differentiation, depend always upon the interaction of external and
internal factors, of the environment and the living substance, and
the resultants of the interaction, namely, the structure of the body
and its parts must necessarily turn out differently, not only when
the germ-substance is different, but when the essential conditions
of development are changed. But that the constitution of the germ is
by far the most potent factor, and that the nature of the results of
development depends on it in a much greater degree than on the external
conditions, has long been known. The conditions, such as warmth, may
vary within certain limits, and yet the frog's egg becomes a frog;
though it does not follow that the result of development may not be
modified through certain changes in the conditions. The interesting
experiments made by Herbst with the eggs of sea-urchins have shown
that, in artificially altered sea-water in which sodium-salts are to a
slight extent replaced by lithium-salts, these eggs develop into larvæ
which only remotely suggest the normal structure, and diverge widely
from it both in external shape and in the form of the skeleton.
Such larvæ are not able to survive, but soon perish; they are,
however, of great interest from the point of view of our theory, for
they show that determinants do not bring forth the same structure
under all circumstances, but that, as I have already said, they are
vital units of specific composition, which play a part in the course
of development, and give rise under normal external influences to
normal parts, while under unusual influences, if these are not such
as to prohibit development altogether, they may give rise to an
abnormally formed part. It must not be forgotten that most composite
parts--indeed, strictly speaking, all the parts--of an animal are not
controlled by a single determinant, but by the many which successively
determine the character of the cells and define the path of development
of the part in question. There are no determinants of 'characters,'
but only of parts; the germ-plasm no more contains the determinants
of a 'crooked nose' than it does those of a butterfly's tailed wing,
but it contains a number of determinants which so control the whole
cell-group in all its successive stages, leading on to the development
of the nose, that ultimately the crooked nose must result, just as the
butterfly's wing with all its veins, membranes, tracheæ, glandular
cells, scales, pigment deposits, and pointed tail arises through the
successive interposition of numerous determinants in the course of
cell-multiplication.
But in both processes we must presuppose _normal conditions of
development_. In regard to the butterfly we know that abnormal
conditions, such as cold during the pupal period, can cause
considerable variation in the colour and marking of the wing, and in
regard to the nose it can scarcely be doubted that, for instance,
persistent pressure on the nasal region would result in a considerable
deviation from the hereditary form.
The case of the lithium-larvæ is similar. Here the chemical conditions
of the first segmentation-cells are modified by the presence of the
lithium-salts, and the determinants which make their way out of the
nucleus in the first and in subsequent cell-generations find an
unusual soil for their activity, which diverges further and further
from the normal with each successive cell-generation. Thus the whole
animal is abnormally formed. The process may perhaps be compared to a
plant which is negatively geotropic and positively heliotropic, that
is, the stem of which tends to grow straight upwards, while all its
green parts grow towards the light. If a plant of this kind have light
shed on it from one side only, the stem with its leaves will grow
obliquely towards that side. If the plant be then turned round so that
it receives light from the other side, the stem in its further growth
will curve in a direction opposite to that which it took before, and so
by continually changing the position of the plant in relation to the
light one could--theoretically at least--produce a plant with a zigzag
stem. But this would not furnish any evidence against the presence of
determinants; there are no 'upright determinants' any more than there
are 'zigzag determinants' or 'crooked nose determinants,' but there are
determinants controlling the nature of the cells which give rise, under
normal conditions of development, to the straight stem, under abnormal
conditions to the zigzag stem, or to a flat nose instead of a crooked
one, and so on.
This consideration should make it clear that plant-galls are not in
the remotest degree a stone of stumbling for the determinant theory,
as some have supposed. Of course there can be no 'gall-determinants,'
for galls are not transmissible adaptations of the plants on which they
occur; they arise solely through the larvæ of the gall-insect which
has laid its eggs within the tissues of the plant. But the specific
nature of the different kinds of plant-cells, predetermined by their
determinants, is such that, through the abnormal influences exercised
upon them by the larvæ, they are compelled to a special reaction which
results in the formation of galls. It is marvellous enough that these
abnormal stimuli should be so precisely graded and adjusted that such a
specifically definite structure should result, and in this case there
is obviously a very different state of matters from that obtaining in
most other processes of development, in which the chief determining
factor is rather implied in the nature of the idioplasm, that is, of
the determinants, than in the nature of the external influences. Here,
however, the specific structure of the gall depends mainly on the
quality, variety, and successive effects of the external influences or
stimuli. In discussing the influences of surroundings I shall return
once more to the galls.
My determinants have generally been regarded as if they were like
grains of seed, from which either nothing may arise, under unfavourable
conditions, or just the particular kind of plant from which the seed
itself originated.
