and through several generations, who have a white lock of hair, in
a particular spot, on an otherwise dark-haired head. This cannot be
referred to external influences, it must depend on a difference in
the germ, on one, too, which does not affect the whole body, not even
all the hairs of the body, but only those of a particular spot on the
surface of the head. It is a matter of indifference whether the white
colouring of the hair-tuft is produced by an abnormal constitution of
the matrix of the hair, or by other histological elements of the skin,
as of the blood-vessels or nerves. It can only depend ultimately on a
divergently constituted part of the germ-plasm, which can only affect
this one spot on the head, and alter it, if it is itself different from
what is usual. On this account I call _it_ the _determinant_ of the
relevant skin-spot and hair-group. In Man such minute local variations
are usually lost after a number of generations, but in animals there
are innumerable phenomena which prove to us that single minute
deviations can become permanent. Thus there lives in Central Europe a
brown 'blue butterfly,' _Lycæna agestis_, which has a little black spot
in the middle of its wing. The same species also occurs in Scotland,
but there, instead of the black spot, it has a milk-white one, and
so-called 'eye-spots' on the under surface of the wing have also
lost their black centres. The species has thus varied transmissibly,
but only in regard to these particular spots on the wing. A slight
variation must therefore have taken place in the germ-plasm which only
affects these few parts of the body, or, to express it otherwise, the
germ-plasms of the ancestral species and of the variety can only be
distinguished by a difference which determines exclusively the scale
colour of these spots. The two germ-plasms differ, I should say, only
as regards the _determinants_ of these wing-scales.
We know from the artificial selection to which Man has subjected and
still subjects his domesticated animals and useful plants, that any
spots and parts of the body which he chooses can be hereditarily
altered, if the desired variations which present themselves are always
selected for breeding, and that this does not necessarily cause
variation in other parts of the body. When, for instance, in the case
cited by Darwin, the comb of a Spanish cock which had previously hung
downwards was made to stand upright because a prize had been offered
for this character, or when a certain breed of hens was 'furnished with
beards,' the results were permanent variations affecting only the parts
on which the fancier's attention had been fixed. In the same way, when
the tail feathers of the Japanese cock are lengthened to three feet
the rest of the plumage does not alter, still less any other part of
the body. Of course there are numerous 'correlated' variations, and
in very many cases the breeder causes a second or third character, on
which he had not fixed his attention, to vary in addition to the one
he was aiming at. But such concomitant variations are not necessary
or inevitable in all cases; and indeed we need not refer them all to
a true correlation of the parts, but may suppose that they depend not
infrequently on the faultiness of our power of observation, which is
not sufficiently keen to control several parts of the body at one
time, and to notice minimal variations in parts on which we have not
specially fixed our attention.
[Illustration: FIG. 13. _Kallima paralecta_, from India; showing the
right under surface in the resting pose. _K_, head. _Lt_, palps. _B_,
limbs. _V_, fore wing. _H_, hind wing. _St_, 'tail' of the latter,
representing the stalk of the leaf. _gl_^1 and _gl_^2, transparent
spots, _Aufl_, remains of 'eye-spots.' _Sch_, a 'mould'-spot.]
So much, at least, is certain, that in all these cases of the
artificial alteration of individual characters the germ-plasm is in
some way changed, but always in such a way that it differs from that
of the ancestral form through such variations alone, and the effect of
these is that only the altered parts are influenced thereby, and not
the whole organism. This again is but another way of saying that only
the _determinants_ of these parts have altered.
We can see from a thousand cases that exactly the same happens in a
state of nature, that there, too, one part changes after another,
until the highest possible degree of adaptation to the conditions has
been attained. In the mimetic resemblance to leaves exhibited among
butterflies this is most clearly seen, for here we are familiar with
the model--the leaf--and we see how one species approximates to it in
a general way only in the total colour, how others develop a brown
stripe crossing the posterior wing obliquely, so that, to a certain
extent, it resembles the midrib of a leaf, how in a third species this
stripe is continued for some distance forward across the anterior wing,
in a fourth it goes a little further, until, finally, in a fifth, it
is continued on to the tip of the anterior wing. This may be seen,
for instance, in the genus _Anæa_, which is rich in species. But even
then a still further increase of the resemblance is possible, for, as
is well known, there are not infrequently imitations of the lateral
veins of the leaf as well, or dark spots which faithfully reproduce the
mould-spot on a damp, decaying leaf, or colourless transparent spots
which probably simulate dewdrops, and so on. All these are variations
relating to individual and distinct groups of wing-scales, which have
varied transmissibly and independently, that is, each of them has been
produced by a variation in the germ-plasm, which brought about a change
in this particular area of the body and in no other.
