essential difference between personal and histonal selection, inasmuch
as the latter can give rise to adaptive structural modifications
corresponding to the needs of the tissue at the moment, but not to
permanent and cumulative changes in the individual elements of the
tissue. If a broken bone heals crookedly, the spongy substance within
the healed portion does not remain as it was before, for the pillars
and arches, which now no longer run in the direction best suited to
their function, break up, and a new system of arches is formed, not
in line with the earlier one, but adapted to the new conditions of
pressure. This is certainly an adaptation through selection, but the
elements, that is the cells which form the bone substance in response
to strain and pressure, or those which in response to the stimulus of
the blood flowing into the spaces form the blood-vessels, or those
which being quite freed from one-sided pressure develop into connective
tissue, must be presupposed. These kinds of cells must be virtually
implied in the germ-rudiment; they are themselves adaptations of the
organism, and can therefore only be referred to _personal selection_.
And this is true of all adaptations of the _elements_ of multicellular
organisms, and thus of the _cells_. Their adaptation according to the
principle of division of labour, their differentiation into muscle,
nerve, and gland cells can only be referred to natural selection in the
Darwin-Wallace sense, and cannot depend upon histonal selection. In
the spongy substance of the bone a better bone-cell does not struggle
with an inferior one and leave behind it by its survival a host of
descendants which are, if possible, better than itself; the struggle
for existence and for descendants, in this case, is between two kinds
of cell which were different from the beginning, and of which one
has the advantage at one spot, another at another. The case may be
compared to that of a flock of nearly allied species of bird, of which
one species thrives best in the plains, another among the hills, and a
third among the mountain forests, all mingled together in a vast new
territory to which they had migrated, and in which all three kinds of
conditions were represented. A struggle would arise among the different
species, in which in every case the particular species would be
victorious which was best adapted to the local conditions. But each
would thrive best in the region in which it was superior to the others,
and very soon the three species would be distributed as they were in
the land from which they came--in the plains, the high lands, and the
mountain forests. This would be the result of a struggle between the
three species, _not between individuals within each species_, and it
could not therefore bring about an improvement of a single species,
but only the local prevalence of one or another. The characters which
made one species adapted for the plain, another for the mountain forest
were _already there_; they can only be referred to personal selection,
which brought about the adaptation of their ancestors in the course
of ages to the conditions of their life. Something similar is true of
the adaptations of the tissues; the differentiation of the individual
kinds of cells is an ancient inheritance, and depends upon personal
selection, but their distribution and arrangement into specially
adapted tissues, so far as there is any plasticity at all, depends
upon histonal selection. Obviously, however, only as far as the tissue
is plastic, that is, with the power of adjusting itself to particular
local conditions. Only adaptations of this kind can be referred to
histonal selection; the ground-plan, even of the most complicated
tissue, such as the large glands of mammals, the kidneys, the liver,
and so on, must have been implicit in the germ, and must therefore be
referred to personal selection. A precise limitation of the respective
spheres of action of personal selection and histonal selection is not
possible as yet, since hardly any investigations on the subject are
available.
Roux undoubtedly over-estimated the influence of his 'struggle
of parts' when he believed that the most delicate adaptations of
the different kinds of cells depended on it. I admit that, for a
considerable time, I made the same mistake, until it became clear to
me, as it did first in regard to the sex-cells, that this is not, and
cannot be the case. How, for instance, could the diverse and minutely
detailed adaptations of the sex-cells--which we are to discuss in a
subsequent lecture--have arisen in this way? As far as the individual
sperm-cell is concerned, it is a matter of indifference whether its
head is a little thinner or thicker, its point a little sharper
or blunter, its tail a little stronger or weaker. This does not
decide whether the cell is to thrive better, or to occur in greater
numbers than some other variety. But it does decide whether it is to
be able to penetrate through the minute micropyle, or through the
firm egg-envelope, into the egg, there to effect fertilization. An
individual with less well formed sperm-cells will be able to fertilize
fewer eggs, and therefore to leave fewer descendants which might
inherit its tendency to produce inferior sperm-cells, and conversely.
