is the fate of all knowledge which rests on an intelligent and
comprehensive working up of facts to be attacked, until it gradually
bears down antagonism by the weight of its truth, and compels at least
a silent recognition.
The fact that natural death made its appearance with the appearance of
a 'body,' a soma, as distinguished from the germ-cells, will sooner or
later compel recognition. When I pointed out above that the explanation
of natural death lay in the fact that it would be superfluous for the
soma to continue to live on unlimitedly, after it had discharged its
germ-cells, and so fulfilled its duty to the species, I only intended
to say that this was the general reason for the introduction of natural
death. I have no doubt that the actual beginning of this phenomenon
could have, and probably did come about in other ways. Many kinds of
cells in higher animals perish as a result of their function; it is,
so to speak, their business to perish, to break up; this is the case
with many glandular and epithelial cells. It may very well be that, in
many of the highly differentiated tissue-cells, such as nerve, muscle,
and glandular cells, the high differentiation in itself excludes the
possibility of unlimited length of life and multiplication. Through
this alone, therefore, the exhaustion of the body and an ultimate
death may be explicable from internal causes. But the deeper cause
remains what I have already indicated, for it is obvious that if the
continued life, that is, the immortality of the soma, were necessary
to the preservation of the species it would have survived through
natural selection; that is to say, had it been so, then histological
differentiations incompatible with immortality would not have made
their appearance; they would always have been eliminated on their way
to development, since only that which is adapted to its end survives.
Only if the immortality of the soma were indifferent for the species
could the soma have become so highly organized that it became subject
to death.
Thus the old song of the transitoriness of life does not apply to
all the forms of life: natural death is a phenomenon which made its
appearance comparatively late in the development of the organic world,
a phenomenon which, up to a certain point, we can quite well understand
from the standpoint of purposefulness.
It would take me too far from the goal towards which we are at present
making if I were now to attempt to show, in connexion with natural
death, that the durability of the soma, or what we usually call
the normal duration of life, is also exactly regulated by natural
selection, so that each species possesses exactly that duration of life
which is most favourable to it, according to its physical constitution,
its physiological capacity, and the conditions of life to which it has
to adapt itself[11]. But, interesting as this subject is, I must not
digress further, but return to our proper subject of study, namely,
reproduction in its relation to inheritance.
[11] See Weismann, _Ueber die Dauer des Lebens_, Jena, 1882. Translated
in _Essays on Heredity_.
We digressed from this study after having seen that all, even the most
complex, multicellular plants and animals, in which the differentiation
of the cells into a number of cell-groups with the most diverse
functions has attained the highest degree of complexity, are able
to produce special cells, the germ-cells, which have the power of
reproducing from themselves another organism of the same species, and
with the same complex structure. It might be thought that such cells
must necessarily be very complex in their own structure, but in most
cases nothing of the kind is to be seen, and the germ-cells often
appear simpler in organization than many of the tissue-cells, such as
the glandular-cells; and where there is an unusual size or complexity
of structure in the germ-cell it usually bears no relation to the
grade of organization of the young creature that is to arise from it,
but is due solely to the special conditions imposed on the particular
germ-cell, if a young organism is to be evolved from it. We shall soon
see what is meant by this.
I must note here that plants and animals do not multiply by means of
germ-cells alone, but that many species--the majority of plants and
the simpler forms of animals--also exhibit multiplication by budding
or division. All animals and plants which do not stop short at the
stage of the individual, the 'person,' but rise to the higher stage
of the 'stock' (or corm), illustrate this. The first person from
which the formation of the stock proceeds gives rise by budding or
division to new persons which remain attached to it, and in turn by
repeated production of buds give rise to a third, fourth, or _n_^{th}
generation of persons, all remaining in connexion with the first, and
together forming the composite individuality of the animal-colony or
plant-stock. Such colonies or stocks are seen in polyps and corals,
Siphonophoræ and Bryozoa, and among plants, according to Alexander
Braun, in all phanerogams which do not consist only of a single shoot.
In these cases we find that definite, or perhaps indefinite groups
of cells in the stock may give rise to a new person, and we have to
inquire how this power may be theoretically interpreted.
New stocks may also have their origin from such buds, or from single
persons of the stock. The fresh-water polyp (_Hydra_) gives rise by
budding to a small stock of at most three or four persons; but the
young animals budded off only remain attached to the parent hydra
until they have attained their full development; then they detach
themselves and settle down independently, and begin to bud off in turn
a similar and transitory stock. Among plants there are many which, like
_Dentaria bulbifera_ and _Marchantia polymorpha_, multiply by so-called
'brood-buds,' that is, buds which fall from the stock and grow into new
plants. The whole horticultural propagation of plants by cuttings also
depends on the process of budding, for what is cut off from the parent
plant and stuck into the earth is a single shoot, that is, a 'person'
which possesses the power of sending down roots into the earth, and by
continual budding giving rise to new shoots or persons which together
make up a new plant-stock.
I must not, however, spend much time over this so-called 'asexual'
reproduction by budding and division, because it does not suggest
any way by which we may penetrate more deeply into the processes of
inheritance, and we may be content if we can bring them into harmony
with other theoretical views which we deduce from other phenomena.
These forms of reproduction were long regarded as the oldest and
the simplest, and it is only since the time of Francis Balfour that
the conviction has gradually gained ground that this cannot be so,
but that they are rather secondary methods of multiplication in the
Metazoa and Metaphyta, which therefore rest on a very complex basis.