Let us for a moment assume the impossible, and suppose that we could
look on at the evolution of such a leaf-butterfly; the beginning of the
leaf-imitation might have its cause in the fact that an ancestral form
of _Kallima_, which had previously lived in the meadows, exhibited on
the part of some of its descendants a migration to the woods, and thus
divided into two groups, with a different manner of life--a meadow form
and a wood form. The latter adapted itself to sitting among leaves, and
the midrib of a leaf developed on its wings. In a germ-plasm without
'primary constituents' this variation could only depend on a uniform
variation of all the parts, for these parts are either alike among
themselves, or at any rate have the same value for every part of the
finished organism. But the germ-plasm of the new breed must somehow
differ from that of the ancestral form, otherwise it could produce no
new variety, but only the ancestral form over again. But how could
an animal differing only in one minute part arise from a germ-plasm
which has varied in all its parts, and how could such little steps of
variation be repeated many times in the course of the phylogeny without
the corresponding variations of the germ-plasm becoming so intense
that not only the wing-markings but everything about the animal would
be altered likewise? And yet these 'leaf-pictures' have not originated
suddenly, but by many small steps, so that the germ-plasm must have
varied _in toto_ a hundred times in succession if there are no primary
constituents.
In the Indian species, _Kallima paralecta_, there are no fewer than
five well-marked varieties, the differences between which depend solely
on the manner in which the leaf-picture on the wing is elaborated,
_for the upper surface of the wing is alike in all_. Even a cursory
observation of a collection of these butterflies shows that the lateral
veins of the leaf-picture are quite different in number, distinctness,
and length in the different individuals. On the right half of the wing
there may be as many as six of them indicated (Fig. 13); and it can
be observed that the three middle ones are the longest, most sharply
defined, and darkest, while those lying near the tip and the base of
the mimic leaf are shorter and often even shadowy. On the left side
the second lateral vein in particular distinctly shows indentations
indicative of the rings, inherited from the ancestral forms, which
surrounded the still visible eye-spots (_Aufl_); the third lateral
vein is quite indefinite and shadowy, but nevertheless it runs exactly
parallel to the first two, and thus heightens the deceptive effect. We
can thus distinguish older and more recent elements in the marking--a
proof of the slow and successive origin of the picture.
This is not reconcilable with the conception of a germ-plasm without
primary constituents, however complex a mixture it may otherwise
be. A substance which had to undergo thousands upon thousands of
variations, arising from each other according to law and in the
strictest succession, in order that it might become a definite
organism, predetermined as to all its thousands of parts down to the
most minute, cannot vary over and over again in its whole constitution
without the consequences showing themselves in numerous, or indeed in
_all_, the parts of the body. Such variations in the germ-plasm would
be comparable to many successive deviations of a ship from her course,
which, although the single ones would only cause a minimal deviation
from the true course, would, when summed up in a voyage of some length,
land the vessel at quite another coast than the one intended. If each
individual adaptation of the species depended on a variation of the
whole germ-plasm the wood _Kallima_ would soon retain no resemblance
to its ancestral form, the meadow species; yet we are acquainted with
species of _Kallima_ which do not show the special resemblance to a
leaf, but, for instance, still exhibit the perfectly developed eye-spot
of the ancestral form, and so forth. It follows, therefore, that the
origin of the leaf-picture has not greatly influenced the general
character of the species; and the fact that the upper surface of the
wings has remained the same in all the varieties is in itself enough to
prove this.
Since, then, the resemblance to a leaf cannot have arisen without
something in the germ-plasm varying, since the germ-plasm of a forest
_Kallima_ and a meadow _Kallima_ must be different in something, and
cannot be any more alike than the germ-plasm of a fantail-pigeon and a
carrier, there _must be 'primary constituents' in the germ-plasm_, that
is, vital units whose variation occasions the variation of definite
parts of the organism, and of these alone.
