THE INDIVIDUAL AND THE SPECIES
What is an individual organism? A Protozoan, such as an _Amœba_ or a
_Paramœcium_, is a single cell: it is an aggregate of physical and
chemical parts, nucleus, cytoplasm, etc., and no one of these parts
can be removed if the organism is to continue to live. The cell can
be mutilated to some extent, but, in general, its life depends on the
integrity of its essential structures, and it cannot be divided without
ceasing to be what it was. It contains the minimum number of parts
which are necessary for continued organic existence.
Such an organism as a _Hydra_ consists of an aggregate of cells which
are not all of the same kind. The outer layer, or ectoderm, is sensory
and protective, and contains organs of aggression; while the inner
layer consists of cells which subserve the functions of digestion and
assimilation. All these parts are, in general, necessary for the life
of the _Hydra_. They can be mutilated; the animal can be cut into
two parts, and each of these parts may regenerate, by growth, the
part that was removed. Yet the existence of ectoderm and endoderm, in
a certain minimum of mass, is necessary for this regeneration. The
higher animal, or Metazoon, is therefore an aggregate of cells, each of
which is equivalent to the individual Protozoon; but these cells are
not all alike--that is, there is differentiation of tissues in the
multicellular organism.
Again, the Cœlenterates provide examples of animals which are
aggregates of parts, each of which is the morphological equivalent
of a single _Hydra_. Such an animal as a Siphonophore, for instance,
consists of zooids, and each of these units has the essential structure
of a _Hydra_. But the zooids are not all alike: some of them subserve
the function of locomotion, others of aggression, others of digestion
and assimilation, and so on. Here, again, the whole organism may be
mutilated; parts may be removed and regeneration may occur; but, as
a Siphonophore, all of the different zooids must be present if the
characteristic functioning of the animal is to continue.
The Protozoon is, therefore, an individual of the first order, the
_Hydra_ an individual of the second order, and the Siphonophore an
individual of the third order. Some such conception of degrees of
individuality will probably be regarded as satisfactory by most
zoologists, yet consideration will show that it is very inadequate.
Many unicellular plants and animals may consist of a number of cells,
which are all alike. The Diatoms and Peridinians reproduce by the
division of their cell bodies and nuclei, and the parts thus formed
may remain in connection with each other. A Diatom may consist of one
cell, or it may consist of a variable number of such connected together
by filaments, or in other ways; and the dissociation of such a series
may occur without interfering in any way with the functioning of the
parts separated. A Tapeworm consists of a “head” or scolex, containing
a central nervous mass and organs of fixation; and organically
continuous with this is a series of segments or proglottides. These
proglottides are formed continuously from the posterior part of the
scolex, and they may remain in connection with each other, and with the
central nervous system and some other organs which are concentrated
in the scolex. Nevertheless, each proglottis contains a complete
set of reproductive organs; it has locomotory organs so that it can
move about, and can fix itself to any surface into which it comes in
contact. It can lead, for a considerable time, at least, an independent
existence apart from that of the scolex and the other proglottides with
which it was originally in continuity. In the majority of Polyzoa, the
common Sea-Mat, for instance, the organism consists of a very large
number of polypes or zooids, each of which secretes an investment of
some kind round itself, but all of which may be connected together
by a common flesh. In many Zoophytes there is essentially the same
structure. In Corals there are very numerous zooids, each of which
lives in a calcareous calyx secreted by itself. Polyzoa, Zoophytes,
and Corals are individuals of the third order, and we might regard
the tapeworm strobila--the scolex with its chain of proglottides--as
belonging also to the same category. Nevertheless, a part of a Polyzoan
or Hydrozoan colony, or a proglottis from a tapeworm, may become
detached, when it will continue to live and reproduce and exhibit all
the characteristic functioning of the species to which it belonged.
Such an animal as a _Hydra_, or a Planarian or Chætopod worm, or a
starfish, may be cut into several pieces, and provided that each of
these pieces exceeds a certain minimum of mass, it will regenerate
the whole structure of the organism of which it formed a part. In the
developing embryo of the Sea-urchin the eight-cell stage may be treated
so that the blastomeres may come apart from each other: each of them
will then begin to segment again and will reproduce the typical larval
Sea-urchin. The parasitic flat-worm, known as the liver-fluke, produces
larvæ which develop to form other larvæ called rediæ. Each redia
normally develops into another larval form, called a cercaria, which
finally develops into the adult worm. But in certain circumstances
each redia may divide and reproduce a number of daughter-rediæ, and
there may even be several generations of these larvæ. In many lower
animals buds may be formed from almost any part of the body, and each
of these buds may reproduce the entire organism. In plants the entire
organism may be grown from a very restricted part or cutting. Thus
the individual, whether of the first, second, or third order, may be
divided without necessarily ceasing to be what it was.
Regeneration of fragments detached from the fully developed adult
body so as to form complete organisms does not, in general, occur
among the higher animals, nor, as a general rule, does reproduction by
bud-formation occur. When such animals reproduce, an ovum develops to
form a large mass of cells, which later on become differentiated to
form the tissues and organs of the adult body. But a relatively small
number of the undifferentiated cells persists in the ovaries of the
females, or in the testes of the males, and each of these cells may
again develop and reproduce the organism. There is apparently no limit
to this process: any animal ovum may become divided successively so
that an infinite geometrical series is produced, and in every term of
this series all the potentialities of the first one are contained.
The physical concept of individuality--that which cannot be divided,
or which may not be divided without ceasing to be what it was--such
individuality as the chemical molecule possesses cannot be applied
to the organism. Any definition that involves the idea of materiality
fails. Obviously the notion of the individual most commonly met with
in zoological writings--that it is the product of the development of
a single ovum--fails, for, logically applied, it would regard the
entire progeny of the ovum, that is, all the organisms belonging to the
species, as the individual. It is clear that the difficulties of the
concept arise from our attempt to identify the life of the organism
with the material constellation in which this life is manifested. In
the course of generation after generation the ovum becomes divided
and grows and is again divided, and so on without apparent limit. But
if we assume that the “organisation” or “entelechy” is material and
is capable of this infinite divisibility without impairment of its
attributes, do we not extend to matter a property which belongs only to
the concepts dealt with by mathematics?
The discussion of individuality with regard to the organism, considered
as a morphological entity, is, indeed, rather a formal one, and it is
valuable only in so far as it has for its object the establishment of
the most convenient terminology. Nevertheless, the notion of organic
individuality is clear to us though it is a notion felt intuitively and
incapable of analysis. We see in nature animals like ourselves, and
we do not doubt that each of them is an entity isolated from the rest
of the universe, and to which the rest of the universe is relative.
We ourselves are primarily centres of action. Motion, or change of
position with respect to some object apart from ourselves in nature, is
only relative, and there is no standard or point in the universe which
is motionless and to which we can refer the motion of a body apart
from our own. But the motion of our own body is something felt or
experienced intuitively, something absolute. As we move, the universe,
our universe rather--that is, all that we _act upon, actually or in
our contemplation_--contracts in one direction and expands in another.
We feel ourselves to be apart from it although we may, to some extent,
control it. We have no doubt that the higher animals have this feeling
of isolation from, and relation to, an universe which is something
apart from themselves; though, of course, the attempt to demonstrate
this leads to all the kinds of difficulties suggested in our attempt to
discuss individuality. It is a conviction so strongly felt that we have
no doubt about it. The organic individual we may then describe as an
isolated, autonomic constellation of physico-chemical parts capable of
indefinite growth or reproduction.[26]
[26] The description is, of course, only a convenient one. The notion
of individuality, as it is expressed in the earlier part of this
paragraph, is an intuitively felt, or subjective, one. It is best
called personality.