This simile is, however, to be taken _cum grano salis_. The whole ovum
is certainly comparable to a grain of seed, but single determinants
or groups of determinants will always be able to adapt themselves
to different influences, and to remain active even under slightly
abnormal conditions, though in that case the resulting structures may
be somewhat divergent. This relative plasticity is indispensable even
in relation to the ceaseless mutual adaptations of the growing parts
of the organism. Not only do the cells which live beside each other
at the same time influence each other mutually, but the influence
extends to the whole cell-lineage. No cell or group of cells develops
independently of all the others in the body, but each has its ancestral
series of cells on whose determinants it is so far dependent, since
these have taken part in determining its own nature, in, so to speak,
supplying the soil in which ultimately its own determinants will
be sown from the nucleus, and whose influence modifies these last
according to its quality. We might therefore say that every part is
determined by all the determinants of its cell-ancestors.
If there be urged against the doctrine of determinants the undoubted
fact of the dependence of individual development on external
conditions, or the capacity that organisms have of functional
adaptation, or especially the power that some parts of the organism
have of taking a different form in response to different stimuli, I can
only say that I see no reason why certain cells and masses of cells
should not be adapted from the first for responding differently to
different stimuli.
Therefore I see no contradiction of the determinant theory when, for
instance, among the higher vertebrates, the cells of the connective
tissue exhibit a great diversity of form, becoming a loose 'filling'
connective tissue in one place, a tense fascia, ligament, or tendon
tissue in another, according as they are subjected to slight pressure
on all sides or to stronger pressure on one side. I see no difficulty
in the fact that this connective tissue forms in one case bone-tissue
with the most accurate adaptation of its microscopic structure to the
conditions of stress and pressure which affect the relevant spot,
or in another case cartilaginous tissue, when the cells are exposed
to varying pressure (as on the surface of joints), or even that it
gives rise to blood-vessels when the pressure of the circulating
blood and the tension of the surrounding tissues supply the necessary
stimulus. It is easy to see how important, indeed how necessary, the
many-sidedness of these cells is for the organism, even leaving out
of account such violent interference as the breaking of a bone, the
irregular healing of broken ends of bones, new joint formation, and
so on, and thinking only of the normal phenomena of growth. While the
bone grows it is continually breaking up in the inside and forming anew
on the surface, and this occurs through the power of the connective
tissue-cells to form different tissues under different influences or
stimuli.
We must therefore assume that there are side by side in the connective
cells of higher vertebrates determinants of bone, of cartilage, of
connective tissue in the narrower sense, and of blood-vessels, and
that one or other of these is liberated to activity according to the
stimulus affecting it. Phenomena occur also in the development of lower
animals which lead us to the same assumption.
Among these is the remarkable behaviour of the primary mesoderm-cells
in the young embryo (gastrula) of the Echinoderms (Fig. 92). At the
point where the primitive gut or archenteron invaginates into the
interior of the hitherto single-layered blastula (Fig. 92, _A_), some
cells are separated off (_M_), and move independently, constantly
multiplying the while, into the clear gelatinous fluid (_G_) which
fills the cavity of the larva, and there they fix themselves, some
on the outer ectodermic layer, others to the various regions and
outgrowths of the archenteron (_Ms_). According as these cells have
established themselves at one or another point, they become connective
tissue, muscle, or skeleton cells of the dermis, or contribute to
the muscular layer of the food-canal and water-vascular system, or,
finally, become skeleton-forming cells of the calcareous ring which
surrounds the gullet of the sea-cucumber. In all this there is nothing
to indicate a determination of the cells in one direction; on the
contrary it seems as if the fate of the individual cells depended on
the chance conditions which may lead them to one place or to another.
[Illustration: FIG. 92. Echinoderm-larvæ. _A_, blastula-stage; the
primary mesoderm-cells (_M_) are being formed at the subsequent
invagination-area of the endoderm (_Ent_). _Ekt_, the ectoderm. _B_,
gastrula-stage; the archenteron (_UD_) has been invaginated (_Ent_),
and between it and the ectoderm (_Ekt_) the mesoderm-cells (_Ms_)
migrate into the gelatinous fluid which fills this cavity. There they
attach themselves partly to the ectoderm, and partly to the endoderm.
After Selenka.]
There are thus three possibilities of development, three kinds of
reaction, implied in these cells, which are all outwardly alike, and
we can only understand their rôle in the building up of this very
symmetrical animal if we assume that of these three only one is in
each case liberated, by the specific stimulus exerted by the immediate
surroundings of the cell, so that it may become, according to the
chance position it takes up after its migration, either a skin-cell, a
muscle-cell, or a skeleton-forming cell.