We have seen that the germ-cells made their appearance along with
the multicellular body, and the step from _Pandorina_ to _Volvox_ is
as small a step as can be well imagined. It is thus proved that the
oldest mode of multiplication among multicellular organisms was that
through germ-cells, at least along this line of evolution. _Volvox_
does not reproduce by dividing, or by the development of buds from
any part of the spherical colony of cells. What is known as budding
among single-celled organisms is only an unequal cell-division, and
has nothing but its external appearance in common with the budding of
higher plants and animals. The latter, therefore, is something new, of
later and independent origin; _the primitive mode is reproduction by
unicellular germs_.
LECTURE XIV
REPRODUCTION BY GERM-CELLS.
Historical--Differentiation of germ-cells into male and
female--Pandorina--Volvox--Sperm-cells and ova in Algæ--Zoosperm form
of the male germ-cells--Zoosperms of the Barnacles--Adaptation of the
sperm-cells to the conditions of fertilization--Daphnids--Spermatozoa
in different animal groups--Their minute structure--Form and structure
of the egg-cell--Adaptation of the ovum to given conditions--Dimorphic
ova in the same species--Nutritive cells associated with
egg-cells--Complex structure of the bird's egg.
IF we now turn to the reproduction of the Metazoa and Metaphyta by
means of germ-cells we find that a great number of lowly plants
produce germ-cells which require nothing more for the development
of a new plant beyond certain favourable external conditions, above
all, moisture and warmth. Such, for instance, are the 'spores' of the
ferns, which are formed on the under surface of the fronds in little
clusters of a brown or yellow colour, easily visible to the naked
eye. These spores are individually very small, so that thousands
go to form one spore-cluster or sporangium, and millions of spores
are given off annually by a single fern. Each spore is a germ-cell
enclosed in a protective capsule, and may, if carried by the wind to
a spot favourable to germination, become a young plant, the so-called
prothallium, from which the fern-plant proper subsequently develops.
This reproduction by spores has been regarded as a form of 'asexual
reproduction' so-called, and has been classed along with budding and
fission under this head. But it has nothing in common with these
forms of multiplication except the negative character that the act
of fertilization, which we shall inquire into later on, does not in
this case occur. This mode of classification has no longer any more
justification than the division of the animal kingdom into backboned
and backboneless animals, in which the negative character of the
absence of vertebræ has led to the slumping of quite heterogeneous
forms in one group. I do not mean to dispute that both these
classifications were fully justified in their own time; indeed
they expressed a step of progress. Nowadays, however, the division
'Invertebrata' or 'backboneless animals' as a scientific conception has
been abandoned, and the same should be done with the category 'asexual
reproduction,' since it groups together quite different things, such
as multiplication by single-celled and many-celled 'germs,' and is
moreover based on a quite erroneous idea of what 'fertilization'
really is. Both terms may very well be retained as a mere matter of
convenience, but it is much to be desired that the two apt designations
proposed by Haeckel--Monogony for asexual, and Amphigony for sexual
reproduction--should come into general use.
Meanwhile it is enough to say that reproduction by 'spores' occurs
normally in Algæ, fungi, mosses, and fern-like plants, and that
there are also animals in which the germ-cells possess the power of
giving rise of themselves to a new individual. But the cases which
I am chiefly thinking of are those of so-called virgin birth or
parthenogenesis, which are not to be compared with multiplication by
spores in regard to their mode of origin; there is a peculiarity in
the origin of this mode of multiplication which I can only make clear
after we have studied the normal forms of what is called 'sexual
reproduction.'
We shall therefore pass on to this mode of reproduction. It is well
known that, in all higher animals, just as in Man, an individual cannot
reproduce by itself; the co-operation of two individuals is necessary,
and these--the male and the female--differ essentially from each other
in many particulars. Their union in the act of procreation induces the
development of a new individual, whether this matures within the mother
in a special receptacle, or whether it is deposited as a 'fertilized
egg,' as in birds, the lower vertebrates, and most 'invertebrates.'
As long as Man has lived he has regarded this process of procreation as
the essential factor in the origin of new individuals, and as he had no
insight into the essence of the process he had necessarily to regard
reproduction as something entirely mysterious, and the co-operation of
the two sexes as a _conditio sine qua non_ of reproduction in general;
thus copulation and reproduction seemed identical.
This was in the main the state of opinion at the time of the discovery
of innumerable minute filaments, the so-called 'spermatozoa' in the
'fertilizing' spermatic fluid of the male. The discovery was made in
1677 by Leeuwenhoek in the case of birds, mammals, and many other
animals. Albrecht von Haller (1708-77) was at first inclined to regard
these spermatozoa as the rudiments of the embryo, but later on, in the
course of his long life, he withdrew this theory, and declared them
to be a kind of parasite in the spermatic fluid without anything to
do with fertilization. The same opinion was expressed in 1835 by K.
E. von Baer, in opposition to the opinion of Prevost and Dumas, who
had rightly interpreted the spermatozoa as the essential elements of
the spermatic fluid. When one follows the matter out in detail, one
finds it almost incredible that such a number of mistakes should have
been made, and so many circuitous paths traversed, before even the
limited knowledge current in the middle of the nineteenth century was
attained--that is to say, enough to give ground for the assertion that
fertilization depends upon the contact of the spermatozoon with the
body of the egg. In 1843 Martin Barry had found the spermatozoa within
the egg-envelope of the rabbit ovum, but it was some time later (1852)
that the investigations of Meissner, Bischoff, and Newport established
the fact that the zoosperm penetrates through the egg-envelope. All
else remained quite obscure, and could not be cleared up as long as it
was believed, on the strength of observations which were in themselves
correct enough, that _several_ zoosperms were always necessary to
fertilize one ovum.