What is reproduction? It is organic growth by dissociation accompanied
in the higher organisms by differentiation and reintegration. To
make this statement clear, we must now consider the phenomena of
reproduction in the lower and higher organisms.
We know purely physical growth. If a small crystal of some suitable
substance be suspended in an indefinitely large quantity of a solution
of the same chemical substance it will begin to grow, and there is no
apparent limit to the mass which it may attain. Such giant crystals
may be grown in the laboratory or they may be found in rock masses.
Growth here is a process of accretion in which a particular form is
maintained. Form in inorganic nature may be essential or accidental.
Accidental forms are such as are partially the result of a very great
number of small and un-co-ordinated causes: the form of an island or a
mountain suffering erosion, or the shape of a river valley or delta,
or the arrangement of the stones forming a moraine at the side of a
glacier. Essential forms are such as are assumed as the result of the
operation of one or a few co-ordinated causes, and such are the forms
of crystals. They are invariable, or they vary within very small limits
about an invariable mean form.
The form of a crystal depends on the structure of the molecules of the
chemical substance from which it is produced. We cannot, of course,
speak of the shape of a molecule, but we know that the atoms of which
it is composed have certain positions in space relative to each
other--positions which are conceptualised in the structural formulæ of
the chemists. In the solution, or mother-liquor, these molecules move
freely among each other, but in the crystal they become locked together
and their motions are restricted. The shape of the crystal depends on
the way in which the molecules are locked together, or on the way in
which they are arranged. A cube may be built up by the arrangement of a
number of very small cubes: obviously we could not make a cube from a
number of very small hexagonal prisms if the latter were to be packed
together in such a way as to occupy the minimum of space. An infinitely
great number of cubes might also be formed by adding single layers of
very small cubes to the faces of an already existing one--that is,
by the accretion of elements of essentially similar form. In every
cube (or crystal) of this infinite number the geometrical form would
be the same, and if we were to measure any one side of any cube of
this series we should find that the total surface would always be a
definite function of the length of this side. The mass of a cube would
also be a function of such a measurement: it would be _al_^3, _a_ being
a constant depending on the unit of mass and on the specific weight of
the substance of which the crystal was composed. If we take a series of
crystals of increasing size, this relation holds for every one of them:
_M_ = _al_^3, _M_ being the mass, _a_ the constant referred to above,
and _l_, the independent variable, being any one length of a side of
the crystal.
If the organism grows by accretion in the same way as does a crystal,
this relation ought also to hold in all the exclusiveness with which
we expect it to hold in the growth of a crystal. But it does not so
grow. Its growth is something essentially different, and none of
the superficial analogies so prevalent nowadays ought to obscure
this difference. The organism may grow by accretion, thus layers of
calcareous matter may be added to the outside of a membrane bone
from the investing periosteum, or it may grow by the deposition
of matter within the actual cell bodies, (the process of growth
by intussusception of the plant physiologists). But the extent of
growth by accretion is strictly limited in all organisms: for each
there is a maximal mass determined by the nature of the animal or
plant, and this mass is that of the unicellular organism itself, or
that of the cells of which the multi-cellular organism is composed.
There may also be growth by accretion in the case of the formation of
skeletal structures, which are laid down by the agency of the cells
of the organism but if we confine our attention to the growth of the
actual living substance we shall see that accretion ceases when the
mass characteristic of the cells has been attained, when growth by
dissociation begins. The cell then divides, and each of the parts
into which it has divided grows to the limiting size, and division
again occurs. This is what happens in the case of the growth of the
Sea-urchin egg to form the larva, or blastula. The ovum segments
into two blastomeres, each of which then grows to a certain extent,
and again segments into two blastomeres. After the completion of ten
divisions there are about 1000 cells which are arranged so as to form a
hollow ball--the blastula.
[Illustration: FIG. 20.--The Sea-urchin Gastrula larva in section.]
Differentiation is now set up. In the blastula stage all the cells
are alike, actually and potentially. But soon one part of the hollow
ball of cells becomes pushed inwards, and the cells of this inturned
layer become different from those of the external layer, while
cells of a third kind appear in the space between the external and
internal layers. This is the process of differentiation leading to the
development of the various tissues--protective, sensory, digestive,
skeletal, etc. The cells still continue to divide and grow to their
maximal size, but when the process of differentiation begins, the
cells which are formed are not quite the same as those from which they
originated. Finally, however, when the rudiments of all the tissues
of the adult body have been laid down, the cells begin to produce
daughter-cells of only one kind. Growth of the embryo consists,
therefore, of the dissociation or division of the substance of the ovum
and blastomeres, followed by a gradually increasing differentiation of
the cells so produced.
Reintegration proceeds all the time. Blastula and gastrula larvæ are
really organisms capable of leading an independent existence--that
is, they are autonomous entities or individuals. The activities of
the parts of which they are composed--ectodermal locomotory cells,
ectodermal sensory cells, endodermal assimilatory cells, and so on,
must be co-ordinated. The cells are in organic material continuity with
each other, and events which occur in any one of them affect all the
rest. Impressions made upon the sensory cells are transmitted to the
locomotory cells, and food-material assimilated by the assimilatory
cells is distributed to all the others. At all stages the growing
embryo is an organic unity. The more fully it is developed, the greater
the morphological complexity of the organism, and the more numerous its
activities, the greater is the differentiation; but the greater also is
the co-ordination of the organs and tissues. In the higher animals this
co-ordination and integration of activities is effected (mainly) by the
central and peripheral nervous systems, but specially differentiated
nervous cells are not necessary for this purpose. Differentiation
during growth is therefore necessarily accompanied by reintegration of
the parts dissociated and differentiated.[27]
[27] Societies and civilisations, the associations of bees and ants,
or the Modern State, obviously exhibit this differentiation. It is
morphological and functional in the case of the Arthropods, since
individuals performing different duties are modified in form. It is
functional only in the case of human societies. Integration of the
activities of the individuals in both kinds of societies is effected
by inter-communication: articulate symbols in the case of the lower
animals, language in the case of man. If the concept of “orders of
individuality” were anything more than a convenient, though artificial,
analysis of naturally integral entities, we might speak of the ideal
state or the insect society as a “fourth order of individuality.”
In the process of organic growth the relation between mass and form
no longer holds in all the exactness with which it applies to the
growth of the crystal. We might spend a lifetime growing tablets of
cane-sugar, but in all cases we should find that the mass of any
crystal was proportional to the cube of a length of a diameter: there
would be a strict relation between mass and geometrical form. But this
strict relation does not hold in the case of a series of organisms
belonging to the same species but differing in size. If we measure,
for instance, the lengths of a great number of fishes of the same
species, we should find that we must describe the law of growth, not
by the simple equation _M_ = _al_^3, but by an empirically evaluated
expression of the form _M_ = _a_ + _bl_ + _cl_^2 + _dl_^3 + ... and
that the constants in this equation would vary with the species studied
and with the conditions in which it is living--that is, the organism
changes in form as it increases in size. This is inconceivable in the
case of purely physical growth by the accretion of molecules, and we
find again that the characters of the organism depend not only on what
it is but also upon what it has been--that is, upon its duration.
Growth, then, in plants and animals implies variability in form, in
general cumulative variability, leading to an indefinite departure from
the typical form.
