Form and Function

E >> E. S. (Edward Stuart) Russell >> Form and Function

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As his ideas developed he laid less stress upon the simplicity and
continuity of the scale; in his supplementary remarks to the
Introduction of 1816 he admits that the series is really very much
branched, and even that there may be two distinct series among animals
instead of one. His last schema of the course of evolution shows no
little analogy with the genealogical trees of Darwinian speculation. It
is headed "The presumed _Order_ of the formation of Animals, showing two
separate partly-branching series," and it reads as follows:--

I.--_Series of Non-articulated_ II.--_Series of Articulated_
_Animals_. _Animals_.
"
I |-- Infusoria.
n | |
s A | Polyps.
e n | |
n i | ----------------
s m | | |
i a | Ascidians. Radiates. Worms.
t l | | |
i s | | --------------
v . | | | |
e | | | Epizoa.
" |-- | | |
| | |
" |-- | | |
S A | Acephala. Annelids. Insects.
e n | | |
n i | | |
s m | Molluscs. -------------
i a | | |
t l | | Arachnids.
i s | Crustacea.
v . | |
e | |
" |-- Cirripedes.

I
n |--
t A |
e n | Fishes.
l i | Reptiles.
l m | Birds.
i a | Mammals.
g l |
e s |--
n .
t

It is interesting to note that Vertebrates are placed between the two
series, and are now not linked on directly to any Invertebrate group.

Lamarck's theory had little success. There is evidence, however, that
both Meckel and Geoffroy owed a good many of their evolutionary ideas to
Lamarck, and Cuvier paid him at least the compliment of criticising his
theory,[345] not distinguishing it, however, very clearly from the
evolutionary theories of the transcendentalists. But, speaking
generally, Lamarck's theory of evolution exercised very little influence
upon his contemporaries. This was probably due partly to the obscurity
and confusion of his thought, partly to his lack of sympathy with the
biological thought of his day, which was preponderatingly morphological.

It was not that men's minds were not ripe for evolution, for in the
early decades of the 19th century evolution was in the air. There were
few of von Baer's contemporaries who had not read Lamarck;[346] Erasmus
Darwin's _Zoonomia_ ran through three editions, and was translated into
German, French and Italian;[247] German philosophy was full of the idea of
evolution.

There was no unreadiness to accept the derivation of present-day species
from a primordial form--if only some solid evidence for such derivation
were forthcoming. Cuvier and von Baer, as we have seen, combated the
current evolution theories on the ground that the evidence was
insufficient, but von Baer at least had no rooted objection to
evolution. In an essay of 1834, entitled _The Most General Law of Nature
in all Development_,[348] von Baer expressed belief in a limited amount of
evolution. In this paper he did not admit that all animals have
developed from one parent form, and he refused to believe that man has
descended from an ape; but, basing his supposition upon the facts of
variability and upon the evidence of palaeontology, he went so far as to
maintain that many species have evolved from parent stocks. In the
absence of conclusive proofs he did not commit himself to a belief in
any extended or comprehensive process of evolution.

Imbued as he was with the idea of development von Baer saw in evolution
a process essentially of the same nature as the development of the
individual. Evolution, like development, was due to a _Bildungskraft_ or
formative force. The ultimate law of all becoming was that "the history
of Nature is nothing but the history of the ever-advancing victory of
spirit over matter" (p. 71). In a later essay (1835) in the same volume
he says that all natural science is nothing but a long commentary on the
single phrase _Es werde!_. (p. 86).

As we shall see, von Baer adopted in later years the same attitude to
Darwinism as he did to the evolution theories in vogue in his youth.

