DE MINIMIS
Immense have been the preparations for me,
Faithful and friendly the arms that have help’d me.
* * * * *
“Before I was born out of my mother generations guided me,
My embryo has never been torpid, nothing could overlay it.”
Walt Whitman.
There are two functions of organic life which are often confused
together, but which it is well to keep distinct in our thought. These
are Growth and Development. The mark of growth is that an organism, by
assimilation from the outside world, becomes _larger than it was_. But
in development it becomes _different from what it was_. The history of
an embryo in the womb presents a succession of phenomena which, when
one comes to realize them, almost stagger thought; for, while remaining
the _same thing_ all through, it is continually becoming a _different
class of thing_—first two cells, then one cell, then a fish, a
quadruped, ultimately a human being. This is Development. Once born, it
is laid hold of by the principle of Growth which lasts until maturity.
Now in the groups called Species, as well as in individuals, we observe
exactly the same distinction. The members of a species multiply and
increase their numbers. This is Growth. But under certain conditions,
which we have now to investigate, they vary in type and ultimately give
rise to new species differing widely from that from which they sprang.
This we call Development or, in the more popular term for the process
when applied to species, Evolution.
The investigation of this process in all its details has been the
master-impulse of biology ever since the fact of the process was
established by the researches of Darwin.
In Darwin’s time the study of evolution was mainly an affair of
what is called Natural History But it has now been realized that
fully to comprehend the processes involved—so far as they can
ever be comprehended—it is necessary to find out of what kind of
material living beings are composed, and how their fundamental
processes take place. “The ultimate problems of sex, fertilization,
inheritance, and development,” says Wilson, have been now “shown to be
_cell-problems_.”[26] Before going further, therefore, we must give
some account of the leading facts connected with the structure and
vital action of the cell.
Since the publication of the Origin of Species, probably the most
important contribution to biological theory is to be found in the
researches of Dr. A. Weismann, and particularly in his large work, The
Evolution Theory, of which a masterly English translation has recently
appeared.[27] Weismann, on one side, represents an heroic attempt to
bring back to the strictly mechanical principles of Darwinism the tide
of biological speculation, which has been flowing more and more in
the direction of recognizing an essential and not a merely fortuitous
connexion between the goal of the evolution of natural forms and the
means taken by nature to attain it. On another side he has brought the
physiology of the cell into true relation with the natural history of
the organism and of the species, and has become the author, or at least
the first great expounder and systematizer, of a theory of heredity—the
now famous Germ-Plasm theory—much of which seems a solid, permanent,
and deeply important contribution to knowledge. But this theory seems
to lead straight to a non-mechanical or psychic conception of the
driving-force of evolution, and Weismann has therefore supplied the
other part which, in the view of the present writer and of many others
better qualified to judge, seems to be of the nature of a baseless and
improbable hypothesis, devised to find a means of avoiding recourse to
any non-mechanical conception of the ultimate nature of evolutionary
processes.
As we shall be much concerned with Weismann’s views, let us place at
the head of our study of them a couple of passages in which his general
attitude towards the phenomena of vital processes is expressed.
“In our time,” he writes, “the great riddle has been solved—the riddle
of the origin of what is best suited to its purpose without the
co-operation of purposive forces.”[28] “We must certainly assume,” he
declares, “that the mechanical theory of life is correct.”[29]
A longer passage shows us what he understands by ‘mechanical’:—
“The living machine differs essentially from other machines in the
fact that it constructs itself; it arises by development from a
cell, by going through numerous stages of development, but none of
these stages is a dead thing, each in itself is a living organism
whose chief function is to give rise to the next stage. Thus
each stage of the development may be compared to a machine whose
function consists in producing a similar but more complex machine.
Each stage is thus composed, just like the complete organism, of
a number of such ‘constellations’ of elementary substances and
elementary forces, whose number in the beginnings is relatively
small, but increases rapidly with each new stage.”[30]
It would have been simpler, but it would not have suited Weismann’s
conception of nature, to say that the “living machine” differs
essentially from other machines in not being a machine at all, or
anything in the least like one. No machine constructs itself. No
machine can do anything but repeat a certain series of movements, each
series exactly similar to the last. What Weismann has described is
_not_ a machine, just because it is a living organism. It is surely as
true in biology as it is in mechanics that in any purely physical chain
of sequences you cannot by any possibility get more out at the end than
you put in at the beginning, unless you take it in upon the way.
