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Dictionary of the History of Ideas

Studies of Selected Pivotal Ideas
  
  
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Historical Development of the Concept of Homol-
ogy.
It is in the light of these goals and difficulties
that one should consider the prevalent evolutionary
definitions of homology and analogy. According to
G. R. De Beer, “The sole condition which organs must
fulfill to be homologous is to be descended from one
and the same representative in a common ancestor.”
G. G. Simpson also defines homology in terms of com-
mon inheritance: “Homology is resemblance due to
inheritance from a common ancestry.” Usually, this
evolutionary sense of homology is contrasted with
analogy, resemblance due to common function. A fre-
quent example of homology is the relation between
a bird's wing and a mammalian forearm; of analogy,
the relation between the wing of a bird and of an
insect.

It has recently been claimed by several authors that
the evolutionary definition of homology is viciously
circular, for “in order to show that a part of one
organism, x, is homologous with a part y of another
organism, it must be shown that they are derived from
a part z in a common ancestor. But homology itself
is invoked in identifying parts x and y with part z
(Jardine, 1967). It will be argued below that the circu-
larity of De Beer and Simpsons' definition is not at
all vicious, but whether it is or not, the circularity
points up the priority of a nonevolutionary, phenotypic
sense of homology, defined solely in terms of pheno-
typic similarity, and logically independent of criteria
of descent.

A nonevolutionary sense of phenotypic homology
was widely utilized prior to Darwin. In 1818, Geoffrey


237

St. Hilaire, in his théorie des analogues (where analogue
is roughly equivalent to phenotypic homology), argued
that animals conform to a common type; the analogy
of parts was to be established by showing that they
occupied corresponding relative positions in different
animals—the principe des connexions. Owen, in his
Lectures... (1843), defined homologue as “The same
organ in different animals under every variety of form
and function.” In 1847 he distinguished special
homology “the correspondence of a part or organ,
determined by its relative position and connections,
with a part or organ in a different animal,” from gen-
eral homology, “a relation in which a part or a series
of parts stands to the fundamental or general type.”
Several authors in addition to Owen, most notably the
poet Goethe, advocated this Platonic idealism of fun-
damental types, or common bauplans (construction
plans) of which diverse organisms were seen as imper-
fect realizations.

Darwin himself utilized Geoffroy St. Hilaire's con-
cept of homology in the Origin of Species... (1859):
“... If we suppose that the ancient progenitor, the
archetype as it may be called, of all mammals, had
its limbs constructed on the existing general pattern,
for whatever purposes they served, we can at once
perceive the plain signification of the homologous
construction of limbs throughout the whole class.”
Darwin, of course, thought homology evidence in favor
of
evolution, but he utilized a nonevolutionary, pheno-
typic concept of homology.

Both the Platonic idealism of fundamental types, and
the nonevolutionary sense of homology soon came
under attack. In 1870, E. R. Lankester argued that St.
Hilaire's and Owen's use of homology “belongs to the
Platonic school.... Professor Owen... would under-
stand by homologue 'the same organ in different ani-
mals under every variety of form and function'....
But how can the sameness (if we may use the word)
of an organ under every variety of form and function
be established or investigated?... to settle this ques-
tion of sameness, an ideal 'type' or a group... had
to be evolved from the human mind,... then it could
be asserted that organs might be said to be the 'same'
in two animals” (“On the Use...,” pp. 34-43). In the
place of this idealist, purely phenotypic concept of
homology, Lankester proposed a definition resting in
part on common ancestry. “Structures which are ge-
netically related, in so far as they have a single rep-
resentative in a common ancestor, may be called
homogenous” (idem). The term “homogeny” was not
accepted, but its definition was used to define
“homology” in the evolutionary sense later advocated
by Simpson, De Beer, and others.

