THE ORGANISM AS AN HISTORICAL BEING
"Of late the attempt to arrange genealogical trees involving
hypothetical groups has come to be the subject of some ridicule, perhaps
deserved. But since this is what modern morphological criticism in great
measure aims at doing, it cannot be altogether profitless to follow this
method to its logical conclusions. That the results of such criticism
must be highly speculative, and often liable to grave error, is
evident."
The quotation is from Bateson's paper of 1886, and it is symptomatic of
the change which was soon to come over morphological thought. New
interests, new lines of work, began to usurp the place which pure
morphology had held so long.
This is accordingly a convenient stage at which to take stock of what
has gone before, to consider the relation of evolutionary morphology to
the transcendental and the Cuvierian schools of thought which preceded
it, and to make clear what new element evolution-theory added to
morphology.
The close analogy between evolutionary and transcendental morphology has
already been remarked upon and illustrated in the last three chapters.
We have seen that the coming of evolution made comparatively little
difference to pure morphology, that no new criteria of homology were
introduced, and that so far as pure morphology was concerned, evolution
might still have been conceived as an ideal process precisely as it was
by the transcendentalists. The principle of connections still remained
the guiding thread of morphological work; the search for archetypes,
whether anatomical or embryological, still continued in the same way as
before, and it was a point of subordinate importance that, under the
influence of the evolution-theory, these were considered to represent
real ancestral forms rather than purely abstract figments of the
intelligence. The law of Meckel-Serres was revived in an altered shape
as the law of the recapitulation of phylogeny by ontogeny; the natural
system of classification was passively inherited, and, by a _petitio
principii_, taken to represent the true course of evolution. It is true
that the attempt was made to substitute for the concept of homology the
purely genetic concept of homogeny, but no inkling was given of any
possible method of recognising homogeny other than the well-worn methods
generally employed in the search after homologies.
There was a close spiritual affinity between the speculative
evolutionists and the transcendentalists. Both showed the same
subconscious craving for simplicist conceptions--the transcendentalists
clung fast to the notion of the absolute unity of type, of the ideal
existence of the "one animal," and the evolutionists did precisely the
same thing when they blindly and instinctively accepted the doctrine of
the monophyletic descent of all animals from one primeval form. Geoffroy
persisted in regarding Arthropods as being built on the same plan as
Vertebrates: Dohrn and Semper did nothing different when they derived
both groups from an ancestor combining the main characters of both. The
determination to link together all the main phyla of the animal kingdom
and to force them all into a single mould was common to evolutionary and
pre-evolutionary transcendentalists alike.
From the fact that all Metazoa develop from an ovum which is a simple
cell, the evolutionists inferred that all must have arisen from one
primordial cell. From the fact that the next step in development is the
segmentation of the ovum, they argued that the ancestral Metazoa came
into being through the division of the primal Protozoon with aggregation
of the division-products. From the fact that a gastrula stage is very
commonly formed when segmentation has been completed, they assumed that
all germ-layered animals were descended from an ancestral Gastraea.
They quite ignored the possibility that a different explanation of the
facts might be given; they seized upon the simplest and most obvious
solution because it satisfied their overwhelming desire for
simplification. But is the simplest explanation always the
truest--especially when dealing with living things? One may be permitted
to doubt it. It is easy to account for the structural resemblance of the
members of a classificatory group, by the assumption that they are all
descended from a common ancestral form; it is easy to postulate any
number of hypothetical generalised types; but in the absence of positive
evidence, such simplicist explanations must always remain doubtful. The
evolutionists, however, had no such scruples.
Phylogenetic method differed in no way from transcendental--except
perhaps that it had learnt from von Baer and from Darwin to give more
weight to embryology. The criticisms passed by Cuvier and von Baer upon
the transcendentalists and their recapitulation theory might with equal
justice be applied to the phylogenetic speculations which were based on
the biogenetic law. There was the same tendency to fix upon isolated
points of resemblance and disregard the rest of the organisation. Thus,
on the ground of a presumed analogy of certain structures to the
vertebrate notochord, several invertebrate groups, as the Enteropneusta,
the Rhabdopleura, the Nemertea, were supposed to be, if not ancestral,
at least offshoots from the direct line of vertebrate descent. And if
other points of resemblance could in some of these cases be discovered,
yet no successful attempt was made to show that the total organisation
of any of these forms corresponded with that of the Vertebrate type.