Thus it is not the sperm-cells of any one individual which are selected
according to their fitness, it is the individuals themselves which
compete with one another in the production of germ-cells which shall
fertilize best, that is, most certainly. The struggle is thus not
intercellular, but a struggle between persons.
The same is true of all cells differentiated for particular functions;
every new kind of glandular, muscular, or nerve cell, such as have
arisen a thousandfold in the course of phylogeny, can only have
resulted from a struggle between individuals which turned on the
possession of the best cells of a particular kind, _not from a struggle
between the cells themselves_, since these would gain no advantage
from serving the organism, as a whole, better than others of their
kind. In regard to the sex-cells we might admit, in addition to
personal selection, the possibility of an internal struggle between the
sperm-cells or egg-cells of the same individual, inasmuch as each of
these cells is the primordium of a new individual, and as those better
adapted for reproduction might transmit their better quality to these
new individuals. I will not here enter into my reasons for regarding
this idea as erroneous, for in any case this interpretation would not
apply to any other kind of cells. If, for instance, it were a question
of the transformation of an ordinary mucus or salivary gland into a
poison gland, it would not matter in the least to the individual cell
whether it yielded a harmless or a poisonous secretion; but individuals
with many poisonous cells would have an advantage in the struggle for
existence.
I agree so far with Plate when he refers the differentiation of
the tissues entirely to personal selection, but not in his further
conclusion that histonal selection does not exist. The ground-plan
of the architectural structure of the organ depends upon personal
selection, but the realization of the plan in particular cases is
not predetermined down to the minutest details, but is regulated by
histonal selection, and is thus to a certain extent an adaptation to
local conditions of stimulus. The direction, strength, and size of
every single bone lamella is not predetermined from the germ, but only
the occurrence and nature of bone-cells and bone lamellæ in general.
The direction and the strength which these bone lamellæ may assume
depends on the local conditions of strain and pressure which affect the
cell-mass, as is shown very clearly by the spongiosa of an obliquely
healed bone, which we have already described.
But let us now turn to the question which is here most important for
us: _whether functional adaptations can be transmitted_. We must
admit that the insight we have so far gained into the causes of these
adaptations does not make it much easier to answer the question.
Histonal selection is a purely _local_ process of adaptation to the
conditions of stimuli prevailing at the moment, and no one will be
likely to suppose that the distorted position of the spongiosa of
a badly healed fracture could reappear in the straight bone of a
descendant; this would be quite contrary to the principle, for the
crooked lamellæ would in that case no longer be the best adapted. Even
the question _whether the strengthening of a muscle through use can be
transmitted_ cannot be answered in the light of the knowledge we have
hitherto gained. The 'trophic effect of the functional stimulus' is
brought into activity through entirely local influences, through which
only the parts most strongly affected by the stimulus can be caused to
vary. Thus the problem remains unaltered, How can purely local changes,
not based in the germ, but called forth by the chances of life, be
transmitted to descendants?
If all species, even in the highest groups, reproduced by dividing
into two, we might imagine that a direct transmission of the changes
acquired in the course of the individual life through use or disuse
took place, though this would presuppose a much more complicated
mechanism than is apparent at first sight. But it is well known that
multiplication by fission is for the most part restricted to simple
organisms, and that the great majority of modern plants and animals
reproduce by means of germ-cells, which develop within the organism in
regions often very remote from the parts, the results of the exercise
of which are said to be transmitted. Moreover, the germ-cells are of
very simple structure, at least as far as our eyes can discern; for
we see in a germ-cell neither muscles nor bones nor ligaments, glands
nor nerves, but only a cell-body consisting of that semifluid living
matter to which the general name of protoplasm has been given, and of
a nucleus, in regard to which we cannot say that it differs in any
essential or definite way from the nucleus of any other cell. How then
could the changes which take place in a muscle through exercise, or
in the degeneration of a joint in consequence of disuse, communicate
themselves to a germ-cell lying inside the body, and do so in such
a fashion that this germ-cell is able, when it grows into a new
organism, to produce of itself, in the relevant muscle and joint, a
change the same as that which had arisen in the parent through use and
disuse? That is the question which forced itself upon me very early,
and in following it up I have been led to an absolute denial of the
transmission of this kind of 'acquired characters.'