The organism, therefore, does not grow simply by the accretion of
material, but, having attained a certain limit of size, it divides or
reproduces. In the lowest plants and animals this process of division
is simple: either the organism (unicellular or multicellular) divides
itself into two approximately equal parts or it divides into a number
of such parts. The first process is represented by the reproduction of
a bacterium or an _Amœba_, or by the division of a Planarian worm; the
second is represented by the division (in many Protozoa, for instance)
of the whole organism into a number of spores. Fundamentally the two
processes are alike: the simple, binary division of the Bacterium is
followed at once by growth by accretion, while in brood-formation (the
cases of multiple division) the parent cell divides, and then each of
the daughter-cells divide, and so on for several generations. After
the completion of these divisions the brood-cells grow by accretion
to their normal size. It is meaningless, in the light of our previous
discussion, to say that the individuality of the mother-cell “is
merged in that of the daughter-cells.” But we may believe that a
_Paramœcium_ possesses some degree of consciousness. Does it possess
personality--that is, the feeling of isolation from the rest of the
universe, and the feeling of oneness with its own past-memory or
conscious duration? If so, its personality, when it divides, becomes
one with that of its daughter-cells. Or is its personality and
conscious past that also of its sister-cells, and also that of the no
longer existent mother-cell, and the cell of which this in its turn was
a part? We must remember that such an organism as a _Paramœcium_ shows
in its behaviour most of the signs of intelligence; that the parts into
which it divides when it reproduces are equally developed; and that the
process of division may not interrupt the conscious duration of either
part. Is there a common personality, or oneness of consciousness,
of all the organisms of this kind which are descended from the same
individual?
Reproduction by division, simple or multiple, does not proceed
indefinitely in the case of the unicellular organisms. Sooner or later
there is a limit, and the cell is then no longer able to continue
dividing. Conjugation then occurs in one of many modes. Essentially two
organisms come into contact and their nuclei fuse, or rather some of
the material of one nucleus is transferred to the other. The cells then
separate and reproduction by division begins again.
This is not necessarily sexual reproduction: it is the conjugation
of essentially similar morphological entities. If two conjugating
_Paramœcia_ possessed distinct personalities we might imagine a
merging or addition of two conscious durations or memories. Sexuality,
however, includes less than this. In this mode of reproduction the
conjugating bodies are not organisms in the usual sense, but rather
modified organisms or highly modified parts of organisms. In some lower
plants the conjugating cells may be modified with respect to the cells
characteristic of the organism, but they may be approximately equal
in size. But in the multicellular plant and animal, in general, the
conjugates are cells detached from the parental body, and differing
chiefly from the cells of the latter in that they show a lack of
differentiation. One of these cells, that detached from the paternal
body, is the spermatozoon (in the case of the animal), or the pollen
cell (in the case of the plant). It is much smaller than the sexual
cell detached from the maternal body: this is the ovum in the case of
the animal, or the oosphere in the case of the plant. In general the
ovum is a relatively large cell, since it contains abundant cytoplasm,
which may also be loaded with yolk or other reserve food material.
The spermatozoon is very much smaller and consists of a nucleus with
a minimal mass of cytoplasm. The ovum is, in general, immobile; the
spermatozoon is generally highly mobile.
The essential process in the sexual reproduction of the unicellular
organisms is therefore the conjugation of the organisms themselves.
In multicellular organisms, modified cells--the germ-cells--become
detached from the bodies of the parents, and these cells conjugate.
In many lower plants and animals phases of sexual and asexual
reproduction alternate, thus _Paramœcium_ reproduces by simple
division, but at intervals conjugation occurs. In plants sporophytic
and gametophytic generations alternate, the sporophyte reproducing
by multiple division--that is, by the formation of spores, and the
gametophyte reproducing by the formation of germ-cells. There are few
organisms--possibly none--in which continued asexual reproduction by
simple or multiple division, spore-formation, bud-formation, etc., can
proceed without limit. In the great majority of cases investigated
asexual reproduction becomes feeble after a time and then ceases, and
it has been held that the stimulus of conjugation of the cells, or that
of sexual reproduction, is necessary for its renewal. In such cases
the organism is said to have become “senescent,” and “rejuvenescence”
by some means becomes necessary. As a general rule rejuvenescence is
effected by the interchange of nuclear matter between two conjugating
organisms, but it may be effected by rest, or by a change of
environment, or by the supply of some unusual food-material to the
liquid in which the dividing organism is contained. Thus, if various
materials be added to the water inhabited by a dividing _Paramœcium_,
the Protozoon may continue to reproduce by simple division for at least
two thousand generations. We must remember, however, that “senescence”
and “rejuvenescence” are only words; what is the essential nature of
the changes denoted by them we do not know.
In sexual reproduction, as it occurs in the great majority of
plants and animals, the ovum, or female germ-cell, is fertilised
or “activated” by the male germ-cell. But this activation by the
spermatozoon is not necessary, for the ovum itself is capable of
division and development to form a complete organism. This occurs
in the cases of natural parthenogenesis among insects and some
other animals, where the ovum proceeds, without fertilisation, to
segmentation and development. In some lower plants, where the size
of the male and female germ-cells is nearly equal, either of them
may undergo parthenogenetic development: in such cases we cannot, of
course, properly speak of sexual differentiation. In the cases of
organisms normally reproducing sexually, the stimulus to development is
afforded by the entrance into the ovum of the spermatozoon--that is,
by the mixture of the male and female germ-plasms; but in some animals
this stimulus may be replaced by the addition to the water in which
they are living of certain chemical substances. This is the process of
artificial parthenogenesis first studied by Loeb in the case of the
eggs of the Sea-urchin; and its analysis suggests that the spermatozoon
conveys some substance into the egg, and that this substance initiates
segmentation by setting up a train of chemical reactions. What
these reactions are exactly, and what is the process of “formative
stimulation” by the spermatozoon, we do not know. It is quite certain,
however, that much more than this process of formative stimulation
is involved in the fertilisation of the egg by the spermatozoon. The
mixture of the male and female germ-plasms resulting from conjugation
confers upon the embryo the characters of both the parents and of their
ancestries.
In an unicellular organism the “body” consists of a single cell
containing a nucleus. The extra-nuclear part of the cell--the
cytoplasm--is modified in various ways: thus it may possess flagella,
or cilia, so that it may be actively locomotory. It is at once a
receptor apparatus, susceptible to changes in the medium in which it
lives, and it is also an effector apparatus, capable of transforming
stimuli received into motor impulses. It may be able to accumulate
available energy by making use of the energy of radiation in the
synthesis of carbohydrate and proteid from the inorganic substances in
solution in the water in which it lives; and it is also able to expend
this energy in controlled movements. All the characteristics of life,
in fact, are exhibited by the unicellular organism, the differentiation
of the cytoplasm corresponding functionally to the differentiation of
the tissues of the multicellular animal or plant.
In the latter the organs, organ-systems, and tissues are composed of
differentiated cells. Development consists essentially of a process of
cell-formation by simple division, and at the end of this process of
segmentation various rudiments (Anlagen) are established. The older
embryologists sought to recognise the formation of three “germ-layers”
in most groups of animals: these were the outer layer or ectoderm,
the middle layer or mesoderm, and the internal layer or endoderm. The
ectoderm, it was held, gave rise to the integument, the central and
peripheral nervous systems, and the sensory organs. The mesoderm gave
rise to the musculature and skeleton, the excretory organs, and some
other tissues. The endoderm gave rise mainly to the alimentary canal
and its glands. The “Gastrea-Theory” of Haeckel sought to recognise
a similar larval form, or “Gastrea,” in the development of most
multicellular animals, and much ingenuity of argument was required for
the establishment of this homology. The newer embryology recognises
the difficulties implied in the application, in all its exclusiveness,
of the Gastrea-theory to the higher phyla of multicellular animals;
so that nowadays it has been necessary to abandon the notion of the
metazoan animal as being built up from these three primary germ-layers.