With the possible exception of the Ascidian theory, all the numerous
theories of vertebrate descent suffered from this irremediable defect,
and none carried complete conviction.
In spite of the efforts of the evolutionists, as of those of the
transcendentalists, the phyla or "types" remained distinct, or at best
connected by the most general of bonds.
The close affinity of transcendentalists and evolutionists is shown very
clearly in their common contrast in habits of thought with the Cuvierian
school. It is the cardinal principle of pure morphology that function
must be excluded from consideration. This is a necessary and unavoidable
simplification which must be carried out if there is to be a science of
pure form at all. But this limitation of outlook, if carried over from
morphology to general biology becomes harmful, since it wilfully ignores
one whole side of life--and that the most important. The functional
point of view is clearly indispensable for any general understanding of
living things, and this is where the Cuvierian school has the advantage
over the transcendental--its principles are applicable to biology in
general.
Geoffroy and Cuvier in pre-evolutionary times well typified the contrast
between the formal and the functional standpoints. For Geoffroy form
determined function, while for Cuvier function determined form. Geoffroy
held that Nature formed nothing new, but adapted existing "materials of
organisation" to meet new needs. Cuvier, on the other hand, was always
ready to admit Nature's power to form entirely new organs in response to
new functional requirements.
The evolutionists followed Geoffroy rather than Cuvier. They laid great
store by homological resemblances, and dismissed analogies of structure
as of little interest. They were singularly unwilling to admit the
existence of convergence or of parallel evolution, and they held very
firmly the distinctively Geoffroyan view that Nature is so limited by
the unity of composition that she can and does form no new organs.
By no one has this underlying principle of evolutionary morphology been
more explicitly recognised than by Hubrecht, who in his paper of 1887,
after summarising the points of resemblance between Nemertines and
Vertebrates which led him to assume a genetic connection between them,
writes as follows:--"At the base of all the speculations contained in
this chapter lies the conviction, so strongly insisted upon by Darwin,
that new combinations or organs do not appear by the action of natural
selection unless others have preceded, from which they are gradually
derived by a slow change and differentiation.
"That a notochord should develop out of the archenteric wall because a
supporting axis would be beneficial to the animal may be a teleological
assumption, but it is at the same time an evolutional heresy. It would
never be fruitful to try to connect the different variations offered,
_e.g._, by the nervous system throughout the animal kingdom, if similar
assumptions were admitted, for there would be then quite as much to say
for a repeated and independent origin of central nervous systems out of
indifferent epiblast just as required in each special case. These would
be steps that might bring us back a good way towards the doctrine of
independent creations. The remembrance of Darwin's, Huxley's, and
Gegenbaur's classical foundations, and of Balfour's and Weismann's
brilliant superstructures, ought to warn us away from these dangerous
regions" (p. 644).
This same prejudice lies at the root of the idea of _Functionswechsel_,
in spite of the general functional orientation of that idea.
Dohrn's constant assumption is that Nature makes shift with old organs
wherever possible, instead of forming new ones. He derives gill-slits
from segmental organs, fins and limbs from gills, ribs from gill-arches,
and so on, instead of admitting that these organs might quite as well
have arisen independently. He objects on principle to the origin of
organs _de novo_. Thus, rebutting the suggestion that certain organs
which are not found in the lower Vertebrates might have arisen as new
formations, he writes:--"Against this supposition the whole weight of
all those objections can be directed that are to be brought in general
against the method of explanation which consists in appealing without
imperative necessity to the _Deus ex machina_, 'New formation,' which is
neither better nor worse than _Generatio equivoca_" (p. 21).