In order to explain how I reached this result, and what it is
based upon, it is indispensable that we should first make ourselves
acquainted with the phenomena of heredity in general, and the
inseparably associated phenomena of reproduction, so that we may form
some sort of theoretic conception of the process of inheritance--a
picture, necessarily provisional and imperfect, of the mechanism
which enables the germ-cell to reproduce the whole organism, and not
merely, like other cells, others like itself. We are thus led to an
investigation of reproduction and heredity, at the conclusion of which
we shall feel justified in returning to the question of the inheritance
of acquired characters, in order to give a verdict as to the retention
or dismissal of the Lamarckian principle.
LECTURE XIII
REPRODUCTION IN UNICELLULAR ORGANISMS
Reproduction by division--In Amœbæ--In Infusorians--Divisions
following one another in immediate succession--Formation of germ-cells
in the Metazoa--Contrast between germ-cells and body-cells--Potential
immortality of unicellular organisms--Beginning of natural
death--Budding and division in the Metazoa.
WE wish to consider the reproduction of organisms with special
reference to the problem of heredity, and it is most instructive to
begin with the lowest forms of life--the unicellulars--because their
structure, as far as we can see with the instruments at our command,
is very simple, and, what is even more important, is relatively
homogeneous.
[Illustration: FIG. 59. An Amœba: the process of division. _A_, before
the beginning of the division. _B_, the nucleus divided into two. _C_,
the two daughter-Amœbæ. Magnified about 400 times.]
Suppose that there are bacteria-like organisms of quite homogeneous
structure, and that these multiply by simply dividing into two, each
rod-like creature dividing transversely in the middle of its length,
the two halves would represent independent daughter-organisms, whose
structure would correspond exactly with that of the mother-organism,
could not indeed in any way deviate from it, and consequently would
take over all its characters, that is, would inherit them. The size
of body is the only feature which is not obviously inherited, but
in reality it is potentially heritable, since the structure of the
divided portions involves the capacity and the limits of their possible
growth. Moreover, the size of body is not invariable in any species;
a particular size is only reproduced under similar conditions of
development. Inheritance here consists simply in a continuation of the
mother-organism into its two daughter-cells.
Even in an Amœba (Fig. 59) we might picture the process of inheritance
as equally simple, though in so doing we should probably be making a
fallacious inference, for the structure of these lowest unicellular
animals probably seems to us simpler and more homogeneous than it
really is. Among Infusorians it is quite obvious that inheritance
implies more than the mere halving of the mother-animal into the two
daughter-cells; something more must be involved. For among these
unicellular animals the differentiation of the body is not only
great, but it is unsymmetrical. The posterior and the anterior ends
are different, and the transverse division of the animal, in which
the process of reproduction here consists, does not produce two
halves, but two very unequal portions. In the division of _Stentor_,
the so-called trumpet-animalcule (Fig. 60), the anterior portion
contains the funnel-shaped mouth and gullet with its complicated
nutritive apparatus, the circular peristome with its spirally curved
rows of composite ciliated plates, the so-called membranellæ, and so
forth; the posterior half contains nothing of all this, but possesses
the foot of the mother-Stentor with its attaching organ, which the
anterior half lacks. But each of the two portions possesses the
power of 'regeneration,' that is, it is able to develop anew the
missing parts, mouth or foot, and so on. So that here there is no
longer merely a simple continuance of the maternal organization in
the daughter-animals, there is something new added, something which
requires explanation; we are confronted with the first riddle of
heredity. Simple growth does not explain the phenomenon, for what has
to be added to complete the halved portions has a different structure,
a different form, different accessory apparatus from any that the
halves themselves possess. It in no way affects this state of matters
that in the normal process of division in Infusorians the formation
of the new mouth and peristome-region begins before the halves have
actually separated, for even if a Stentor be cut in two artificially
the cut halves form complete animals. And, indeed, a Stentor may be cut
into three or four pieces, and in certain conditions each piece will
develop into a complete animal. These pieces must therefore possess
something more than the mere power of growth. We shall try later on to
discover whether this marvellous invisible transmission of characters,
this implication of the whole in each of the parts, can be in any
way theoretically expressed and included in our scheme of conceptual
formulation.