“Development,” writes Weismann, “is an expression of life.”[31] But
“life,” again, is merely “a chemico-physical phenomenon.”[32] To say
that development is an expression of a chemico-physical phenomenon does
not seem a very illuminating or helpful generalization. The fact is
that the statement that life is a chemico-physical phenomenon does not
take us further towards an understanding of the subject than when we
say, what is equally true, that chemical and physical phenomena are a
manifestation of life. Life is everywhere. We use it as a convenient
term for the energies associated with ‘living’ protoplasm, because
we observe that when it is present protoplasmic structures act and
react (as in the phenomena of nutrition, for instance) in certain
chemico-physical ways, while, if it be absent, the same protoplasm
acts in other ways, also chemico-physical, but quite different from
the former, and analogous to the ways of minerals and of gases into
which dead protoplasm finally resolves itself. The chemico-physical
actions and reactions appear in a living plant or animal to be under
the direction of a force devoted to the preservation of that particular
organism. The smallest atom of organic life includes not only a
chemical compound but a chemist. In the mineral world we may say
broadly that there is no individuality of parts.[33] With protoplasmic
structure, therefore, a stage is reached in the evolution of life which
we may rightfully call ‘life’ _par excellence_, but there has been no
breach of continuity, and it is highly probable, as Weismann himself
suggests, that far below the limits of microscopic observation the
transformation of ‘dead’ into ‘living’ matter is continually going
forward. When, therefore, we speak of the action of living protoplasm
the distinction is rather between this action and that of a piece of
mechanism than between protoplasm and minerals or gases.
The phenomena of cell-growth, reproduction, and heredity are those
which lie at the basis of all organized protoplasmic life, and in
all the forms of that life, vegetable as well as animal, they are
extraordinarily similar; there is, in fact, nothing which all the
species of living things have so much in common. One of the most
wonderful and fascinating chapters in the whole range of science is
that which contains the account of these processes, and it is only
within the last few years that it has been possible to write it.
Weismann, in a certain section of his Evolution Theory, has brought the
facts together in a manner which, for its lucidity and mastery of the
subject-matter, deserves to be called a classic example of scientific
exposition.[34] To understand the basis of the higher manifestations
of life, these processes, as we have said, must first be understood,
and an account of them, based on Weismann, and accepting his germ-plasm
theory so far as it seems to accord with established facts, will be
given, of course only in the broadest outlines.[35] At the same time
it will be attempted, here and there, to throw some light on the
_rationale_ of the processes described.
All animal and vegetable structure arises from cellular tissue, and in
fact _is_ either cellular tissue or, as in the case of bones, scales,
etc., the mineral deposit formed by the action of cells. The simplest
living forms are composed of single cells, and the most complex and
huge of them were each once nothing more than a single cell, possessed
of the powers of development and growth. In multicellular organisms,
this single originating cell is usually formed by the fusion of two
imperfect cells by what is indifferently called conjugation, sexual
reproduction, or ‘amphimixis.’ All cells, whether they are the product
of conjugation or not, grow, when they do grow, fundamentally in the
same way, and this way must now be described.
The contents of the typical cell are broadly differentiated into (1)
a more or less hardened envelope containing (2) a substance called
cytoplasm (Gk. κύτος, a cell), and (3) a small, rounded, dark-coloured
body called the nucleus. Until recently nothing more than this was
known of the structure of the cell, and nothing at all of the functions
of the nucleus. Now, keener microscopic research and better instruments
have thrown a flood of light on cell-organization, and the nucleus
is revealed as a powerful factor in the vital processes of the cell
and the bearer of its hereditary substance[36]—that which makes it a
cell of some particular organism, plant or animal, and of no other.
This hereditary substance, divined by the botanist Nägeli, and since
observed by Weismann and others, is called ‘chromatin’ (from the fact
that it is observed by means of the stain it takes from the addition of
an aniline dye), or ‘idioplasm’ (Nägeli’s appellation), which might be
rendered the ‘selfhood substance’ of the cell.
Cellular structure begins, as has long been known, by the division
of a cell into two, each of the parts then proceeding to grow by the
assimilative power of protoplasm and in due time to divide in its
turn. A mass of these cells is called ‘cellular tissue.’ The so-called
‘budding’ of a small cell from the side of the parent is, of course,
simply a form of division. The process of division and redivision
goes on, accompanied by a differentiation in the shape and function
of the different cells or groups of cells which are formed, until the
structure of the plant or animal is completed. In these operations
the nucleus plays the principal part. The division of the cell is
essentially the division of the nucleus. A detached portion of a cell
which contains nothing of the nucleus can reproduce itself no more; it
perishes.