At the conclusion of segmentation, then, the embryo consists of a mass
of cells similar to each other in structure, but differing in fate
and in potency. Some of these cells are destined to give rise to the
integument, the nervous system, and the sense-organs; others become the
skeleton and musculature; and others again the organs of digestion,
assimilation, and excretion. A primary arrangement of these groups of
cells into three layers is indeed set up in many cases of development,
but it is plain that this arrangement is far from being an universal
one. Modern embryology shows in the clearest possible manner that at
the end of segmentation the embryo consists of a group of cells each
of which has normally a different fate in subsequent development. What
precisely each cell will become depends on its position with regard to
the others. But each cell is capable of becoming more than it normally
becomes: its potency is greater than its actual fate. If the normal
course of development is interrupted, a cell, which would usually have
given rise to a part of the skeleton, may give rise to a part of the
alimentary canal. The cells of the developing embryo are autonomous.
In the normal course of development most of the cells existing at the
end of segmentation give rise to the “body” of the organism, undergoing
differentiation as they so develop. But a few embryonic cells persist
without structural modification throughout the development of the
animal. They divide and grow and become greater in number, but
remain unchanged in other respects. These cells become the essential
reproductive organs, or gonads, of the adult animal--that is, the
ovaries of the female and the testes of the male. In the females of the
higher animals (the mammals, and perhaps some of the Arthropods) these
cells only divide and grow during the early stages of development, and
long before the beginning of adult life the number of ova in the gonads
has become fixed. In all males, and in the females of most animals,
however, the reproductive cells appear to be capable of unlimited
multiplication.
The essential cells of the gonads, the ovarian mother-cells or the
sperm mother-cells, constitute the germ-plasm. In modern, speculative,
biological literature the term germ-plasm is, however, restricted to
the chromatic material in the nuclei of the reproductive cells, the
cytoplasm being regarded as non-essential for the transmission of the
hereditary qualities of the organism. It seems clear, however, that
this distinction between the cytoplasm and the chromatic matter of
the nucleus is not always a valid one, so that it is best to speak of
the whole cell as constituting the germ-plasm. The embryonic cells,
therefore, have different fates: some of them become transformed
during development into the body or _soma_, and others remain
unmodified throughout life as the _germ_. The soma enters into intimate
relationships with the environment; it is affected by the vicissitudes
of the latter; and it may actively respond to them. The germ-cells may
possibly migrate through the body, perhaps, it has been suggested,
developing fatally and irresponsibly into the mysterious, malignant
tumours of adult life. Normally, however, they remain segregated in
the reproductive glands, secluded from the outer environment. Their
activities are inherent in themselves, are rhythmic, and become
functional only on the assumption by the soma of the phase of sexual
maturity. From the point of the species the soma is only the envelope
of the germ-cells. It is affected by every change of the environment,
and being usually cumulatively affected by the latter it becomes at
length an unfit envelope. Somatic death then follows as a natural
consummation, but the germ-cells are, in a sense, immortal in that they
remain capable of indefinite growth by division.
In the sexual reproduction of the higher organism a part of the
germ-plasm becomes detached, undergoes growth, and develops into an
organism exhibiting the parental organisation. But in the development
of the offspring, part of the germ-plasm received from the parent
persists unchanged, is transmitted to another generation, and so on
without apparent limit. _Something is transmitted from parent to
offspring._ This something we regard as a cell exhibiting a definite
chemical and physical structure; but while the germ-cell differs in
certain respects from an ordinary somatic cell, these structural and
chemical differences are insignificant when they are compared with
the differences in the potentialities of the cells. The somatic cells
are, in general, capable of reproducing only the general character of
the tissues of which they form part. Some of them, the cells of the
grey matter of the central nervous system, for instance, appear to be
incapable of division and growth. But again the facts of regeneration
appear to point to the possession by the somatic cells of more than
this restricted power of reproducing the tissues of which they form
part: to this extent the regeneration experiments tend to remove
the essential distinction between the somatic and germinal cells.
Neglecting these results in the meantime, we see that the germ-cells
contain within themselves the potentiality of reproducing the entire
organism in all its specificity. That which is transmitted from the
parent to the offspring is the parental organisation in all its
specificity; and to say that this organisation is a material thing is,
of course, to state a hypothesis, not a fact of observation.
This transmission of a specific form and mode of behaviour from
generation to generation is what a hypothesis of heredity attempts to
explain--that is, to describe in the simplest possible terms, making
use of the concepts of physical science. “Twelve years ago,” says
Jacques Loeb, “the field of heredity was the stamping ground for the
rhetorician and metaphysician; it is to-day perhaps the most exact and
rationalistic part of biology, where facts cannot only be predicted
qualitatively, but also quantitatively.” Let the reader examine
for himself the meagre array of facts on which this apotheosis of
mechanistic biology is based.
Two modern hypotheses of heredity demand attention--Weismann’s
hypothesis of the continuity of the germ-plasm, and Semon’s “Mnemic”
hypothesis. In the latter it is assumed that the basis of heredity
is the unconscious memory of the organism: modes of functioning are
“remembered” by the germ-plasm and are transmitted. This notion
presents many points of similarity to that which we consider later on,
so that it need only be mentioned here. Weismann’s hypothesis--like
Darwin’s hypothesis of Pangenesis--is a corpuscular one, and has
obviously been suggested by the modern development of the concepts of
molecules and atoms in the physical sciences. It supposes that that
which is handed down is a material substance of a definite chemical
and physical structure. This is not the germ-cell, nor even the nucleus
of the latter, but a certain material contained in the nucleus. The
latter contains protein substances containing a greater proportion of
phosphoric acid than does the cytoplasm of the cells in general; these
proteins are known as nucleo-proteins, though our knowledge of their
chemical structure is, so far, not very exact. It is not, however,
these that are the germ-plasm, but a substance in the nucleus _which
becomes visible when the cell is killed in certain ways, and which
becomes stained by certain basic dyes_. It is distinguished by this
character alone and on that account is loosely called “chromatin.” This
substance Weismann identifies as “the material basis of inheritance.”
When a cell divides, a very complex train of events usually occurs.
This process of “Mitosis” exhibits many variations of detail, and
without actual demonstration it is rather difficult to explain clearly.
But its essential feature is evidently the exact halving of all the
structures in the cell which is about to divide. In the ordinary cell
which is not going to divide immediately, the chromatin is diffused
throughout the nucleus as very numerous fine granules, recognised only
by their staining reactions. They may be concentrated at some part of
the nucleus, so that a division through a plane of geometrical symmetry
of the cell would not, in general, exactly halve the chromatin. Prior
to division, therefore, this substance becomes aggregated as granules
lying along a convoluted filament of a substance called “linin,” which
is characterised principally by the fact that _it does not stain with
the dyes that stain the chromatin_. The filament breaks up into short
rods, called Chromosomes, and these rods become arranged in the equator
of the nucleus. The rods then split longitudinally, and one-half of
each moves towards one pole of the nucleus, the other half moving
towards the other pole. Various other modifications of the cell and
nucleus occur concomitantly with these changes, but the essential thing
that happens seems to be the halving of all the structures of the cell,
and this is the simplest explanation of the phenomena of mitotic cell
division. Two daughter-cells are then formed by the division of the
mother-cell, and each of these daughter-cells receives one-half of each
of the chromatin granules that were contained in the mother-cell.