Are Lankester's points well taken? It is true that
Owen's and Goethe's fundamental type or common
bauplan has the features of a Platonic Ideal of which
all organisms of the type were imperfect realizations.
Their major motivation for the supposition of a funda-
mental type was the belief that in order to class a group
of organisms together, the organisms must jointly share
some common features, the fundamental type.

This understanding of the nature of universals has
been criticized by the philosophers Ludwig Wittgen-
stein and (more recently) Morton Beckner. Wittgen-
stein points out that we apply the concept “game” to
diverse objects which share different attributes in par-
tially overlapping ways but have no single attribute
in common. Classification of objects together requires
only that we have repeatable criteria for their classifi-
cation. Thus phenotypic homology need not lead to
idealism.

Furthermore Lankester's definition of homogeny
(homology) failed to meet his own criticisms of Owen,
for to establish that two structures have a single repre-
sentative in a common ancestor requires showing that
the two structures are homologous to that single an-
cestral representative; and the only criterion of homol-
ogy to which Owen can allude is phenotypic homology.
If phenotypic homology inevitably led to idealism, then
Owen's redefinition of homology would fail to escape
the disaster.

There were, however, more valid grounds for the
introduction of Lankester's sense of homology. It was
argued that evolutionary theory provided a criterion
by which to distinguish “true” from accidental homol-
ogy. Because the traits of organisms overlap in complex
ways, phenotypic clusters can be made discrete only
somewhat arbitrarily. Hence, phenotypic homology is
somewhat arbitrary. However, evolution of organs in
one species took place, presumably, in only one way.
By utilizing the criterion of descent of two structures
from the same representative in a common ancestor,
evolutionists like Simpson and De Beer felt they were
providing a less arbitrary and more biologically mean-
ingful sense of homology. With the wide adoption of
this evolutionary sense of homology, the meaning of
the term had changed. In its new sense, homology
required the theory of evolution for its definition. This
is one of many cases in science in which a theory built
in part with data described by one concept, later is
used to change the meaning of that concept itself.

But the clarity of aim of the evolutionists to intro-
duce a concept of homology which overcame the ar-
bitrariness of mere phenotypic homology does not
completely attain its goal. To establish that two struc-
tures are homologous, in Simpson's sense, requires


238

showing that a phylogenetic hypothesis is true. But part
of the evidence that the hypothesis is true stems from
phenotypic resemblance of structures in fossil and
living organisms. And that phenotypic homology
suffers from the arbitrariness which the evolutionist
hoped to avoid. Nevertheless what the evolutionist does
do is to construct and attempt to verify phylogenetic
hypotheses on the basis of all the available information,
including not only phenotypic homology but also tem-
poral relations among structures in the fossil record,
as well as considerations about possible sequences of
change in structures, which minimize the total number
of changes which must be supposed to have occurred.
For the evolutionist, the statement that two structures
are homologous is the result of a great deal of theory
building. Because the evolutionist utilizes other criteria
in addition to phenotypic similarity in asserting that
two structures are homologous, he is able to say that
two phenotypically dissimilar structures are homol-
ogous; for example, the bones of the mammalian mid-
dle ear and their homologues in fish.

The evolutionary sense of homology has held sway
for nearly a century. It is currently facing vigorous
attack by systematists who wish to substitute a purely
phenotypic sense of homology. The argument is not
merely semantic, for it expresses very different con-
victions about how best to do systematics.

The pheneticists (who wish to classify organisms on
the basis of clusters of phenotypic traits) raise three
major arguments against the traditional evolutionary
taxonomist. 1. The evolutionary sense of homology is
claimed to be viciously circular, for homology must
be utilized to show that two structures are descended
from the same representative in an ancestor. 2. The
pheneticists argue that the traditional evolutionist has
poorly defined criteria for asserting the phenotypic
similarity of organisms. 3. Worse, it is argued, the
traditional taxonomist interjects speculative phylo-
genetic hypotheses into his very phenotypic classifica-
tion schemes, thus rendering them biased and unscien-
tific, and rendering suspect any purported evolutionary
homologies derived from the data.