[Illustration: Fig. I.
This illustration, which (by permission of The Macmillan Co.) I take
from Wilson’s work, The Cell, is one of remarkable interest, for in it
the microscope has caught, in a piece of actual tissue from the skin
of the salamander, _Amblystoma_, three nuclei in different stages of
mitotic division. Most of the nuclei, which are seen as large, roundish
objects in their respective cells, show the chromatin in its ‘resting’
condition interspersed through the nucleus. The nucleus under _a_ shows
the chromatin gathered into chromosomes. At _b_ the centrosomes with
their astral figures (which can barely be detected) have been formed,
the chromosomes have carried out their longitudinal division, and are
being attracted half towards one centrosome and half towards the other.
A little above this the process has been carried further, and the sides
of the cell are beginning to contract, preparatory to forming two new
ones. In Fig. 2 will be found a clear representation of the astral
figures.
_To face p. 40._]
[Illustration: Fig. 2.
The above illustration from Wilson’s The Cell shows in more or
less diagrammatic form the stage of nuclear division in which the
chromosomes, as yet undivided, have arranged themselves in the centre
of the nucleus. The centrosomes with their astral figures have been
formed, and have taken their places near each pole of the nucleus. The
next stage is represented at _b_ in Fig. 1.]
When a cell is about to divide, an organ of recent discovery, termed
the ‘centrosome,’ comes into play. This appears as the core of a sort
of rayed or star-like figure, and it takes up its position beside the
nucleus. When the cell is resting, the chromatin is dispersed through
the nucleus in a mass of broken lines, forming a kind of network. When
division begins, this broken-up substance forms itself into a series of
small threads, sometimes straight, sometimes looped or curved. These
are called ‘chromosomes.’ There are always a definite and invariable
number of chromosomes for every species of plant or animal—the cell
of a man has so many,[37] of a grasshopper so many, of a lily so
many. The chromosomes range themselves in a belt across the centre of
the nucleus, and the centrosome breaks into two parts, which take up
a position one at each end of the nucleus. Regarding the nucleus as
a tiny globe, we may say that the chromosomes lie in the equatorial
plane, while the two parts of the centrosome move towards the North and
South Poles respectively.
The centrosomes, at the two poles of the nucleus, are surrounded each
with a halo of ray-like processes (the centrosphere), and on the sides
next each other these rays penetrate the nucleus and join, forming a
spindle-shaped figure with a centrosphere at each end. This spindle
figure appears to be the organ by which the division is accomplished,
for each of the chromosomes now splits itself in two longitudinally,
as one cleaves a log of wood, and one half passes over to each
centrosphere, thus making an exact division of the whole chromatin or
hereditary substance. An indentation now appears in the outer wall of
the cell and also in the nucleus—it deepens and deepens, and finally
two cells appear instead of one, each with a nucleus, a centrosome, and
a supply of chromatin, the latter now breaking up into its original
condition of diffusion through the nucleus. In multicellular organisms
the two new cells, of course, do not separate, but a wall is formed
between them. Some plant-cells contain several nuclei; in this case
division of the nucleus is not necessarily followed by that of the
cell.[38]
Throughout the processes of cell division it is apparent that the
utmost care is taken to ensure an exact partition of the chromatin
between the two new cells. This partition has to be qualitative as well
as quantitative; for one chromosome may, and no doubt does, differ in
function and influence from another, and has various elements within
itself. The _longitudinal_ division of each chromosome, in which
the elements are arranged like beads on a rosary, ensures that the
different elements of the whole hereditary substance shall appear in
each new cell in exactly the same relative proportion as in the parent
cell; just as if two persons had to divide between them a dozen apples
of different varieties, and secured perfect equality, not by taking
six apples each, but by dividing every apple in two. _This is the
fundamental cause of the fixity of species_, which means the production
of offspring having the same specific characteristics as their parents.
How, under these conditions, the _mutability_ of species is brought
about must be discussed later. It is first of all necessary to inquire
more closely into the composition of the chromatin, and to study the
special phenomena of cell-growth in connexion with conjugation, where
new and extraordinary features come to light.
A chromosome is not, or is not usually, a simple body. In all but the
very lowest organisms it is composed, as we have said, of a number
of elements. Each of these elements is styled a ‘determinant,’ and
it controls the form, colour, and function of some definite part of
the future plant or animal. Weismann believes the determinants to be
grouped into complex bodies called ‘ids,’ each id containing all the
determinants necessary for a whole being, and each chromosome being
composed of a number of ids. These ids are microscopically visible;
they form the beads on the rosary already referred to; but their
exact composition and potency are largely conjectural at present.