[Illustration: FIG. 60. _Stentor rœselii_, trumpet-animalcule.
Process of division. _wsp_, ciliated spiral leading to the mouth
(_m_); _cv_, contractile vacuole. _A_, in preparation for division,
the nucleus (_k_) has coalesced into a long twisted band. _B_, a
second ciliated spiral (_wsp´_) has begun to be formed; the nucleus
(_k_) is contracted. _C_, just before the constricting off of the two
daughter-Stentors. Magnified about 400 times. After Stein.]
Now that we have become familiar with these facts it will no longer
surprise us to learn that the reproduction of unicellular animals does
not always depend on _equal_ division, but that unequal spontaneous
divisions are also possible, so that one or several smaller portions of
the cell-body, containing a portion of the cell-nucleus, can separate
off from the mother-animal. This occurs especially among the suctorial
Infusorians or Acinetæ. In relation to the phenomena of inheritance the
problem raised by the equal division of the Infusorians repeats itself,
and it is in no way affected by the fact that equal division can take
place several times, or many times in succession, so that from one
animal a large number of small pieces of the same size may be produced
in rapid succession. The characteristic marks of the mother-animal are
not infrequently lost sight of, wholly or partially, when this occurs,
and the divided portions seem to consist of a homogeneous cell-body
and nucleus; but they possess the power of regenerating themselves, or
of developing, if the expression be preferred, into animals similar
to the maternal-organism. Such divided portions might very well be
called germs, only it must not be forgotten that the relation of the
mother-animal to these germs is a different one from that of a higher
animal or plant to its germ-cells; the unicellular animal breaks up by
continued division into these 'germs,' while the Metazoon continues its
individual existence unimpaired by the production of its germ-cells.
The beginning of a so-called 'spore-formation' is to be found in many
Infusorians. Thus the holotrichous species, _Holophrya multifiliis_
(Fig. 61), reproduces by first becoming enclosed in a cyst or capsule,
and then dividing many times in rapid succession, so that 2, 4, 8,
16, &c. individuals arise consecutively, and subsequently burst forth
from the cyst (Fig. 61, _B_). In the Gregarines and other Sporozoa the
period of division lasts much longer, and the encysted animal divides
into 128, 256, or even more portions; but in this case also each part
or 'spore' receives a piece of the maternal cell-body and cell-nucleus,
so that there is no difference in principle between this and the simple
division into two exhibited by _Stentor_; as in that case, so here,
it is not the fully differentiated structure of the animal which is
handed on to the divided parts; it is only the power to redevelop this
anew on their own account. Thus here again we are face to face with the
fundamental problem of heredity: How is it possible that the power of
reproducing the complex whole can be inherent in the simple parts?
[Illustration: FIG. 61. _Holophrya multifiliis_, an Infusorian
parasitic on the skin of fishes. _A_, in its usual condition; _ma_,
macronucleus; _mi_, micronucleus; _cv_, contractile vacuole; _m_,
mouth. _B_, after binary fission has been several times repeated within
the cyst (_cy_); _tt_, results of the division. _C_, one of these units
much enlarged.]