The chromosomes, or “Idants,” are seen to consist of discrete granules,
and these are (generally) the bodies known as the “Ids.” The id
cannot be resolved by the microscope into any smaller structures: it
lies on the limits of aided vision; but the hypothesis assumes that
it is composed of parts called “Determinants,” and the determinants
are further supposed to consist of “Biophors.” The biophors are the
ultimate organic units or elements, and they are of the same order
of magnitude as chemical molecules. We must suppose them to be more
complex than a protein molecule, and the latter contains many hundreds
(at least) of chemical atoms. Now it is possible to calculate the
number of atoms contained in a particle of the same size as the id:
such a calculation may be made by different methods, all of them
yielding concordant results. This calculated number of atoms may be
less than that which we must suppose to be present in the biophors, of
which the hypothetical id is composed![28]
[28] “But,” says Weismann, referring to an objection of this nature,
“it should rather be asked whether the size of the atoms and molecules
is a fact, and not rather the very questionable result of an uncertain
method of investigation.”
The id is supposed to contain all the potentialities of the
completely developed organism. It is composed of a definite number of
determinants, each of the latter being a “factor” for some definite,
material constituent of the adult body. There would be a determinant
for each _kind_ of cell in the retina of the eye, one for the lens,
one for the cornea (or rather for each kind of tissue in the latter),
one for each kind of pigment in the choroid and iris, and so on;
every particular kind of tissue in the body would be represented by
a determinant. Thus packed away in a particle which lies just on the
limits of microscopic vision are representatives of all those parts
of the body which are chemically and physically individualised, each
of these hypothetical “factors” being a very complex assemblage of
chemical atoms. In development the determinants become separated from
each other, so that whatever parts of the body are formed by the first
two blastomeres are represented by determinants which are contained in
those cells, and which are sifted out from each other and segregated.
As development proceeds this process of sifting becomes finer and
finer, until when the rudiments of each kind of tissue have been laid
down a cell contains only one kind of determinant. This consists of
biophors of a special kind, and the latter then migrate out from the
chromatin into the cytoplasm of the cells in which they are contained,
and proceed to build up the particular kind of tissue required.
The nucleus of the germ-cell is thus a mixture of incredible
complexity, but in addition to this material mixture there must exist
in it the means for the _arrangement_ of the determinants in the
positions relative to each other occupied by the adult organs and
tissues. A mechanism of unimaginable complexity would be required for
this purpose, and it must be a mechanism involving only known chemical
and physical factors. It is safe to say that absolutely no hint as to
the nature of this mechanism is contained in the hypothesis.
The determinants must be able to grow by reproduction, or by the
accretion of new biophors, since in each generation new germ-cells are
formed. If we say that they grow by reproduction in the sense that an
organism grows by reproduction, we beg the question of their means
of formation. Do they grow by the addition of similar substances in
the way that a crystal grows? If so, the molecules of which they are
composed must exist in the lymph stream bathing the germ-cells--that
is, the biophors themselves must already exist in this liquid, for if
we suppose that the biophors are able to divide and grow by making
use of the protein substances which we know are present in the lymph
stream, then we confer upon these bodies all the properties of the
fully developed organism. If they are present in the blood, then the
composition of the latter must be one of inconceivable complexity,
since it must contain as many substances as there are distinct tissues
in the animal body. We know, of course, that this is not the case. How,
then, are the biophors reproduced?
We must leave this field of unbridled speculation (which cannot surely
be “the most exact and rationalistic part of biology.”) What the study
of the reproduction of the organism does show is that something--which
we call the specific organisation--is handed down from parent to
offspring, and that this something _may_ possess a high degree of
stability. No apparent change of significance can be observed in
the very numerous generation of organisms (the 2000 generations of
_Paramœcium_, for instance, which were bred by Woodruff) which can
be produced by experimental breeding. Some species of animals--the
Brachiopod _Lingula_, for instance--have persisted unchanged since
Palæozoic times. Throughout the incredibly numerous generations
represented by this animal series, the specific organisation must have
been transmitted in an almost absolutely unchanged condition. The
germ-plasm is therefore continuous from generation to generation, and
it possesses an exceedingly great degree of constancy of character.
This conception of the continuity and stability of the specific
organisation is the feature of value in Weismannism, and all that
we know of the phenomena of heredity confirms it. But it is pure
speculation to regard the organisation as an aggregate of chemically
distinct substances, or if we say that this speculation is rather a
working hypothesis, then it must justify itself by leading us back
again to the results of experience.
It is, however, not quite accurate to say that the organisation
persists unchanged from generation to generation. The offspring is
similar to the parent--that is, the organisation has been transmitted
unchanged. But the offspring also differs just a little from the
parent--that is to say, the organisation is modified by each
transmission. In these two statements we formulate in the simplest
manner the law of organic variability. Organisms may obviously be
arranged in categories in such a way that the individuals in any one
category resemble each other more closely than they resemble the
individuals belonging to another category. We may, by experimental
breeding, produce an assemblage of organisms all of which have had
a common ancestor, or a pair of ancestors. Now the individuals
composing such an assemblage would exhibit a close resemblance
to each other, such a resemblance as our categories of naturally
occurring organisms are seen to exhibit. We should also find that the
individuals of our naturally occurring assemblage would be able to
interbreed among themselves, just as in the case of the experimentally
produced population. It may be concluded, then, that the naturally
occurring population is also the product of a pair of ancestors. This
inter-fertility, as well as the close morphological resemblance of the
individuals, are the facts on which the hypothesis of the common origin
and unity of the assemblage, or species, is formed.
The morphological resemblance between the individuals, either in
the natural or the artificial populations, is not absolute. If we
take any single character capable of measurement we shall find that
it is variable from organism to organism. This important concept of
organic variability may be made more clear by a concrete example.
Examination of a large number of cockle shells taken from the same
restricted part of the sea-shore, and therefore belonging presumably
to the same race, will show that the number of the radiating ridges
on the shell varies from 19 to 27, and that the ratio of the length
to the depth of the shell also varies from 1 : 0.59 to 1 : 0.85. In
the former case the most common number of ridges is 23, and in the
latter case the most common ratio of length to depth is 1 : 0.71.
These are the characteristic or modal values of the morphological
characters in question, and the other or less commonly occurring
values are distributed symmetrically on either side of the mean or
modal value, forming “frequency distributions.”[29] The value of
the first character changes by unity in any distribution: obviously
there cannot be a fraction of a ridge; and this kind of variation is
called “discontinuous.” The value of the second character may change
imperceptibly, and it is therefore called “continuous,” a term which
is not strictly accurate, since in applying it we assume that the
numerical difference between two variates may be less than any finite
number, however small. In this assumption we postulate for biology the
distinctive mathematical concept of infinite divisibility.
[29] See Appendix, p. 350.
The difference from the mode, or mean, with respect to a definite
character in a fully grown organism may be due to the direct action of
the environment, in the sense in which we have regarded the environment
as influencing the organism; or it may be due to the changes in the
organism resulting from the increased or decreased use of some of its
parts. The conditions with regard to nutrition, for instance, will not
be the same for all the individuals composing a cluster of mussels
growing on the sea-bottom. Those in the interior of the cluster do not
receive so abundant a supply of sea-water as those on the outside of
the cluster; and since the amount of food received by any individual
depends on the quantity of water streaming over it in unit time, we
shall find that the internally situated individuals will be stunted
or dwarfed, while those on the outside will be well grown. Such
variations are acquired ones, but even when we allow for them, even
if we take care that all the organisms studied live under conditions
which are as nearly uniform as possible, there will still be some
degree of variability. We cannot be sure that this absolute uniformity
ever exists; and the notion of the environment of an organism may be
extended so as to include the medium in which embryonic development
took place, and even the parental body which formed the environment
for the germ-cells from which embryonic development began. But it is
probably the case that even with an uniform environment, or with one
in which the differences were insignificant, variability would still
exist. The variations that might be observed in such a case would
belong to two kinds--“fluctuating variations,” and “mutations.”
[Illustration: FIG. 21.]