How far the subdivision of determinants may go, it is, of course,
impossible to ascertain. We cannot say, for instance, whether there
is a determinant for every hair of the head, or one for the hirsute
covering in general, or one for each of the different sections of the
scalp. But the division is very minute. Each of the ids may be a very
complex body, as we see by the manner in which, in some families,
small physical signs like a patch of hair differing from the colour of
the rest, or a tiny pit or mole on the skin of a certain part of the
body, may be handed down, in that precise position, for generations.
There may be, and, in fact, in the higher plants and animals there
_must_ be, a number of determinants for each part of the structure,
and the final characteristics of that part must be the resultant of
a blend of all these determinants, the more powerful predominating
in proportion to their vitality and force. The whole body of the
chromosomes may therefore be said to represent one or more complete
beings in diagrammatic form, each part of the complete animal or plant
being represented by some part of a chromosome, though of course not
physically resembling it. And we thus strike on the very curious and
startling fact that, as far as we can see, _every_ cell in every
organism throughout the world of life contains all the elements of the
whole being to which it belongs, and is, potentially, that being.[39]
All the higher organisms possess two kinds of cells—reproductive cells
which have the faculty of fusing together to reproduce their kind,
and ‘somatic’ or body cells, which, although they all originate in a
reproductive cell, multiply only by division, and have the function of
forming the various parts of the bodily structure. Of the nucleus of a
germ cell “we cannot say that it differs in any essential or definite
way from the nucleus of any other cell.”[40] All possess the chromatin
or hereditary substance of the organism, though, according to Boveri,
the germ cells alone receive _all_ the chromatin of the parent cell,
the derived somatic cells having to part with some of it.[41] There may
be some distinction, though on what it may be based it is at present
impossible to say, between cells that are capable of developing into a
complete organized creature and those that are not.
Every somatic cell is doomed to perish, but every reproductive cell
now upon the globe is united, not metaphorically, not by a chain
of successive originations or impulses, but by actual identity of
substance, with the first beginnings of protoplasmic life in the abyss
of time; and it has before it a potential immortality commensurate with
life itself. It is not, as used to be thought, a physiological product
of the organism in which it dwells; it is a part of the original
reproductive cell from which that organism sprang.
To understand these conceptions we must now study the phenomena of
reproduction in the light of recent discoveries.
The lowest form of the reproductive process is, of course, by simple
division and redivision. This is characteristic of many of those
organisms which consist only of a single cell, and it may co-exist,
even in these, with a considerable degree of structural complexity,
as in the ‘trumpet animalcule,’ _Stentor raselii_. But among the
lowest of these unicellular organisms a curious process is sometimes
observed to take place, in which we may doubtless recognize the origin
of sexual reproduction. Two, three, or more _Amœbæ_[42] approach
each other, partially coalesce, and remain united for some time. They
then separate again. No new creatures are formed by this contact;
there are no visible results at all. But that _something_ which is
for the advantage of the organisms takes place during this period of
union is certain, and in the light of what is known of processes in
other organisms we can make a very good guess at what this something
is. Each _Amœba_ parts with some of its chromatin to some other and
receives an equivalent in exchange. The creature is thus reconstituted.
The element of _change_, which always provides so marked a stimulus
to vital processes, has been obtained. The process has actually been
observed in a certain Infusorian, _Noctiluca_. Two _Noctilucas_
coalesce, and then proceed to divide _at right angles to the plane of
contact_. This necessarily has the effect of giving to each of the
two new _Noctilucas_ which result from the division half the nucleus
and chromatin of one parent and half of the other. There is, however,
no actual new birth or multiplication of beings; there are only two
_Noctilucas_ as before.
We can now imagine that if a certain class of unicellular organisms
are in the habit of approaching each other for the purpose of this
interchange of portions of their chromatin, they might occasionally,
under the influence of the approaching conjugation, expel those
portions of chromatin _before_ another cell was in a position to
receive it. What would happen if two cells, each of which had thus got
rid of half its chromatin, were to come into contact? Plainly, they
would fuse together; they would not separate again; they would become a
new organism. Each would have supplied just what the other lacked.