This case may be compared in some respects with the permanent
colour-adaptation of those caterpillars, in regard to which Poulton
demonstrated that they become almost black if they are reared on
blackish-brown bark, light brown on light bark, and green if they are
kept among leaves, and in all cases permanently so. In this case also
the implicated pigment-cells of the skin may develop in three ways,
according to whether this or that quality of the light releases this or
that determinant.
But in many cases we do not know the quality of the liberating
stimulus, and must content ourselves with imagining it. This is so in
the case of dimorphism of the sexes. It is clear that in the males
of a species the germ-cells develop quite otherwise than they do in
the females, that different determining elements attain to activity
in each sex, and since the primary constituents of both sexes must be
contained in most animals in the ovum and in the spermatozoon, we must
assume that in both there are at once 'ovogenic' and 'spermogenic'
determinants, of which, however, only _one_ kind becomes active in a
given individual. There are, however, both among plants and animals
hermaphrodite individuals, in which both kinds of sexual products are
developed simultaneously or successively.
It is not only the primary sexual characters, however, that compel
us to the assumption of double determinants in the germ-plasm, the
secondary sexual characters do so too. We know very well in relation
to ourselves that 'the beautiful soprano voice of the mother may be
transmitted through the son to the grand-daughter, and that the black
beard of the father may pass through the daughter to the grandson.'
Thus both kinds of sexual characters _must be present in every sexually
differentiated being_, some visible, others latent. In animals the
determinants are sometimes handed on from germ-plasm to germ-plasm
through several generations in a latent state, and only make their
appearance again in a subsequent generation. This is the case in the
water-fleas (Daphnids) and the plant-lice (Aphides), in which several
exclusively female generations succeed one another, and only in the
last of them do males occur again side by side with the females.
The germ-plasm of the ovum which is ripe for development must thus
contain not only the determinants of the specific ova and sperms of the
species, but also those of all the male and female sexual characters,
which we discussed at length in the section on sexual selection. I
then showed that these secondary sexual characters differ greatly
in range and in strength, that among lower animals they are almost
entirely absent, and that among higher forms, such Crustaceans,
Insects, and Birds, they attain to very different grades of development
even among the same species. Thus the birds of Paradise are in most
species brilliantly coloured and adorned with decorative feathers
only in the male sex, while the females are simply blackish-grey, but
there is a single species in which the males are almost as soberly
coloured as the females. Conversely, too, we find that in parrots both
sexes are usually coloured alike, but a few species exhibit a totally
different colouring in the two sexes. In the same way the secondary sex
differences may affect only a few parts of the animal or many, while
in a few species the sexes are so divergent in structure that almost
everything about them may be called different. Examples of this are
the dwarf males of most Rotifers, and the males, more minute still in
proportion to the females, of the marine worm _Bonellia viridis_ (p.
227).
We have now to inquire what theoretical explanation of these facts we
can arrive at in accordance with the germ-plasm theory. That double
determinants, male and female, for the differently formed parts of the
two sexes must be assumed to exist in the germ-plasm has been already
said, and we have to suppose that the same stimulus--usually unknown
to us--which incites the determinants of the primary sexual characters
to activity also liberates those of the secondary characters. But we
may safely go a step further and conclude that there are male and
female _ids_, that is, that the male and female determinants belong
to different ids. I infer this from the fact that in some groups,
such as the Rotifers and certain plant-lice, the ova are sexually
differentiated even at the time of their origin. Males and females
of these animals arise from different kinds of eggs, which are even
externally recognizable. Both develop parthenogenetically, so that
fertilization has nothing to do with it; from the first, therefore,
they must contain ids which consist of determinants of one sex alone.
If this conclusion be correct, then the sexual equipment of the
determinants of the sexual characters must have taken place in the
course of phylogeny in such a way that each id was affected in one
direction only, and we should thus have to assume male and female ids,
even before the separation of the sexes as males and females, and the
same conclusion must be extended to the primary sexual characters. Only
in this way can we understand the fact that differences between the
sexes, at first small, have increased in the course of phylogeny to
such complete divergence of structure as is now exhibited in the forms
we have named, _Bonellia_, the Rotifers, and some parasitic worms.
But there is not only sexual dimorphism, there is also dimorphism
of larvæ, e.g. green and brown caterpillars in certain species of
hawk-moth (_Sphinx_), and there are sometimes not only two but three
or more forms of a species; and in all these cases determinants of the
differential parts must be represented twice, thrice, or several times
in each germ-plasm, in each fertilized ovum, at least in all cases in
which the different forms live together on the same area. In discussing
mimicry we spoke of species of butterfly which were everywhere alike
or nearly so in the male sex, while the females were not only quite
different from the males, but differed greatly in many respects among
themselves. Three different forms of females of _Papilio merope_ occur
in the same region of Cape Colony, each of these resembling a protected
model. All three forms have been obtained from the eggs of one female.
