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Reprinted, without change of paging, from the Journal of Heredity (Organ of the American Genetic
Association}, Vol. VI, No. 10, Washington, D. C., U.S.A., October, 1915.

UNIT CHARACTERS

Reality of Their Existence is Fundamental to Study of Evolution, But Has Never
Been Proved — Independent Variability of Parts and Independent
Transmissibility of Variations Open to Question1

S. J. HOLMES
Associate Professor of Zoology, University of California, Berkeley, Cal.

THE doctrine of unit characters
is one that has figured largely
in speculations on heredity and
evolution from the time of
Darwin to the present. According to
this doctrine an organism is a sort of
mosaic of parts each of which is depend-
ent for its development upon some kind
of discrete entity in the germ cell.

The germ cell is therefore considered a
complex of organic units more or less
independent of one another in their
activities and transmission.

The unit character hypothesis is
founded on (1) the assumed independent
variability of the parts of an organism,
and (2) the assumption that characters
are capable of independent transmission.
Independent variability was appealed
to by Darwin in support of his hypo-
thetical gemmules, by De Vries in his
Intracellular Pangenesis, and especially
by Weismann who has adduced a
formidable array of facts in support
of this doctrine upon which he founds
much of his argument for the complex
organization of the germ plasm.

"There are human families," says
Weismann, "in which individuals occur
repeatedly, and through several genera-
tions, 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 geim, 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 coloring
of the hair-tuft is produced by an
abnormal constitution of the matrix of
the hair, or by other histological ele-
ments of the skin, as of the blood-
vessels or nerves. It can only depend
ultimately on a divergently constituted
part of the germplasm, 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 . ' '

"There must be .as many of these
(determinants) as there are regions in
the fully-formed organism capable of
independent and transmissible varia-
tion, including all the stages of develop-
ment."

Weismann has no quarrel with epige-
nesis2 as a theoretic possibility. The
complexity of the germ plasm is to be
measured by the amount of independent
variability occuring in the parts of the
organism. How great this amount is,
how many parts are capable of under-
going heritable changes independently
of the others is a question to be answered
only through extensive observation,
but one nevertheless capable, at least
theoretically, of being answered.

WEISMANN'S ARGUMENT

Weismann argues with great plausi-
bility that the number of independently

1 Read before a joint meeting of the zoological section, American Association for the Advance-
ment of Science, and the American Genetic Association, at Stanford University, August 4, 1915.

2 Older naturalists imagined that a minute but complete embryo must be preformed and in-
cased in either the egg or the sperm. In 1759 C. F. Wolff enunciated the doctrine of epigenesis
which, modified by later discoveries, is still accepted by the world of science. As at present
understood, it declares that there is no pre-existence of an organism as such, but that the embryo
is a new thing created as the result of the union of egg and sperm cells. — THE EDITOR.

473,

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474

The Journal of Heredity

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heritable variations presented by organ-
isms must be great, because it would be
impossible to have complex organs
evolving simultaneously, as they ob-
viously have done, unless the improve-
ments in the one did not modify or
interfere with improvements in the
others. If every variation making to-
ward the perfection of the eye were tied
lip with a variation in the ear, the
organs of digestion, and the structure
of the limbs, it seems inevitable that
there would be so much interference
with one another's progress that any
progressive evolution of a number of
complex organ systems would be prac-
tically impossible. Variations accumu-
lating toward the perfection of any one
organ, argues Weismann, would in all
probability, work toward the undoing
of various other organs. Independent
variability of parts must, therefore be
assumed in order to make the evolution
of a complex organization possible
through variation and natural selection.

There is much apparent force in this
argument for the conception of the
organism as a mosaic product. Its
real weight is difficult to estimate,
plausible as it may appear, because we
know so little of the possibilities of
organismal variability. However the
assumption of any particular kind of
variability may increase or lighten the
task of explaining how evolution takes
place, it is obviously our first duty to
inquire whether or not organisms ac-
tually vary in the way alleged. Since
so much has been built upon the doctrine
of independent variability of parts, the
the burden of proof may fairly be held
to rest with those who espouse this
theory.