In contrast to the unicellular organisms, the great majority of the
multicellulars, the Metazoa and Metaphyta, many-celled animals and
plants, differ not only in the multitude of their cells, but even
more in the manifold differentiation of these cells according to the
principle of division of labour, so that the various functions of
the animal are not performed by all the cells uniformly, but each
function is relegated to a particular set of cells specially organized
with reference to it. Thus there is differentiation between motile,
nutritive, and reproductive cells, and there may also be glandular,
nerve, muscle, and skin cells, and we know how this differentiation
into a great number of different kinds of cells with highly specialized
functions has arisen, especially among the higher animals, in a
multiplicity which cannot easily be overlooked. Thus we find a large
number of the most diverse kinds of cells, all of which serve for
the maintenance of the body, in contrast to the simply reproductive
cells or germ-cells. These alone possess the power of reproducing,
under certain conditions, a new individual of the same species. We can
contrast with these germ-cells, which serve, not for the maintenance of
the individual, but only for that of the species, all the other kinds
of cells under the name of somatic or body-cells. The problem which we
have to solve now lies before us in the question, How comes it that
the germ-cell is able to bring forth from itself all the other cells
in definite sequence and arrangement, and is thus able to build up the
body of a new individual?
[Illustration: FIG. 62. _Pandorina morum_; after Pringsheim. I, A young
colony, consisting of 16 cells. II, Another colony, whose cells have
reproduced daughter-colonies; all the cells uniformly alike. III, A
young Volvox-colony; _sz_, somatic cells; _kz_, germ-cells.]
The similarity of this problem to that formulated in regard to
unicellular organisms is at once obvious, but it becomes still more
emphatic when we remember that the gulf between unicellular organisms
and the higher animals and plants is bridged over by certain transition
forms which are of the greatest interest, especially in relation to the
problems of inheritance.
Among the lower Algæ there is a family, the Volvocineæ, in which the
differentiation of the many-celled body on the principle of division of
labour has just set in; in some genera it has been actually effected,
though in the simplest way imaginable, and in others it has not yet
begun. Thus in the genus _Pandorina_ the individual consists of sixteen
green cells, united into a ball (Fig. 62, I), each one exactly like the
other, and all functioning alike. They are all united into a spherical
body, a whole, by a gelatinous matrix which they all secrete, and thus
they form a cell-colony, a cell-stock, a many-celled individual; but
each of these cells has not only all the typical parts--cell-body,
nucleus, and contractile vacuole--but each possesses a pair of flagella
or motor organs, an eye-spot, and a chlorophyll body which enables
them to assimilate nourishment from the water and the air. Each one
of these cells thus performs all the somatic functions, that is, all
that are necessary to the maintenance of the individual life. But each
also possesses the power of reproducing the whole colony from itself,
that is, it also performs the function of reproduction necessary to
the maintenance of the species. When such a colony, whose sixteen
cells are continually growing, has led for some time a free-swimming
life in the water, the cells retract their flagella, and each begins
to multiply by dividing into 2, 4, 8, finally into 16 cells of the
same kind, which remain together, forming a spherical mass enclosed in
a gelatinous secretion (Fig. 62, II). Thus there are now, instead of
sixteen cells in the mother-colony, sixteen daughter-colonies, each
with sixteen cells which soon acquire flagella and eye-spots, and are
then ready to burst forth from the dissolving jelly of the maternal
stock as independent individuals. This _Pandorina_ shows no trace of
a differentiation of its component cells to particular and different
functions, but a nearly allied genus of the same family, the genus
_Volvox_ (Fig. 62, III), consists of two kinds of cells--on the one
hand of small cells (_sz_) which occur in large numbers and compose the
wall of the hollow gelatinous mass, forming, so to speak, the skeleton
of the _Volvox_; and, on the other hand, of a much smaller number of
cells which are very much larger (_kz_). The former, the 'body' or
'somatic' cells, are green, and have a red 'eye-spot' and two flagella;
they are connected with each other by processes from their cell-bodies,
and are able, by means of the co-ordinated action of their flagella,
to propel the whole colony with a slow rotatory movement through the
water. Many of my readers are doubtless familiar with these light green
spheres, which are quite recognizable with the naked eye, and people
our marsh pools and ponds in Spring in such abundance that it is only
necessary to draw a glass of water to procure a large number of them.