Although in the twenty or thirty years before the publication of the
_Origin of Species_ (1859) no evolution theory of any importance was
published, and although the great majority of biologists believed in the
constancy of species, there were not wanting some who, like von Baer,
had an open mind on the subject, or even believed in the occurrence of
evolutionary processes of small scope. Isidore Geoffroy St Hilaire, the
son of the great Etienne Geoffroy St Hilaire, seems to have held that
species might be formed from varieties. The law which L. Agassiz thought
he could establish,[349] of the parallelism between palaeontological
succession, systematic rank, and embryological development, tended to
help the progress of evolutionary ideas. J. V. Carus, who afterwards
became a supporter of Darwin, seems already, in 1853, to have inferred
from Agassiz's law the probability of evolution.[350]

But no evolution theory was taken very seriously before 1859, when the
_Origin of Species_ was published.

Like Lamarck, Charles Darwin was, neither by inclination nor by
training, a morphologist. In his youth he was a collector, a sportsman
and a field geologist. His voyage round the world on the _Beagle_
aroused in him keen interest in the problem of species--their variety,
their variation according to place and time, their adaptedness to
environment. The conviction gradually took possession of his mind that
the puzzling facts of geographical range and geological succession which
he observed wherever he went were explicable only on the hypothesis that
species change. He was not satisfied with the theories of evolution that
had been proposed by his grandfather, by Lamarck, and by E. Geoffroy St
Hilaire--he did not indeed understand these theories any too well. He
resolved to work out the problem in his own way, for his own
satisfaction. He tells us all this very clearly in his autobiography.
"During the voyage of the _Beagle_ I had been deeply impressed by
discovering in the Pampean formation great fossil animals covered with
armour like that on the existing armadillos; secondly, by the manner in
which closely allied animals replace one another in proceeding
southwards over the continent; and thirdly, by the South American
character of most of the productions of the Galapagos archipelago, and
more especially by the manner in which they differ slightly on each
island of the group; some of the islands appearing to be very ancient in
a geological sense.

"It was evident that such facts as these, as well as many others, could
only be explained on the supposition that species gradually become
modified; and the subject haunted me. But it was equally evident that
neither the action of the surrounding conditions, nor the will of the
organisms (especially in the case of plants) could account for the
innumerable cases in which organisms of every kind are beautifully
adapted to their habits of life--for instance, a woodpecker or a
tree-frog to climb trees, or a seed for dispersal by hooks or plumes. I
had always been much struck by such adaptations, and until these could
be explained it seemed to me almost useless to endeavour to prove by
indirect evidence that species have been modified."[351]

All Darwin's varied subsequent work revolved round these, for him,
essential problems--How do species change, and how do they become
adapted to their environment? He never ceased to be essentially a field
naturalist, and his theory of natural selection would have been an empty
and abstract thing if his vast knowledge and understanding of the "web
of life" had not given it colour and form. He never lost touch with the
living thing in its living, breathing reality--even plants he rightly
regarded as active things, full of tricks and contrivances for making
their way in the world. No one ever realised more vividly than he the
delicacy and complexity of the adaptations to environment which are the
necessary condition of success in the struggle for existence. Almost his
greatest service to biology was that he made biologists realise as they
never did before the vast importance of environment. He took biology
into the open air, away from the museum and the dissecting-room.

Naturally this attitude was not without its drawbacks. It led him to
take only a lukewarm interest in the problems of morphology. It is true
he used the facts of morphology with great effect as powerful arguments
for evolution, but it was not from such facts that he deduced his theory
to account for evolution. It is questionable indeed whether the theory
of natural selection is properly applicable to the problems of form. It
was invented to account for the evolution of specific differences and of
ecological adaptations; it was not primarily intended as an explanation
of the more wonderful and more mysterious facts of the _convenance des
parties_ and the interaction of structure and function. Perhaps Darwin
did not realise this inner aspect of adaptation quite so vividly as he
did the more superficial adaptation of organisms to their environment.
It was, perhaps, his lack of morphological training and experience that
led him to disregard the problems of form, or at least to realise very
insufficiently their difficulty.

It is in any case very significant that only a small part of his _Origin
of Species_ is devoted to the discussion of morphological
questions--only one chapter out of the fourteen contained in the first
edition.

Though the theory of natural selection took little account of the
problems of form, Darwin's masterly vindication of the theory of
evolution was of immense service to morphology, and Darwin himself was
the first to point out what a great light evolution threw upon all
morphological problems. In a few pages of the _Origin_ he laid the
foundations of evolutionary morphology.