In place of these putatively objectionable practices,
the pheneticist, and in particular, the numerical tax-
onomist, wishes to substitute less biased, more reliable
methods (Sokal and Sneath, 1963); therefore, they have
introduced “operational homology” (Sokal and Camin,
1965). Unit characters, such as eyes: red, blue, green,
etc., are chosen (Colless, 1967); and, on the basis of
a set of such characters and their “states” for each
organism in a sample, similarity among groups of orga-
nisms is computed. Many techniques of calculation of
similarity have been generated (Sokal and Sneath),
depending upon whether each unit character is con
sidered of equal or differing weight. Diverse functions
on these characters are computed as measures of simi-
larity or “distance” between organisms, and diverse
types of cluster and factor analysis are utilized to
generate clusters.

The claims for scientific respectability of these tech-
niques are: 1. The criteria by which organisms are
judged similar are explicit and repeatable, in contrast,
the pheneticists allege, to the unclear criteria by which
traditional taxonomists talk of similarity among orga-
nisms. 2. The phenotypic clusters found by these
techniques are supposed to be free of phylogenetic
speculations, or, more strongly, free of theory, and
unvarnished data on which to build theory.

Can these arguments be maintained by the pheneti-
cist against the traditional evolutionary taxonomist?
First, the circularity ascribed to the evolutionary sense
of homology also applies to the purely phenotypic
sense of homology. The evolutionist's homology is
circular because of his reference to the “same” an-
cestral structure; but that “same” is just the circle the
phenotypic sense of homology requires. If it is a vicious
circle, both evolutionist and pheneticist would be
trapped, but the circle is benign. Many concepts can
only be defined by reference to a self-defining set of
terms. For example, “homologus,” “similar,” “resem-
bling,” “almost the same,”... etc. Benign circles are
not limited to biological concepts, a “rule” cannot
be understood without the ideas of “correct” and
“wrong,” nor they without it.

The pheneticist's hope for a “theory-free” opera-
tional homology is illusory. No operation is completely
free of theory; measuring the same ear length again
requires a theory about measuring rods not changing
length in these circumstances. Nor are the pheneticists'
similarity measures free of phylogenetic bias. Different
choices of unit characters or different computation
schemes will yield different phenetic clusters consistent
with diverse phylogenetic sequences. Suppose the
pheneticist deduced from his clusters a presumptive
phylogenetic sequence which happened to be incon-
sistent with the temporal relations in the fossil record.
Faced with disconfirming evidence, he might doubt the
deductions, doubt the fossil record, or doubt the ade-
quacy of his clustering techniques or choice of unit
characters. If he were willing to consider altering his
unit characters or clustering techniques, then he would
be doing what he accuses the traditional evolutionist
taxonomist of doing, namely, redefining his phenotypic
classification scheme to fit with other phylogenetic data
or hypotheses. If he would never change his clustering
technique, he might be asked to justify his position,
and would find it hard to do so.

Finally, the pheneticist is wrong in asserting that


239

the traditional evolutionist taxonomist must be unsci-
entific since he cannot make explicit and simple the
criteria by which he judges organisms similar. An art
critic can correctly recognize a Picasso, but would
probably be unable to make all his criteria explicit and
simply measurable. While the numerical taxonomist's
measures are probably more repeatable, and perhaps
more easily learned than those of the traditional tax-
onomist, numerical methods nevertheless are not nec-
essarily more meaningful measures of phenotypic simi-
larity than the traditional evolutionist methods.

In brief, the differences between those who support
a phenotypic and those who favor the evolutionary
sense of homology is not as great as the furor of current
debate makes it appear. Both utilize a phenotypic sense
of homology. The pheneticist must admit that his oper-
ational homology is neither theory-free nor unbiased
with regard to phylogenetic hypotheses, and that the
evolutionary taxonomist is not necessarily unscientific
for his lack of simple measuring operations.