[Illustration: FIG. 17. Caterpillar of _Selenia tetralunaria_ on a twig
of birch. _K_, head. _F_, feet. _m_, protuberances resembling dormant
buds. Natural size.]
It is on such considerations as these that my assumption, that _the
germ-plasm is composed of determinants_, depends. There must be as many
of these as there are regions in the fully-formed organism capable
of independent and transmissible variation, including all the stages
of development. Every part, for instance, of the butterfly's wing,
which is capable of independent and transmissible variation, must,
so I conclude, be represented in the germ-plasm by an element which
is likewise variable, the determinant; but the same must be true of
every independently and transmissibly variable spot of the caterpillar
from which the butterfly developed. We know how markedly caterpillars
are adapted in form and colour to their environment. Let us assume
that the caterpillar of the butterfly which we chose as an example
of wing-marking had the habit of feeding only by night and during
the daytime of resting on the trunk of a tree, or, more precisely,
in the crevices of the bark. It would then resemble the caterpillar
of the moths of the genus _Catocala_ or the Geometers (Geometridæ),
and possess the colour of the bark of the tree in question; the
determinants of the skin would thus have varied to correspond with this
mode of life on the part of the caterpillar, so that the skin would
appear grey or brown. But there cannot be only _one_ determinant of
the caterpillar skin in the germ-plasm, for the bark-like colour of,
for instance, a Geometer caterpillar is not a uniform grey, but has
darker spots at certain places and lighter whitish spots at others,
such as are to be seen on the bark of the twig on which the caterpillar
is wont to rest, or brown-red spots, like those on the cover-scales of
the buds, or little warts and protuberances which exactly correspond
to similar roughnesses on the twigs, to cracks in the bark, and so on.
All these markings are constant, and are to be found in the same spot
in every caterpillar of the species. A large number of regions of the
caterpillar skin must therefore be independently determined by the
germ-plasm; the germ-plasm must contain parts the variations of which
bring about variations only of an independently variable region of the
caterpillar skin. In other words, in the germ-plasm of the butterfly
ovum there must not only be determinants for many regions of the
butterfly's wing, but also for many regions of the caterpillar's skin.
This line of argument, of course, applies to all the bodily parts and
organs of the butterfly and of the caterpillar, as well as to all the
stages of development of the species as far as these parts are able
to vary in such a way that the variation reappears in the following
generation, that is to say, as far as it is transmissibly variable.
But all parts must be transmissibly variable which have exhibited
independent variation in relation to their ancestors. When, for
instance, the eggs of a butterfly (_Vanessa levana_) bear a deceptive
resemblance to the flower-buds of the stinging-nettle on which
the caterpillar lives, not only in form and colour, but in their
pillar-like arrangement, we may conclude that these eggs have varied
transmissibly from those of their ancestors, which had not acquired
the habit of living on the stinging-nettle, in these three respects
independently, that is, uninfluenced by any other variations the
species may have undergone; and that, consequently, the germ-plasm
must contain determinants for the egg-shell, egg-colouring, and so
on. The manner of laying the eggs in the form of pillars depends on a
modification of the egg-laying instinct, which must in its turn depend
on the variations of certain nerve-centres, and we learn from this that
there must be in the germ-plasm determinants for the individual centres
of the nervous system.
It may, perhaps, be suggested that matters could be explained in a
simpler way--that it is enough to assume the presence in the egg of
determinants for all the parts of the caterpillar, and that those of
the butterfly are only formed within the caterpillar.