We have here to consider his interpretation of morphological facts and
its relation to the current morphology of his time.

The sketch of his theory, written in 1842,[352] shows a very significant
division into two parts--the first dealing with the positive facts of
variability and the theory of natural selection, the second with the
general evidence for evolution. It is in the second part that the
paragraphs on morphological matters occur. In paragraph 7, on affinities
and classification, Darwin points out that on the theory of evolution
homological relationship would be real relationship, and the natural
system would really be genealogical. In the next paragraph he notes that
evolution would account for the unity of type in the great classes, for
the metamorphosis of organs, and for the close resemblance which early
embryos show to one another. It is of special interest to note that he
definitely rejects the Meckel-Serres theory of recapitulation. "It is
not true," he writes, "that one passes through the form of a lower
group, though no doubt fish more nearly related to foetal state" (p.
42). The greater divergence which adults show seems to him to be due to
the fact that selection acts more on the later than on the embryonic
stages. He realises very clearly how illuminative the theory of
evolution is when applied to the puzzling facts of embryonic
development. "The less differences of foetus--this has obvious meaning
on this view: otherwise how strange that a horse, a man, a bat should at
one time of life have arteries, running in a manner which is only
intelligibly useful in a fish! The natural system being on theory
genealogical, we can at once see why foetus, retaining traces of the
ancestral form, is of the highest value in classification" (p. 45).

Abortive organs, too, gain significance on the evolutionary hypothesis.
"The affinity of different groups, the unity of types of structure, the
representative forms through which foetus passes, the metamorphosis of
organs, the abortion of others, cease to be metaphorical expressions and
become intelligible facts" (p. 50).

In general, organisms can be understood only if we take into account the
cardinal fact that they are historical beings. "We must look at every
complicated mechanism and instinct as the summary of a long history of
useful contrivances much like a work of art" (p. 51).[353]

Already in 1842 Darwin had seized upon the main principles of
evolutionary morphology: the indications then given are elaborated in
the thirteenth chapter of the _Origin of Species_ (1st ed., 1859). A
good part of this chapter is given up to a discussion of the principles
of classification, only a few pages dealing with morphology proper. But,
as Darwin rightly saw, the two things are inseparable.

We note first that there is no hint of the "scale of beings"--Darwin
conceives the genealogical tree as many branched. Animals can be classed
in "groups under groups," and cannot be arranged in one single series.

He discusses first what kind of characters have the greatest
classificatory value. Certain empirical rules have been recognised, more
or less consciously, by systematists--that analogical characters are
less valuable than homological, that characters of great physiological
importance are not always valuable for classificatory purposes, that
rudimentary organs are often very useful, and so on. He finds that as a
general rule "the less any part of the organisation is concerned with
special habits, the more important it becomes for classification" (p.
414), and adduces in support Owen's remark that the generative organs
afford very clear indications of affinities, since they are unlikely to
be modified by special habits. These rules of classification can be
explained "on the view that the natural system is founded on descent
with modification; that the characters which naturalists consider as
showing true affinity ... are those which have been inherited from a
common parent, and, in so far, all true classification is genealogical;
that community of descent is the hidden bond which naturalists have been
unconsciously seeking, and not some unknown plan of creation, or the
enunciation of general propositions, and the mere putting together and
separating objects more or less alike" (p. 420).

In general, then, homological characters are more valuable for
classificatory purposes because they have a longer pedigree than
analogical characters, which represent recent acquirements of the race.

Coming to morphology proper, Darwin takes up the question of the unity
of type, and the homology of parts, for which the unity of type is but a
general expression.

He treats this on the same lines as E. Geoffroy St Hilaire, and Owen,
referring indeed specifically to Geoffroy's law of connections. "What
can be more curious," he asks, "than that the hand of a man, formed for
grasping, that of a mole for digging, the leg of a horse, the paddle of
the porpoise, and the wing of the bat, should all be constructed on the
same pattern, and should include similar bones, in the same relative
positions? Geoffroy St Hilaire has strongly insisted on the high
importance of relative position or connection in homologous parts; they
may differ to almost any extent in form and size, and yet remain
connected together in the same invariable order" (p. 434).