In this case the female ids of the germ-plasm must be represented by
three different sets, one of which, when it is in the majority in the
fertilized ovum, gives rise to the _Danais_-form, the second to the
_Niavius_-form, and the third to the _Echeria_-form of the species.
Phylogenetically considered, it is probable that each of these three
kinds of ids originated by itself, on a more limited area on which the
protected model lived in abundance; but with a wider distribution the
different female ids mingled together, were united through the males
into a single germ-plasm, and now occasionally exhibit all three forms
on the same area. I doubt whether there is any other theory that can
offer an interpretation of these facts, and I regard them, therefore,
as affording further evidence of the real existence of ids.
The polymorphism of social insects must be thought of as similarly
based in the germ-plasm.
In bees there are in addition to the males and females the so-called
workers, and this can only depend on the existence of special kinds
of ids. Those of the workers were originally truly female, but as
many of their determinants underwent variations advantageous for
the maintenance of the species, they were modified into special
'worker-ids.' I postpone for the present any inquiry into the causes by
which these ids come to dominate the ontogeny; obviously it cannot be
by the mere fact of being in a majority over the rest of the ids, as I
indicated in the case of the butterflies with polymorphic females.
In many ants the division of labour goes further still; there are two
kinds of workers in the colony, the ordinary workers and the so-called
'soldiers,' and in this case the worker-id must have developed in two
different directions in the course of phylogeny, and have separated
into two kinds of ids, so that the germ-plasm of these species must
contain four kinds of ids.
I might cite many more cases in regard to which the assumption of two
or more kinds of determinants seems imperative, but I believe that what
has been said is enough to enable any one to think out other cases for
himself.
LECTURE XIX
THE GERM-PLASM THEORY (_continued_)
Co-operation of the determinants to form an organ: insect
appendages--Venation of the insect-wing--Deformities in
Man--Apex of the fly's leg--Proofs of the existence of
determinants--Claws and adhesive lobes--Difference between a
theory of development and a theory of heredity--Metamorphosis of
the food-canal in insects--Delage's theory--Reinke's theory of the
organism-machine--Fechner's views--Apparent contradiction by the facts
of developmental mechanics--Formation of the germ-cells--Displacement
of the germinal areas in the hydro-medusoid polyps, a proof of the
existence of germ-tracks.
IT would be futile to attempt to guess at the arrangement of the
determinants in the germ-plasm, but so much at least we may say, that
the determinants do not lie beside each other in the same disposition
as their determinates exhibit in the fully-formed organism. This may be
inferred from the complex formative processes of embryogenesis in which
many groups of cells, which in their origin were far apart, combine
together to form an organ. Thus the arrangement of the determinants
in the germ-plasm does not correspond to the subsequent arrangement
of the whole animal, nor are primary constituents of the _complete_
organs contained within the germ-plasm. The organ is undoubtedly
_predetermined_ in the germ-plasm, but it is not _preformed_ as such.
Here, again, the history of development gives us a certain basis of
fact from which to work. Let us consider, for instance, the origin
of the appendages in those insects which in the larval state possess
neither legs nor wings, but exhibit a gradual emergence of these
structures from concealment underneath the integumentary skeleton.
In these cases, as I have already shown in regard to the wings, the
development of the limbs arises from definite groups of cells in the
skin. These must therefore be regarded as the formative, and therefore
as the most important and indispensable, parts of the rudiments, and
may be designated the imaginal disks, as I many years ago proposed[22]
(Fig. 89, _ui_ and _oi_).
[22] _Die Entwicklung der Dipteren_, Leipzig, 1864.
But these disks of cells do not contain the _whole_ leg, but only the
skin-layer of it, the 'hypodermis,' which, however, in this case
undoubtedly determines the form. But the internal parts of the leg,
especially the nerves, tracheæ, and probably also the muscles, are
formed from other cell-groups and grow into the imaginal disk from
outside. Something similar probably takes place in the case of all
organs which are made up of many parts; they are, so to speak, shot
together from several points of origin, from various primordia; and
determinants are brought into co-operation whose relative value in
determining the form and function of the organ may be very diverse.
[Illustration: FIG. 89. Anterior region of the larva of a Midge
(_Corethra plumicornis_). _K_, head. _Th_, thorax. _ui_, inferior
imaginal disks. _oi_, superior imaginal disks. _ui_^1, _ui_^2, and
_ui_^3, the primordia of the limbs. _oi_^2 and _oi_^3, the primordia of
the wings and 'balancers.' _g_, brain. _bg_, chain of ventral ganglia
with nerves which enter the imaginal disks. _trb_, tracheal vesicle.
Enlarged about 15 times.]