Whether the variations observed in a population of organisms are
fluctuations or mutations can only be determined by experiment.
Let us suppose that we are dealing with a human population, and
that the variation studied is that of stature. Let the men with
statures considerably over the mean value marry the women who are
correspondingly tall, then it will be found that the children from
these unions will, when grown up, exhibit a stature which is greater
than that of the whole population, but not so great as that of their
parents--that is, regression towards the mean of the whole population
takes place.
This is shown in the above diagram, where the lines above and below
the mean one indicate the proportion (relative to the value or
frequency of the mean) of people of each grade of stature. The latter
is proportional to the distance from the mean measured along the
vertical line, distances below this line indicating statures below the
mean, and _vice versa_.
If, on the other hand, the men and women with statures considerably
below the mean marry, their children will ultimately exhibit statures
which are greater than that of their parents, but which are less than
that of the whole population. Regression again occurs, but in the
opposite direction, and such a case would be represented by the above
diagram reversed. Continued selection of this kind would lead to an
immediate increase in the mean stature (or the opposite, if the “sign”
of the selection were reversed) in one or two generations, but after
that the amount of change would be very small, while if the selection
were to cease the race produced would slowly revert to the mean, which
is characteristic of the whole population from which it arose. It is
very important to grasp this result of the practical and theoretical
study of heredity--the selection of the ordinary variations shown by a
general population leads at once to a small change in the mean value
of the character which is selected, but continued selection thereafter
makes very little difference to this result, while the race slowly
reverts to the value of that from which it arose on the cessation of
the selection.
Races which “breed true” do, of course, exist; thus the mean height of
the Galloway peasant is greater than that of the Welsh. In the cases
of “pure races”--that is, races which breed true with respect to one
or more characters, we have to deal with another kind of variation,
one which shows no tendency to revert to the value from which it
arose. Let the observed variability of stature in a human population be
represented by the frequency distribution _A_, and let the individuals
at _N_--that is, those in which the stature was greater than the mean
by the deviation _ON_--intermarry. It might then happen that the
variability of the offspring of these unions would be represented by
the frequency distribution _B_, in which the value of the mean is also
that of the stock, at _N_, from which the race originated. It does not
matter now from what variants in _B_ a progeny of the third generation
arises: the mean height of the latter will be that of the pure race. In
this case the individuals from which the pure race originated (those at
_N_ in _A_) have exhibited a mutation. The stature of the individuals
of this new race will continue to exhibit fluctuating variations, and
the range of this variability may be as much as that of the stock from
which it arose, _but the mean stature of the new race_ will continue to
be that of the original mutants.
[Illustration: FIG. 22.]
It is well known that de Vries himself considered fluctuating
variations and mutations as something quite different. The former he
considered as nothing new, only as augmentations or diminutions of
something previously existing; and he regarded fluctuations as due to
the action of the environment, following in their distribution the laws
of chance.[30] Mutations, on the other hand, were something quite new.
Now future analysis of variability will not, we think, bear out the
validity of this distinction. It is far more likely that a fluctuation
is a variation which is the result of some causes the action of
which is variable. (We are regarding variability now as subject to
“causation” in the physical sense, for only by so regarding it can we
attempt its analysis). As a rule this process results in a fluctuation,
but if its extent, or degree of operation, exceeds a certain “critical
value” a mutation is produced. We may, following the example of the
physicists, illustrate this by a “model.”
[30] See Appendix, p. 351.
[Illustration: FIG. 23.]
This model is a modification of Galton’s illustration of the degrees of
stability of a species. It is a disc of wood rolling on its periphery.
We divide it into sectors, and the arcs _ab_, _cd_, _ef_, and _gh_ have
all the same radius, 10, 20, 30, and 40. Then we flatten the sectors
_bc_, _de_, _fg_, and _ha_, so that their radii are greater than are
those of the other arcs. Now let us cause the disc to roll about the
point 8 as a centre. It will oscillate backwards and forwards about a
mean position 8. Let us think of these oscillations as fluctuations.
Suppose, however, that we cause the disc to roll a little more
violently, so that it oscillates until either of the points 3 or
4 are perpendicularly beneath the centre _O_. In either of these
positions the disc is in a condition of “unstable equilibrium,” and an
infinitesimal increase in the extent of an oscillation will cause it
to roll beyond the points 3 or 4. But if it does pass either of these
critical points it will begin to oscillate about either of the new
centres 5 or 7, thus rolling on one of the arcs, _ha_ or _de_. This
assumption of a new condition of stability we may compare with the
formation of a mutation.
All this is merely a conceptual physical model of a process about which
we know nothing at all. It is meant to illustrate the view that the
organisation of a plant or animal is not something absolutely fixed and
invariable. The organism in respect of each recognisable and measurable
character oscillates about a point of stability, that is to say
exhibits fluctuating variations about the mean value of this character.
If the stability of the organisation is upset, so that it oscillates,
or fluctuates about a new centre, that is, if the variations deviate
in either direction from a new “type” or mean, a mutation has been
established. A mutation is not, therefore, necessarily a large
departure from “normality.” It is not necessarily a “discontinuous
variation,” nor a “sport” nor a “freak.” It is essentially a shifting
of the mean position about which the variations exhibited by the
organism fluctuate.
Such a mutation will, in general, involve the creation of an
“elementary species.” We have considered only one character, say
stature, in the above discussion, but it generally happens that the
assumption of a new centre of stability involves _all_ the characters
of the mutating organism. An elementary species therefore differs a
little in respect of all its characters from the species from which it
arose, or from the other elementary species near which it is situated.
This is what we do usually find in the cases of the “races,” or “local
varieties,” of any one common species of plant or animal. That we
do not recognise that most, or perhaps all, of the species known to
systematic biology are really composed of such local races is merely
because such results involve an amount of close investigation such as
has not generally been possible except in the few cases studied with
the object of proving such variability; or in the case of those species
which are studied with great attention to detail because of their
economic importance. Thus the herrings of North European seas can be
divided into such races, and it is possible for a person possessing
great familiarity with these fishes to identify the various races or
elementary species--that is, to name the locality from which the fish
were taken--by considering the characteristics in respect of which the
herrings of one part of the sea differ from those of other parts.
The term “variety” has rather a different connotation in systematic
biology from that which is included by the term “elementary species.”
The meaning of the latter is simple and clear. Two or more elementary
species are assemblages of organisms, in each of which assemblages
the mean positions about which the various characters fluctuate is
different. The term “variety” cannot so easily be defined. The progeny
of two different species (in the sense of the term as it is usually
applied by systematists) may be called a hybrid variety of one or other
of the parent species. In the case of the ordinary species of zoology
such a hybrid would, in general, be infertile, or if it did produce
offspring these would be infertile. In the case of ordinarily bred
offspring from parents of the same species a large deviation from the
parental characters might be a malformation, or the result of some
irregularity of development. An “atavistic” variation we may regard as
the reappearance of some character present in a more or less remote
ancestor. Thus dogfishes and skates are no doubt descended from some
elasmobranch fish which possessed an anterior dorsal fin. This fin
persists in the dog-fishes, but has been lost in the skates and rays.
Yet it may appear in the latter fishes as an atavistic variation.
In a variety (following de Vries’ analysis) a character which
disappears is not really lost: it is only suppressed, and it still
exists in a latent form. Some flowers are coloured, for instance, but
there may be varieties in the species to which they belong in which the
flowers are colourless. It may not be quite correct, in the physical
sense, to say that the colour has been lost, but we may put it in this
way. These flowers are then coloured and colourless varieties of the
same species. Colour or lack of colour is not, however, fixed in the
variety, for the individual plant bearing colourless flowers also bears
in its organisation the potentiality of producing coloured flowers.