The unity of plan cannot be explained on teleological grounds, as Owen
has admitted in his _Nature of Limbs_, nor is it explicable on the
hypothesis of special creation (p. 435). It can be understood only on
the theory that animals are descended from one another and retain for
innumerable generations the essential organisation of their ancestors.
"The explanation is to a large extent simple on the theory of the
selection of successive slight modifications--each modification being
profitable in some way to the modified form, but often affecting by
correlation other parts of the organisation. In changes of this nature,
there will be little or no tendency to alter the original pattern or to
transpose the parts.... If we suppose that the ancient progenitor, the
archetype as it may be called, of all animals, had its limbs constructed
on the existing general pattern, for whatever purpose they served, we
can at once perceive the plain significance of the homologous
construction of the limbs throughout the whole class" (p. 435).

We may note three important points in this passage--first, the
identification of the archetype with the common progenitor; second, the
view that progressive evolution is essentially adaptive, and dominated
by natural selection; and third, the _petitio principii_ involved in the
assumption that adaptive modification brings inevitably in its train the
necessary correlative changes.

In his section on morphology Darwin shows clearly the influence of Owen,
and through him of the transcendental anatomists. He refers to the
transcendental idea of "metamorphosis," as exemplified in the vertebral
theory of the skull and the theory of the plant appendage, and shows
how, on the hypothesis of descent with modification, "metamorphosis" may
now be interpreted literally, and no longer figuratively merely (p.
439).

Very great interest attaches to Darwin's treatment of development, for
post-Darwinian morphology was based to a very large extent on the
presumed relation between the development of the individual and the
evolution of the race. Just as he kept clear of the notion of the scale
of beings, so he avoided the snare of the Meckel-Serres theory of
recapitulation, according to which the embryo of the highest animal,
man, during its development climbs the ladder upon the rungs of which
the whole animal series is distributed, in its gradual progression from
simplicity to complexity. The law of development which he adopts is that
of von Baer, which states that development is essentially
differentiation, and that as a result embryos belonging to the same
group resemble one another the more the less advanced they are in
development. There can be little doubt that he was indebted to von Baer
for the idea, and in the later editions of the _Origin_ he acknowledges
this by quoting the well-known passage in which von Baer tells how he
had two embryos in spirit which he was unable to refer definitely to
their proper class among Vertebrates.[354]

Not only are embryos more alike than adults, because less
differentiated, but it is in points not directly connected with the
conditions of existence, not strictly adaptive, that their resemblance
is strongest (p. 440)--think, for instance, of the arrangement of aortic
arches common to all vertebrate embryos. Larval forms are to some extent
exceptions to this rule, for they are often specially adapted to their
particular mode of life, and convergence of structure may accordingly
result. All these facts require an explanation. "How, then, can we
explain these several facts in embryology--namely, the very general, but
not universal, difference in structure between the embryo and the
adult--of parts in the same individual embryo, which ultimately become
very unlike and serve for different purposes, being at this early period
of growth alike--of embryos of different species within the same class,
generally but not universally, resembling each other--of the structure
of the embryo not being closely related to its conditions of existence,
except when the embryo becomes at any period of life active and has to
provide for itself--of the embryo apparently having sometimes a higher
organisation than the mature animal, into which it is developed" (pp.
442-3). Obviously all these facts are formally explained by the doctrine
of descent. But Darwin goes further, he tries to show exactly how it is
that the embryos resemble one another more than the adults. He thinks
that the phenomenon results from two principles--first, that
modifications usually supervene late in the life of the individual; and
second, that such modifications tend to be inherited by the offspring at
a corresponding, not early, age (p. 444).