Let us therefore consider some of the
alleged instances of independent varia-
tion. Take the classical case cited by
Weismann, of the small pit in the ear
which ran through several successive
generations. As this is an inherited
character, the germ plasm of the
person transmitting it must be slightly
different from that of a' person without
this defect. But does it follow that
"it can only depend ultimately on a
•divergently constituted part of the
.germplasm, which can only affect this

one spot on the head, and alter it, if it
is itself different from what is usual?"
If variations such as this could come
and go, leaving the rest of the organism
unmodified, we should be logically led,
I believe, to adopt Weismann 's con-
clusion that these variations depend on
independent carriers of some sort in the
germplasm. Weismann's reasoning is
good, so far as his doctrine of deter-
minants goes, if we grant his funda-
mental assumption. If a small pit in
the ear were absolutely the sole heredi-
tary difference between two human
beings we might be forced to consider it
as a unit character depending on a
special determinant, determiner, or other
germinal unit or entity. But do we
know that the facts are as Weismann
assumes ?

STUDIES NOT DEEP ENOUGH

I am quite sure that these people
with a pit in the ear have never been
very critically studied to find whether
or not this small character may not be a
mere expression of more general dif-
ferences in constitution. It might very
well be that this pit is simply a relatively
obvious manifestation of a very slight
difference which affects the organism as
a whole. The same may be true of the
white lock of hair and numerous other
characters which appear to vary in-
dependently of the rest of the body.

The now neglected study of correlated
variability has revealed numerous cases
in which what appear as single varia-
tions have far-reaching connections.
Supernumerary horns in sheep are said
by Youatt to go along with great
"length and coarseness of the fleece."
In mammals in general there is a strong
tendency for variation to affect simul-
taneously hair, teeth and hoofs or
claws. Darwin points out that the
white star in the forehead of horses is
generally correlated with white feet,
and that in " white^rabbits and cattle,
dark marks often co-exist on the tips of
the ears and on the feet." |*oly-
dactylism; as is well known, tends to
affect both hands and feet. How are
we to interpret these correlations?
If hands and feet vary together do the
intervening parts of the skeleton re-

Holmes: Unit Characters

475

main unaffected? If tip of ears and
hind feet show parallel variations in
color does it not suggest that we are
here dealing with a sort of outcropping
of a color variation which is really
present, but less conspicuously expressed
in other parts of the skin ? That bodily
changes of a general nature may
manifest themselves to ordinary ob-
servation in one or at least a very fewl
characteristics is clearly shown in the'
effects of many diseases. Infectious
diseases may have their characteristic
symptoms in certain form-changes while
leaving the rest of the body apparently
unaffected. Hutchinson's teeth3 in
children for instance are the index of a
general bodily disease which may have
no other very obvious sign.

Introduce some toxin of disease into
the body and you produce certain
specific characters. Introduce a change
affecting all the cells and certain parts
only will reveal the fact by noticeable
modifications. The appearance of in-
dependent variability of parts may thus
result from variations that are in reality
organismal in their extent. Not only
have so-called particular variations not
been studied sufficiently to establish the
fact that they are really independent,
but numerous cases are known in which
variations which to casual observations
would seem to affect but a single part,
are nevertheless correlated with minor
changes of wide extent. We contend
therefore that the alleged independent
variability of parts upon which Darwin,
De Vries, Weismann and others have
based so much of their argument for the
existence of discrete germinal units
rests upon an insecure foundation.

INDEPENDENT TRANSMISSION

The question of the independent
transmission of characters may be dealt
with more briefly. Owing to the inde-
pendent way in which so-called char-
acters such as tallness and dwarf ness,
flower color, characters of seed coat and
various other parts of peas may be
separated and combined almost at will
according to the fancy of the breeder it

has become customary to look upon
these characters as discrete entities
borne by discrete elements in the germ
cells, and to consider the organism as a
mosaic of independently heritable parts.
From this viewpoint organisms may be
likened to brick buildings in which the
bricks may be taken out and replaced
by others without materially affecting,
except secondarily, the bricks which
make up the rest of the structure. But
although the facts of Mendelian in-
heritance are usually interpreted ac-
cording to the mosaic conception, they
do not I believe necessitate the adoption
of this standpoint. When the Anlage4
of a green pea is separated from that of a
yellow one we are not compelled to
assume that something in the germ cell
that stands for just greenness is separ-
ated from something that is the repre-
sentative of mere yellowness. We need
assume only that what are separated
are the Anlagen of organisms as wholes
possessing the characteristics in ques-
tion. In other words green and yellow
represent organismal variations; ex-
pressed in Weismannian terms, green
and yellow depend not on determinants,
but upon ids, the hereditary bases of
whole organisms.