This process, forming the bridge from mere cell division to sexual
reproduction, is a hypothetical one; it has not, I believe, been
actually observed in unicellular organisms, but it is exactly what we
find to be taking place when we reach the stage of sexual reproduction
among multicellulars. Multicellular organisms of more or less elaborate
structure plainly cannot, without breaking up, fuse together like
single cells. How, then, are they, as a species, to gain the advantages
of the temporary union and interchange of elements which we have
observed in the low unicellular organisms? Only in one way—by producing
special cells for this purpose. These cells must represent the whole
parent, they must be capable of shedding half their chromatin, and,
when they have fused, must be capable of growing into a complete
organism like the parent. When these specialized cells have been
formed, the others, the somatic cells, will at the same time have
been specialized for other functions, and will thus naturally lose
the original capacity for interchanging chromatin with other cells,
_i.e._ for conjugation. We see the significance, then, of Weismann’s
remark, “germ cells made their appearance along with the multicellular
body.”[43] They are an instance of that differentiation of structure
and function which takes place in all highly organized life. We
must note also that the benefits of conjugation which are realized
_individually_ by the lowest unicellular forms are only realized _as a
species_ by the multicellulars. A species must, then, be regarded as in
some sense an organic whole, and not as a mere aggregate of individuals.
In some very curious cases which stand on the borderland between
sexual and non-sexual reproduction, the same organism is capable of
employing both methods. Thus, among the lower seaweeds (_Algæ_), the
genus _Pandorina_ consists of a colony of sixteen green cells contained
in a kind of gelatinous matrix which the cells excrete. Each cell is
ordinarily capable of recreating the whole organism by division. But
after this process has gone on for some time, the need of conjugation
is felt, the colony breaks up and cells begin to fuse with each other,
though never with those of the same colony. In _Pandorina_ the two
conjugating cells are similar in appearance, but in the genus _Volvox_
we begin to see a difference in the appearance of the two kinds of
conjugating cells. What may be called the ‘female’ cells (germ cells)
are large and quiescent; the ‘male’ (sperm cells) are smaller and
active. The primary meaning of this is that the larger cells have
stored up a supply of nutriment for the young organism, and are
therefore bulkier and less active, while the others contain only the
bare elements of cell-structure and are therefore able, as they are
obliged, to be active in order to search out their quiescent mates.
A strictly vegetable organism, in this stage, may therefore possess
organs of locomotion, and be as free-moving as a fish. A remarkable
fact has come to light respecting those organisms (like some _Algæ_
among vegetables and Infusorians among animals), which are capable
both of conjugation and of reproduction by division, namely, that the
supply of nutriment often determines which method shall be followed.
If nutriment is abundant, division is practised; if it becomes
scanty, an impulse appears to be given to conjugation. Infusorians,
which ordinarily conjugate at pretty regular intervals, can be kept
indefinitely from doing so, and confined to division, by the simple
process of supplying abundance of nutritive matter in the water in
which they live.
“As far as we can see from an a priori point of view,” writes
Dr. E. B. Wilson in his great work on cell structure and cell
phenomena, “there is no reason why, barring accident, cell-division
should not follow cell-division in endless succession in the stream
of life. It is possible, indeed probable, that such may be the
fact in some of the lower and simpler forms of life where no form
of sexual reproduction is known to occur. In the vast majority of
living forms, however, the series of cell-divisions tends to run
in cycles in each of which the energy of division gradually comes
to an end and is only restored by an admixture of living matter
derived from another cell. This operation, known as fertilization,
or fecundation, is the essence of sexual reproduction, and in it we
behold a process by which, on the one hand, the energy of division
is restored, and by which, on the other hand, two independent lines
of descent are blended into one. Why this dual process should take
place we are as yet unable to say.”[44]
The actual mechanism of sexual reproduction is essentially the same
wherever it occurs, whether in a seaweed or a human being. Two cells
have to play their part in it, the Germ cell and the Sperm cell, and
these, in the higher orders of organized beings, come to be located
respectively in distinct classes or sexes of individuals. Reproduction
begins by the fusion of a sperm, or male cell with a germ, or female
cell.
These cells originally resemble the other cells of the same species,
containing the same number of chromosomes. If this number was, say,
sixteen, which is believed to be the number in man, then a fusion of
two complete cells, if it were possible, would produce a cell with
thirty-two chromosomes, and that would mean a different species of
animal. What happens is that each of the reproductive cells, male
and female, prepares itself for conjugation by getting rid of half
its chromosomes. Two divisions of the nucleus take place, _not_ as
in the ordinary fashion of cell-division, when the chromosomes split
longitudinally, but in such a way that, in each division, four of
the sixteen chromosomes are bodily expelled from the nucleus and
from the cell, when they either perish or, in some cases, appear to
help in forming an envelope of nutritive matter round the germ cell.