Thus, applying these principles to a hypothetical case of the origin of
new species of birds from a common stock, he writes:--"... from the many
slight successive steps of variation having supervened at a rather late
age and having been inherited at a corresponding age, the young of the
new species of our supposed genus will manifestly tend to resemble each
other much more closely than do the adults, just as we have seen in the
case of pigeons"[355] (pp. 446-7).

Since the embryo shows the generalised type, the structure of the embryo
is useful for classificatory purposes. "For the embryo is the animal in
its less modified state; and in so far it reveals the structure of its
progenitor" (p. 449)--the embryological archetype reveals the ancestral
form. "Embryology rises greatly in interest, when we thus look at the
embryo as a picture, more or less complete, of the parent form of each
great class of animals" (p. 450)--a prophetic remark, in view of the
enormous subsequent development of phylogenetic speculation.

We may sum up by saying that Darwin interpreted von Baer's law
phylogenetically.

The rest of the chapter is devoted to a discussion of abortive and
vestigial organs, whose existence Darwin naturally turns to great
advantage in his argument for evolution. Throughout the whole chapter
Darwin's preoccupation with the problems of classification is clearly
manifest.

On the question as to whether descent was monophyletic or polyphyletic
Darwin expressed no dogmatic opinion. "I believe that animals have
descended from at most only four or five progenitors, and plants from an
equal or lesser number.... I should infer from analogy that probably all
the organic beings which have ever lived on this earth have descended
from one primordial form, into which life was first breathed" (p. 484).

Darwin rightly laid much stress upon the morphological evidence for
evolution,[356] which he considered to be weighty. It probably contributed
greatly to the success of his theory. Though he himself did little or no
work in pure morphology, he was alive to the importance of such work,[357]
and followed with interest the progress of evolutionary morphology,
incorporating some of its results in later editions of the _Origin_, and
in his _Descent of Man_ (1871).

In his morphology Darwin was hardly up to date. He does not seem to have
known at first hand the splendid work of the German morphologists, such
as Rathke and Reichert; he pays no attention to the cell-theory, nor to
the germ-layer theory. His sources are, in the main, Geoffroy St
Hilaire, Owen, von Baer, Agassiz, Milne-Edwards, and Huxley.

Perhaps his greatest omission was that he did not give any adequate
treatment of the problem of functional adaptation and the correlation of
parts. It is not too much to say that Darwin not only disregarded these
problems almost entirely, but by his insistence upon ecological
adaptation and upon certain superficial aspects of correlation,
succeeded in giving to the words "adaptation" and "correlation" a new
signification, whereby they lost to a large extent their true and
original functional meaning.

It is true that Darwin himself, as well as his successors, believed that
natural selection was all-powerful to account for the evolution of the
most complicated organs, but it may be questioned whether he realised
all the conditions of the problem of which he thus easily disposed. He
says, rightly, in an important passage, that "It is generally
acknowledged that all organic beings have been formed on two great
laws--Unity of Type, and the Conditions of Existence. By unity of type
is meant that fundamental agreement in structure which we see in organic
beings of the same class, and which is quite independent of their habits
of life. On my theory, unity of type is explained by unity of descent.
The expression of conditions of existence, so often insisted upon by the
illustrious Cuvier, is fully embraced by the principle of natural
selection. For natural selection acts by either now adapting _the
varying parts of each being to its organic and inorganic conditions of
life_:[358] or by having adapted them during past periods of time: the
adaptations being aided in many cases by the increased use or disuse of
parts, being affected by the direct action of the external conditions of
life, and subjected in all cases to the several laws of growth and
variation. Hence, in fact, the law of the Conditions of Existence is the
higher law; as it includes, through the inheritance of former variations
and adaptations, that of Unity of Type" (_Origin_, 6th ed., Pop.
Impression, pp. 260-1). It is clear that Darwin took the phrase
"Conditions of Existence" to mean the environmental conditions, and the
law of the Conditions of Existence to mean the law of adaptation to
environment. But that is not what Cuvier meant by the phrase: he
understood by it the principle of the co-ordination of the parts to form
the whole, the essential condition for the existence of any organism
whatsoever (see above, Chap. III., p. 34).

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