The logical consequence of this stand-
point we have presented is that all
Mendelian characters are really general
and constitutional, however they may
appear to be limited to a particular
feature of the organism. Many Men-
delian characters are quite patently
constitutional while others are ap-
parently very limited in their extent
like pea and rose comb in poultry.
Attention has been so taken up .with
characters per se that I doubt if much
careful scrutiny has been given to the
possible correlations of characters in
other parts of the body. Has anyone
for instance very carefully looked for
any more general attributes which may
be associated with pea or rose comb, or
with the smooth and wrinkled coats of
peas? Correlations may be difficult to
detect, not only for the reasons pre-
viously mentioned, but because the

3 Hutchinson's teeth are a form of incisor teeth indicativa of jhereditary syphilis.

4 Anlage is a German term much used by genetists to denote the hypothetical something in
the germ-cells which determines the nature of a given part of the adult organism. — THE EDITOR.

476

The Journal of Heredity

associated characters may not im-
probably have different relations of
dominance or recessiveness from that of
their more obvious correlates. The
question which we have raised can, at
least theoretically, be decided by obser-
vation and . experiment. However it is
decided will make little practical dif-
ference with most of the problems that
confront the investigator in genetics.
But there are certain problems of
genetics, I suspect, in regard to which
it will be found to have an important
bearing, although its chief importance
is in the way it influences our views on
certain fundamental problems of onto-
geny and evolution. I can here indicate
but a few cases in point :

BEARING ON EVOLUTION

Since I have come to see more clearly
the implications of the question I have
discussed, I have been surprised to find
how many of the difficulties urged
against the theory of natural selection
disappear when we consider variations
as organismal instead of limited pri-
marily to particular parts. Most discus-
sions, I find, consider evolutionary
problems from the standpoint of the
doctrine of unit characters. How com-
mon it is to find speculations as to how
this or the other character could have
been developed through natural selec-
tion, as if each part were somehow
separately improved by a series of
fortunate survivals. If each character
is considered as the summation of a
series of variations which primarily
concern that character alone, and if the
nature of the variations that are
integrated is determined by natural
selection, we should expect most attri-
butes of an organism to be of a useful
kind. If, on the other hand, variations

of any one part involve variations1,
throughout the organism, then the
preservation of favorable variations in
any one organ would of necessity entail
changes in other organs which for the
most part would probably have no
relation to utility. On this view a
considerable ingredient of non-adoptive
characters would naturally be expected,
and it is probable that, through correla-
tion, parts might be evolved to a con-
siderable degree of complexity without
having any important use in the life
of the organism, provided they did not
become positively dangerous to their
possessors. Much of the evidence ad-
dt^qed for orthogenesis is what we should
e:< wet to find if evolution occurred
through the selection of organismal
variations. Much of the difficulty about
the beginnings of structures and their
development up to the point where they
acquire selective value would, I believe,
also be removed. The wonder is not so
much that selection should produce a
large amount of what Haeckel would
call dysteleological structures, but that
it is able to produce (if we grant that it
does produce) so much that is so nicely
coadapted, and especially that it is
able to carry on the simultaneous
elaboration and perfection of numerous
separate systems of organs.

In these days of attack upon evolu-
tionary problems through direct obser-
vation and experiment, I hope I may be
pardoned for presenting anything so
atavistic as an academic discussion of
the method of evolution. But even
with our present accumulation of facts
bearing on this much discussed problem
there is still something to be gained by
reflection, and if our reflection suggests
new things to look for it will assuredly
not be in vain.

3426G3

BlOLOGf
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