These divisions are called ‘maturation divisions,’ and until they
are accomplished, fecundation is impossible. When a sperm cell after
maturation comes into the neighbourhood of a germ cell, it penetrates
into its substance, using the long _flagellum_, or tail-like process,
with which it is equipped as an organ of locomotion. The two nuclei
come into contact and coalesce, and we have thus a new cell with its
sixteen chromosomes complete. This cell is the origin of the new being.
It divides in two, and each part divides and redivides, different cells
gradually differentiating themselves as muscular tissue, cartilage,
blood-corpuscles, nerves, reproductive cells, and so forth, until the
whole animal is built up and is ready for birth. One point of cardinal
importance must here be noted. The originating cell, as we have seen,
has eight of its sixteen chromosomes from one parent and eight from
another. When division takes place, these chromosomes, as we have seen,
split longitudinally, and the result is that each new cell gets exactly
the same mixture of chromatin as that of the originating cell—half
from each parent. This principle of division is carried on throughout
the whole process of building up the new being—every cell of the
latter, down to the minutest details of its structure, containing an
exactly equal quantity of hereditary elements from each of its parents.
It will be seen from the above account that the old conception of the
germ-cell as a passive body, incapable of a change till ‘fertilized’ by
a male or sperm cell, was altogether wrong. Both male and female cells
prepare themselves for conjugation long before it takes place, and
neither of them can be said to be a more active agent in fertilization
than the other. Not ‘fertilization’ but ‘fusion’ is the keyword of the
process. The mystical conception, as old as Plato, of the male and
female as representing respectively the two halves of a complete being,
turns out to be no poetic metaphor. As regards the essential features
of reproduction, it is a literal fact.
If we now ask why and by what mysterious law all these exact and
elaborate choric movements take place Weismann and his school refer us
to “chemotactic forces,” the nature of which is yet unknown. Chemotaxis
means simply the effect of the _presence_ of certain substances on
vital organisms without specific chemical action. The really essential
fact is that these special chemotactic forces are working _in living
protoplasm_. Life is not the product or the slave of any chemotactic
forces, but their maker and steersman.
The following passage from a work of the late Prof. Geo. Rolleston may
be pertinently quoted here:—
“There exists, as is well known, a tendency to resolve all
physiological into physico-chemical phenomena: undoubtedly many
have been, and some more may still remain to be, so resolved; but
the public may rest assured that in the kingdom of Biology no
desire for a rectification of frontiers will ever be called out by
any such attempts at, or successes in the way of, encroachment; and
that where physics and chemistry can show that physico-chemical
agencies are sufficient to account for the phenomena, there their
claim upon the territory will be acceded to, as in the cases
we have been glancing at [certain animal poisons], and where
such claims cannot be established and fail to come up to the
quantitative requirements of strict science, as in the cases of
continuous and of discontinuous development or self-multiplication
of a contagious germ, and in some others, they will be
disallowed.”[45]
This was written in 1870. A generation later the attempt to reduce
life to a physico-chemical phenomenon had not made much way, as may
be judged by the following passage from Strasburger’s Text Book of
Botany:—[46]
“Vital phenomena are essentially bound up with the living
protoplasm. No other substance exhibits a similar series of
remarkable and varied phenomena, such as we may compare with
the attributes of life. As both physics and chemistry have been
restricted to the investigation of lifeless bodies, any attempt to
explain vital phenomena solely by chemical and physical laws could
only be induced by a false conception of their real significance,
and must lead to fruitless results. The physical attributes of
air, water, and of the glasses and metals made use of in physical
apparatus, can never explain qualities like nutrition, respiration,
growth, irritability and reproduction.”
And Wilson concludes his work by the admission that
“the study of the cell has on the whole seemed to widen rather than
to narrow the enormous gap that separates even the lowest forms of
life from the inorganic world.”[47]
“The lowest _observed_ forms of life” would have been a more exact way
of stating the fact.
Many questions of detail will occur to the reader at this point, which
he will find answered in the pages of Weismann or other investigators.