The petals of a flower may be smooth or covered with hairs, and in the
same stock both of these varieties may occur. But we must not speak
of the presence or absence of hairs as constituting a difference of
kind: the smooth-petalled flowers might be regarded as containing the
epidermal rudiments of hairs. So also coloured and colourless flowers
may be regarded as containing the same kinds of pigment, but these
pigments are mixed in different proportions. Such a view enables us to
look upon these contrasting characters in the same way as we look upon
fluctuating variations, that is, as quantitative differences in the
value of the same character.
Such a suppression of a character is not really a loss. An organism
belonging to an elementary species in which, say, monochromatic flowers
are usually produced may produce flowers which are striped. The progeny
of the plant may still produce monochromatic flowers, but we must
think of it as also possessing the potentiality of producing striped
flowers. In the terminology of Mendelism the characters are dominant
and recessive ones.
In discussing Mendelian varieties we consider the manner in which two
contrasting characters--one present in the male parent and one in the
female--are transmitted to the offspring. The characters in question
may be the tallness of the male parent and the contrasting shortness
of the female; or the brown eyes of the male and the blue eyes of the
female; or the brown skin of the female parent and the white skin of
the male one. These characters may be inherited in two ways: either
they may be blended or they may remain distinct in the offspring. The
children of the brown mother and the white father are usually coloured
in some tint intermediate between those of the parents. The mulatto
hybrid is fertile with either of the parent races, and again the
offspring may take a tint intermediate between those of the parents,
and so on through a number of generations. But somewhere in this
series the concealed or recessive brown colour may appear in all its
completeness, showing that it has been present in the organisations
of all the intervening generations. The progeny of a tall male parent
and a short female parent are not, in general, intermediate in stature
between the parents; some of them may be tall and others short. The
children of a brown-eyed mother and a blue-eyed father do not usually
have eyes in which the colours of the parental eyes are blended: they
are blue-eyed or brown-eyed. The contrasting characters are spoken of
as dominant and recessive: if tallness is transmitted to offspring,
which may nevertheless produce dwarf offspring, the latter character
is said to be recessive to tallness. The contrasting characters of the
parents therefore remain distinct in the progeny, some of the latter
exhibiting the one character and some the other; while it may happen
that the one character or the other may be segregated, so that it only
appears in, and is transmitted by, the offspring. There are numerical
relationships between the numbers of the offspring in which the
contrasting characters appear.
Obviously, tallness and dwarfness are not characters which differ in
_quality_: they are different degrees of the same thing. Brown eyes
and blue eyes are not necessarily different in quality, for we may
think of the same kinds of pigment as being present in the iris but
mixed in different proportions. But the terminology of this branch of
biology appears to suggest that the contrasting characters are, each
of them, something quite different from the other: there are “factors”
for “tallness,” “dwarfness,” for blue eyes and brown eyes, and so on.
These qualities are called “unit-characters,” and they are supposed to
possess much the same individuality in the germ-plasm as the “radicles”
of the chemist possess in a compound. Sodium chloride, for instance,
is not a blend of sodium and chlorine: the two kinds of atoms do not
fuse together but are held together merely. The analogy is, however,
very imperfect, for in the chemical molecule the characters are not
those of either of the constituents but something quite different,
whereas in the Mendelian cross the characters remain distinct, but one
of them is patent while the other is latent. In the molecule, however,
the atoms are regarded by the chemist as lying beside each other in
certain positions, and the Mendelian factors are also spoken of as if
they lay side by side in the germ-plasm. This terminology is useful,
perhaps necessary, in the work of investigation, but we must not forget
that it symbolises, rather than describes, the results of experiment.
If the factors are identified with certain morphological structures
in the nuclei of the germ-cells, obviously all the objections that
may be urged against the Weismannian hypothesis as an hypothesis of
development apply also to the Mendelian hypotheses as descriptions of a
physical process of the transmission of morphological characters.
It should clearly be understood what is implied in the construction of
such a hypothesis. Certain processes are observed to take place when a
somatic cell divides: these processes we have regarded as having for
their object the exact division of all the parts of the cell into two
halves. This process of somatic cell division is modified when a germ
cell divides prior to maturation (the process fitting it to become
fertilised). Then the cell nucleus divides into four daughter-nuclei.
One of these remains in the cell substance which is to become the ovum,
and the other three, each of them invested in a minimal quantity of
cytoplasm, are eliminated as the “polar bodies.” Also the number of
chromosomes in the mother-cell becomes halved, so that the mature ovum,
or spermatozoon, possesses only one-half of the number of chromosomes
which are present in the ordinary somatic cell. Now let the reader
puzzle out for himself what may be meant by this behaviour of the
germ cells, and he will certainly see that several interpretations are
possible. But suppose that the chromatin consists of an incredibly
large number of bodies differing in chemical structure from each
other, and occupying definite positions with regard to each other;
and suppose that there is a mechanism of unimaginable complexity in
the cell capable of rejecting some of these chemically individualised
parts, and of “assembling” or arranging the others in much the same way
as an engineer assembles the parts of a dynamo when he completes the
machine. _Then_ we may regard the hypothetical discrete bodies which
form the hypothetical nuclear architecture as the material carriers of
Mendelian characters. It is strange that the correspondence of such a
logically constructed mechanism with the effects which it would produce
if it existed should be regarded as a proof that it does exist, yet
biological speculation has actually made use of such an argument. “It
seems exceedingly unlikely that a mechanism so exactly adapted to
bring it” (the separation from each other of the Mendelian material
“factors” of inheritance) “about should be found in every developing
germ cell if it had no connection with the segregation of characters
that is observed in experimental breeding.” Put quite plainly this
argument is as follows: there is a certain segregation to be seen
in experimental breeding, and certain processes may be observed to
occur in the developing germ cell. Add to these processes many others
logically conceivable, and add to the observed material structure of
the cell another structure also logically conceivable. Then the assumed
mechanism and structure is “exactly adapted” to produce the effects
which are to be explained. Therefore the mechanism and structure do
actually exist!
That which renders the son similar to the father--the specific
organisation--is undoubtedly very stable, and it may persist in the
face of a variable environment. But now and then the son differs
from the father. The differences may be “accidental” and may not be
transmitted further--then we have to deal with an unstable fluctuation;
or the differences may be permanent--then we have to deal with a stable
mutation. What “produces” a mutation? A change of the environment, it
may be said: if so, the mutation is an active change or adaptation of
the organism to a change in its surroundings, and this adaptation is a
permanent one and is transmitted. Or the mutation may be a spontaneous
change of functioning. If this disturbance of the stability of the
organisation is _general_, if it affects all the characters of the
organism, we have to deal with the establishment of a new elementary
species. But if the disturbance affects only one, or a few characters,
then we need not recognise that a new elementary species has come into
existence. Men and women remain men and women (in their morphology),
although some time or other among the brown eyes characteristic of a
race blue eyes may have appeared. The result of the disturbance, in
this case, has been to cause one, or a few, of the characters that
fluctuate to surpass their limits of stability.
The idea of the elementary species is a clear and simple one. It is
a group of organisms connected by ties of blood relationship: all
have descended from one pair of ancestors. The individuals exhibit
certain characters, all of which are variable. This variability is
not cumulative; in generation after generation the individuals of
the species display variations which fluctuate round the same mean
values. Two or more elementary species may have had the same origin--a
common ancestor or ancestors--but the organisms in one species exhibit
characters which, although similar in nature to those of the other
species, yet fluctuate about different mean values.
This is not the “species” of the systematic biologist. The Linnean or
systematic species is a concept which is much more difficult to define:
it is a concept indeed which has not any clear and definite meaning, in
actual practice.