This suggestion seems justifiable if we confine ourselves to
superficial considerations. We read in every handbook of entomology
that the wings only arise during the life of the caterpillar, and
in a certain sense this is true, for the primary constituents or
primordia of wings only develop into the fully formed wing during the
larval period. But even if these primordia were only formed during
the caterpillar-stage, what could they develop from? Only out of the
material parts of the caterpillar, that is, from some of its living
cells or cell-groups. The constitution of the wings would therefore
be dependent on that of the cells of the caterpillar from which they
arose, so that if these varied transmissibly through the variation
of their determinants contained in the germ, the determinants of the
butterfly which were just developing would vary with them; every
transmissible variation of the caterpillar would necessarily cause a
similar variation in the butterfly, and this does not happen. If any
one hazarded the assumption that the determinants of the butterfly
develop only in the caterpillar, but quite independently of its
constitution, he would either be making an absurd statement, namely,
that the characters of the butterfly were not transmissible at all,
or he would be unconsciously admitting that the determinants of the
butterfly were already contained in the parts of the caterpillar, and
come direct from the germ-plasm.
That the characters of the butterfly do vary independently of those
of the caterpillar I demonstrated many years ago, when we were still
very far away from the idea of the germ-plasm or of determinants. I
demonstrated then that the constancy of the markings of a species can
be quite different in the two chief stages; that the caterpillar may be
very variable, while the butterfly or the moth may be very constant in
all its markings, or conversely. I called attention to the dimorphic
caterpillars which are green or brown, and yet become the same moth
(for instance, _Deilephila elpenor_ and _Sphinx convolvuli_); I cited
the case of the spurge hawk-moth (_Deilephila euphorbiæ_), whose dark
but at the same time motley caterpillars occur in the Riviera at
Nice as a local variety (_Nicæa_), and there wear quite a different
dress--pale clay-yellow, with a double row of large conspicuous dark
yellow eye-spots--while the moth does not differ from our variety
in a single definite character, except in its larger size. At that
time, too, I instituted experiments with the caterpillars of the
smallest of our indigenous Vanessa species (_Vanessa levana_), of
which the majority are black with black thorns, while a minority are
yellowish-brown with yellow thorns; reared separately, both yielded the
same butterfly, though in this case one would be inclined to suppose
that there was some internal connexion between the colour of the
caterpillar and that of the butterfly, since the butterfly also occurs
in two colours. It was shown, however, that the colour of the butterfly
had nothing to do with that of the caterpillar, for it is known to be
dependent on the season, and is a seasonal dimorphism, 'while the two
forms of caterpillar may occur side by side at all times of the year.'
Subsequently I made a similar experiment with the dimorphic
caterpillars of the 'fire'-butterfly (_Polyommatus phlæas_), and it
yielded the same result. The pure green caterpillars became the same
butterflies as those marked with broad red longitudinal stripes, and in
this case we can definitely describe both colours as protective, for
the green form is adapted to the green under surface of the leaf, the
red-striped to the green red-edged stalk of the lesser sorrel (_Rumex
acetosella_).
There was really no necessity for special proofs that the caterpillar
and butterfly vary transmissibly in complete independence of each
other, for the facts of metamorphosis alone are enough to prove it. How
would it have been possible otherwise that the jaws adapted for biting
should, in the primitive insects, and in the locusts which are nearest
to them, remain as a biting apparatus throughout life, while in the
caterpillar they are modified during its pupal stage into the suctorial
proboscis of the butterfly? The parts of insects, therefore, must be
capable of transmissible variation in the stages of life independently
of each other. Not only have the jaws of the leaf-eating caterpillars
remained unaltered, while in the sexually mature animal they have been
gradually modified into a very long and extremely complex suctorial
apparatus, but when at a much later time this proboscis became
superfluous in a species, because the butterfly or moth, from some
cause or another, lost the habit of taking any nourishment at all, its
degeneration exercised no effect on the jaws of the caterpillar, as we
can observe in many hawk-moths, silk-moths and Geometridæ. How could
such a degeneration become transmissible if the caterpillar's jaws,
from which those of the adult are developed, remain the same? We are
thus forced to assume that there is something in the latter which can
vary from the germ, without the jaws themselves being altered thereby.
This 'something' it is which I call 'determinants,' vital particles,
which--however we may try to picture them--are indeed contained in the
cells of the caterpillar's jaws, but are there inactive and do not
influence the structure of these, while, on the other hand, it is their
constitution which determines the form and structure of the suctorial
proboscis of the butterfly down to the minutest details. It must be
these alone which cause the suctorial proboscis to develop, and in some
cases to degenerate again, without bringing about any change in the
corresponding parts in the caterpillar.
[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.]