Here we must confine ourselves to what has a distinct bearing on the
objects of this study. One of the points which may be briefly touched
on is the question how it comes that two germ cells, once having
passed through their maturation divisions, cannot fuse and form a
new being; nor can two sperm cells. Were this possible we might have
‘self-fertilization,’ and virginal conception or parthenogenesis,
whenever two germ cells in the ovary of a female animal or in that of
a plant happened to come into contact. But since the object of fusion
is the union of (more or less) _unlike_, and not closely related,
elements, we find that even when a kind of self-fertilization occurs,
as in some plants, the sperm or pollen cells are differentiated
visibly, and probably still more invisibly, from the germ cells. But,
apart from this, the object of preventing the union of reproductive
cells of the same sex is mechanically attained by a very curious
device. The cell-organ by which division is carried out is the
centrosome. But in the course of the two maturation divisions of the
germ cell, that cell _loses its centrosome_, which seems to be absorbed
into the protoplasmic substance of the cell when once its task is
accomplished. No fusion of any number of such cells can therefore lead
to any further change or growth, for growth is based on cell division,
and the centrosome is the organ of division. The sperm cell, on the
other hand, does not lose its centrosome; it retains it to form the
organ of division for the new cell after conjugation. But, reduced
as it is to little more than a bare nucleus without any envelope of
nutritive matter, the sperm cell cannot support the intense vital
activity called for in the initial stages of the life of a new being,
and therefore sperm cells, like the germ cells, though for a different
reason, would be incapable of mutual conjugation, even if the element
of mutual attraction existed among them.
Another point of interest is the question of the determination of sex.
The known facts afford a strong corroboration of the general theory of
reproduction outlined above. It has not been ascertained, nor is it,
perhaps, ascertainable, whether the sperm cells of the male contain in
their chromatin a preponderance of male, while the germ cells provide
chiefly the female determinants.[48] However this may be, it is certain
that determinants which severally control the formation both of male
and of female structure are _always_ present in every combination of
the sperm and germ cells, those which exhibit the greatest energy and
vitality probably prevailing in the determination of the sex of the
future being. This accounts at once not only for the cases (rare in
the higher animals) of actual hermaphroditism, when the sex is really
indistinguishable, but for the universal occurrence in all male animals
of rudimentary female organs (such as mammæ) and in all females of
rudimentary male organs. Both sets of determinants are always present;
the more powerful prevail, but the weaker have a deflecting influence
on the total result. When the primary sexual characters of the embryo
are determined, they appear to communicate a stimulus which starts into
activity the appropriate secondary characters, such as colouring and
other modifications not directly sexual. An extraordinary case, which
I take from Beddard’s Animal Coloration,[49] is that of a chaffinch
which was found to have on the left side of its body the plumage of a
hen bird and on its right that of a cock. On dissection the meaning of
this freak of physiology was revealed. The bird was an hermaphrodite,
having the female organs of generation on the left side of its body and
the male on the right. Hermaphroditism is not in itself a very uncommon
phenomenon in birds (though here it is a monstrosity, not, as in slugs
and snails, a natural and useful condition); but the way in which in
this instance it governed the distribution of colour is most peculiar;
and of course it strongly reinforces Weismann’s conception of distinct
determinants for the various details of bodily structure.[50]
This brings us to the recognition of a _competition among determinants_
which is an important, indeed a cardinal, feature in Weismann’s theory
of evolution. He makes, as I am forced to believe, an illegitimate
and extravagant use of it, but the principle may really exist and
be operative without furnishing the master-word to the riddle of
organized being. The master-word, as I shall try to show, is nature’s
will to live. But before going fully into this argument, let us fix
in our minds the _rationale_ of those processes of elementary organic
life which have been described in this chapter. Protoplasmic life may
be supposed to have originated, and perhaps to be still originating,
in certain molecular combinations of matter. In other words, the
combination, when it took place, developed certain peculiar forces
through which it was enabled to maintain itself and to grow, by the
processes called assimilation and nutrition. These forces, then, were
potentially present in nature before the molecules combined to evoke
them. They are among the latent powers of life. They waited, ready
to be called into action when the required external form should be
arrived at in the play of molecular energy. Life first originated, no
doubt, in unconnected and inconceivably small units of protoplasm.
Between the units thus formed and their combination into the elaborate
structure which we now know a cell to be—packed as full of varied
energies, it has been said, as an ironclad is of machinery—there is
evidently a very wide gap. All we know is that when we have got the
cell, we find it in possession of a complex apparatus for subdivision,
which, taken together with the faculties of nutrition and growth,
enable any one cell to multiply indefinitely by producing replicas of
itself. To _life_ and _growth_, then, has been added the faculty for
_multiplication_. Here we strike on a veritable mystery. Why should
any new movement ever take place? Why should a cell ever divide in
two? We can only say that it is its property to do so.[51] It does
so because it is alive. Did this property first arise as one of a
multitude of aimless movements—the only one which ensured permanence
and multiplicity to the organisms which exhibited it? If so, then
Nature, at the time when life began on the earth, behaved in a manner
most unlike that in which she behaves at present. If we are to
interpret the processes hidden in the remote past by the light of what
we see at present, we shall conclude that, at bottom, the will to live
made molecular action—and the same force incorporated itself in the
combinations which originated protoplasmic life, ordered the structure
of the cell, and gave it the need and the power to multiply. Nature is
for ever changing, for ever straining after new life, after more life.