We often forget how very young the science of systematic biology
is, and how intimately its progress has been dependent on that of
human invention and industrial enterprise. Physics and mathematics
might be studied in a monastic cell, but the study of systematic
biology can only be carried on when we have ships and other means of
travelling--the means, in short, of collecting the animals and plants
inhabiting all the parts of the earth’s surface. Until a comparatively
few years ago the fauna and flora of great tracts of land and sea were
almost unknown: even now our knowledge of the life of many parts of the
earth is scanty and inaccurate. Systematic biology has therefore had to
collect and describe the organisms of the earth, and in so doing it has
set up the Linnean species of plants and animals. These we may describe
as, in the main, categories of morphological structures. The older and
more familiar species are clearly defined in this respect: such are
cats and dogs, rabbits, tigers, herrings, lobsters, oysters, and so
on: the individuals in each of these categories are clearly marked out
with respect to their morphology, and the limits of the categories are
clearly defined. In all of them the specific organisation has attained
a high degree of stability so that the individuals “breed true to
type”; and it has also attained a high degree of specialisation, so
that it does not fuse with other organisations.
Yet, in the majority of the systematic species of biology, this
criterion of specific individuality--this recognition of the isolation
of the species from other species--cannot be applied. Very many species
have been described from a few specimens only, many from only one. How
does a systematist recognise that an organism with which he is dealing
has not already been classified? It differs from all other organisms
most like it, that is, he cannot identify it with any known specific
description. But the differences may be very small, and if he had a
number of specimens of the species most nearly resembling it he might
find that these differences were less than the limits of variation in
this most closely allied species, and he would then relegate it to
this category. But if he has to compare his specimen with the “type”
one, that is, the only existing specimen on which the species of
comparison was founded, the test would be unavailable. The question to
be answered is this: are the difference or differences to be regarded
as fluctuations, or are they of “specific rank”? Now certainly many
systematists of great experience possess this power of judgment, though
they might be embarrassed by having to state clearly what were the
grounds on which their judgment was based. But on the other hand hosts
of species have been made by workers who did not possess this quality
of judgment; and even with the great systematists of biology confusion
has originated. Slowly, very slowly, the organic world is becoming
better known, and this confusion is disappearing.
The species, then, whether it is the systematic group of the biological
systems, or the elementary species based on the study of variability
and inheritance, is an intellectual construction: an artifice designed
to facilitate our description of nature. This is clearly the case with
the higher orders of groups in classifications: genera, families,
orders, classes, and phyla express logical relationships, or describe
in a hypothetical form our notions of an evolutionary process. But
species, it may be said, have an actual reality: there are no genera in
nature, only species. These categories of organisms really exist; they
have individuality, a certain kind of organic unity, inasmuch as the
individuals composing them have descended from a common ancestor. Yet
just as much may be said of genera, families, and the other groupings.
One species originates from another by a process of transmutation:
a genus is a group of species which have all had a common origin; a
family is a similarly related group of genera, and so on. The higher
categories of biological science are intended to introduce order and
simplification into the confusion and richness of nature as we observe
it, but obviously the concept of the species has the same practical
object. Must we then say that there are no species in nature, only
individuals? If so, we are at once embarrassed by the difficulty of
forming a clear notion of what is meant by organic individuality. Does
it not indicate that life on the earth is really integral, and that our
analysis of its forms--species, genera, families, and so on--are only
convenient ways of dealing actively with all its richness?
Systematic biology is a very matter-of-fact occupation, and one is
surprised to find upon reflection how he, in his handling of the
concepts of the science, follows the methods of ancient philosophy.
In classical metaphysical systems mutability was an illusion. Behind
the confusion and change given to sensation there is something that
is immutable and eternal. If there is change there is something that
changes; or, at least there ought to be something that changes when it
is perceived through the mists of sensation, just as the image of a
well-known object on the horizon wavers and is distorted by refraction.
This immutable reality is the Form or Essence of the Platonic Idea:
that which is in some way degraded by its projection into materiality,
so that we become aware of it only through our imperfect organs of
sense. We do not see the Form itself, but its quality rather, the Form
with something added or something taken away from it.
The Form itself is only a phase in a process of transmutation.
Everything that exists in time flows or passes into something else.
But it is not a momentary or instantaneous view of the flux that we
see, but rather a certain aspect of the reality that flows, that in
some way expresses the nature of the transmutation from one Form
into another. The sculptor represents the motion of a man running by
symbolising in one attitude all the actions of body and limbs; so that
from our actual, sensible experience or intuition of the movement of
the runner we see in the rigid marble all the plasticity of life. The
instantaneous photograph shows us a momentary fixed attitude of the
runner--an attitude which is strange and unfamiliar. The Idea does not,
then, represent a moment of becoming like the photograph, but rather a
typical or essential phase of the process of transmutation, just as the
sculptor represents in immobile form the characteristic leap forward of
the runner. Just as our intuitive knowledge of the actions of our own
bodies enables us to read into the characteristic attitude represented
in the marble all the other attitudes of the series of movements, so
our experience enables us to expand the formal moment of becoming into
the action which it symbolises.
This action has a purpose, an intention or design which was
contemplated before it began. There is therefore the threefold meaning
in the Platonic Idea: (1) an immutable and essential Form of which
we perceive only the quality; (2) the characteristic phase in the
transmutation of this Form into some other one; and (3) the design or
intention of the transmutation.
This was, as Bergson says, the natural metaphysics of the intellect.
It was, in reality, the “practical” way of introducing order and
simplification into the confusion of the sensible world--all that is
presented to us by our intuitions. And in the effort to reduce to order
the welter of the organic world biology has followed the same method,
so that it represents the species with the threefold significance of
the Platonic Idea. That which is expressed in the term species is
an assemblage of organisms each of which is defined by an essential
form and an essential mode of behaviour--the characters indicated in
the specific diagnosis. But organisms are variable, their specific
characters fluctuate round a mean, and in saying this we suggest that
there is something which varies--there _ought to be_ an essential form
from which the observed forms of the individuals deviate, something
invariable which nevertheless varies accidentally. This is (1) the
quality of the specific idea. So also we never do actually observe
the essential individual; what we do see is the embryo, or the young
and sexually immature organism, or the sexually mature one, or the
senescent one: there is continual change from the time of birth to
that of senile decay. This confusion is unmanageable, and for it we
substitute the characteristic form and functioning, and that phase in
the life-history of the organism which suggests all that the previous
phases have led up to, and all that subsequent phases take away. Thus
there is contained in our idea of the species (2) the notion of a
typical moment in an individual transformation. It is not a “snap-shot”
of some moment in the life-history that we make: in identifying a
larval form as some species of animal we are identifying it with all
the other phases of the life-history.
Since we accept the doctrine of transformism, the specific idea also
includes that of an evolutionary process. For the organic world is
a flux of becoming, and species are only moments in this becoming.
It does not help us to reflect that if the hypothesis of evolution
by mutations is true the process is a discontinuous one: mutability
is the result of periods of immutability during which the change was
germinating, so to speak. In this flux of becoming we seize moments at
which the specific form flashes out--not as instantaneous views of the
flux, but as aspects of it which suggest the steps, the morphological
processes, by which the transmutation of the species has been effected.
Thus our specific idea represents not only a phase of becoming in
an individual life-history, but also a phase of becoming in an
evolutionary history.
Whether we consider this evolutionary movement as the working out of a
Creative Thought, or as the development of elements assembled together
by design, or as the results of the action of a mechanism working by
itself, we must suppose that underlying it there is design, or purpose,
or determinism. All is given, therefore, and our comparison between
the metaphysical Platonic Idea and the modern concept of the species
becomes complete.