Having arrived at the cell with its powers of division, the next step
was the power of conjugation between cells with their interchange of
vital substance, bringing about, in Weismann’s words, “a wealth and
diversity of organic architecture which without it would have been
unattainable.” It takes place by means of physical energies, but the
process is entirely inexplicable unless we assume that it exists to
satisfy a need, a _Drang_, for life. And this need, although of course
it displays itself in physical processes, is not in itself a physical
process. At the very beginnings of structural life, if not before it,
we are obliged to pass beyond physics in order to comprehend physical
phenomena. Whenever we find an aggregate of living units, such as a
_Pandorina_ colony, living with a communal life which is other than
the sum-total of the lives of the individual units, we are in presence
at once of the necessity for a metaphysical conception, to render
intelligible the unity in diversity which we perceive.
The response of living protoplasm to the stimuli it receives from the
outside world is normally directed to the maintenance of the life and
form of the organism. The response of what is called ‘lifeless’ matter
is of another nature; not because it is really lifeless, for if it
were it would not respond at all, but because it has no organisms to
protect and foster. We all know the nature of the action of gravity on
Newton’s apple. It was treated as a dead substance, like a stone, and
gravity acted upon it as upon all other ponderable matter. But when
it had fallen to the earth, had decayed, and one of its pips began to
grow, the action of gravity began to be manifested in a quite different
and very peculiar fashion. It has been ascertained by a series of
ingenious experiments that gravity is the force which obliges the roots
of a plant to sink downwards into the earth. This does not, of course,
mean that the roots are drawn downwards by attraction of the earth,
but that the pull of gravitation gives a certain stimulus to the cells
concerned which makes them grow in that direction. Precisely the same
stimulus communicated to the cells of the stem has the very opposite
effect—these it causes to grow upright into the air and light. Thus
the roots are, as it is termed, positively, and the stems negatively,
geotropic. The substance of the root cells and of the stem cells is
the same, the stimulus is the same, but the effects on growth agree
in only one point, that they are respectively what the plant requires
them to be. There is no doubt that if a species of plants were placed
in such a position that it would serve them for the roots to grow
upwards, then upward-growing roots would eventually be evolved; in
fact, this is actually the case in the lateral underground roots of
certain mangroves which rise to the surface and become modified as
breathing organs, and in the aerial roots of various orchids, etc.[52]
When a change of habitat takes place calling for new developments of
structure to meet new conditions, these developments are not, as a
matter of actual observation, found to be mechanically ‘selected’ from
a mass of random movements and modifications of tissue—they reach their
goal, it is true, by a series of _gradual_ approximations, but the
goal is in sight from the beginning. In other words, _adaptability is
a fundamental character of life_. Hence the fact that multicellular
organisms which cannot, as a whole, fuse with others, adapt themselves
to these conditions by the allotment of special cells for that purpose;
while, again, the production of multicellular organisms is itself an
adaptation to Nature’s need for the higher organization of life.
“The botanist Reinke,” writes Weismann, “has recently called
attention once again to the fact that machines cannot be directly
made up of primary physico-chemical forces or energies, but that,
as Lotze said, forces of a superior order are indispensable, which
so dispose the fundamental chemico-physical forces that they must
act in the way aimed at by the purpose of the machine.... Organisms
also [according to Reinke] are machines which perform a particular
and purposeful kind of work, and they are only capable of doing
so because the energies which perform the work are forced into
definite paths by superior forces; these superior forces are thus
‘the steersmen of the energies.’”[53]
Weismann admits that there is “undoubtedly a kernel of truth in this
view,” but he is content with this perfunctory acknowledgment. His
main efforts are devoted to the substitution of fortuitously developed
“constellations” of molecular energy for any force which can be deemed
to have the slightest tincture of intelligence or purpose. “In our
time,” as he writes, “the great riddle has been solved—the riddle
of the origin of what is best suited to its purpose without the
co-operation of purposive forces.” The nature of the proposed solution
can be best described and discussed in another chapter, when we shall
be in a position to consider it in relation to the whole history
of organic development from its origin in protoplasmic life to the
evolution of species in plants and animals.