Of a similar nature was the objection to convergence.[456]
Why, we may ask, were morphologists so unwilling to admit the creative
power of life? Dohrn, for instance, was fully aware of the great
transforming influence exerted by function upon form--his theory of
_Functionswechsel_ regards as the most powerful agent of change the
activity of the animal, its effort to make the best use of its organs,
to apply them at need in new ways to meet new demands. Why then did he
not go a step further and admit that the animal could by its own
subconscious efforts form entirely new organs? Why did most
morphologists join with him in belittling the organism's power of
self-transformation?
The reasons seem to have been several. There is first the fundamental
reason, that the idea of an active creative organism is repugnant to the
intelligence, and that we try by all means in our power to substitute
for this some other conception. In so doing we instinctively fasten upon
the relatively less living side of organisms--their routine habits and
reflexes, their routine structure--and ignore the essential activity
which they manifest both in behaviour and in form-change.
We tend also to lay the causes of form-change, of evolution, as far as
possible outside the living organism. With Darwin we seek the
transforming factors in the environment rather than within the organism
itself. We fight shy of the Lamarckian conception that the living thing
obscurely works out its own salvation by blind and instinctive effort.
We like to think of organisms as machines, as passive inventions[457]
gradually perfected from generation to generation by some external
agency, by environment or by natural selection, or what you will. All
this makes us chary of believing that Nature is prodigal of new organs.
Other causes of the unwillingness of morphologists to admit the new
formation of organs are to be sought in the main principle of pure
morphology itself, that the unity of plan imposes an iron limit upon
adaptation, and in the powerful influence exercised at the time by
materialistic habits of thought. Teleology had become a bugbear to the
vast majority of biologists, and all real understanding of the Cuvierian
attitude seems, in most cases, to have been lost, although, curiously
enough, teleological conceptions were often unconsciously introduced in
the course of discussions on the "utility" of organs in the struggle for
existence.
Evolutionary morphology, being for the most part a form of pure or
non-functional morphology, agreed then in all essential respects with
pre-evolutionary or transcendental morphology.
But it contained the germ of a new conception which threw a new light
upon the whole science of morphology. This was the conception of the
organism as an historical being.
We have seen this thought expressed with the utmost clearness by Darwin
himself (_supra_, p. 233). In his eyes the structure and activities of
the living thing were a heritage from a remote past, the organism was a
living record of the achievements of its whole ancestral line. What a
light this conception threw upon all biology! "When we no longer look at
an organic being as a savage looks at a ship as something wholly beyond
his comprehension; when we regard every production of Nature as one
which has had a long history; when we contemplate every complex
structure and instinct as the summing-up of many contrivances, each
useful to the possessor, in the same way as any great mechanical
invention is the summing-up of the labour, the experience, the reason,
and even the blunders of numerous workmen; when we thus view each
organic being, how far more interesting--I speak from experience--does
the study of natural history become!" (_Origin_, 6th ed., pp. 665-6).
Sedgwick expressed the same thing from the morphological point of view
when he wrote, with reference to the ancestral significance of the
blastopore:--"If there is anything in the theory of evolution, every
change in the embryo must have had a counterpart in the history of the
race, and it is our business as morphologists to find it out" (p. 49,
1884).
By the evolution-theory the problems of form were linked indissolubly
with the problem of heredity. Unity of plan could no longer be explained
idealistically as the manifestation of Divine archetypal ideas; it had a
real historical basis, and was due to inheritance from a common
ancestor. The evolution-theory gave meaning and intelligibility to the
transcendental conception of the unity of plan; in particular it
supplied a simple and satisfying explanation of those puzzling vestigial
organs, whose existence was such a stumbling-block to the teleologists.
It enabled the biogenetic law to be substituted for the laws of
Meckel-Serres and von Baer, as being in some measure a combination and
interpretation of both.
Where the concept of evolution proved itself particularly useful was in
the interpretation of structures which were not immediately conditioned
by adaptation to present requirements, such as, for instance, the
arrangement of gill-slits and aortic arches in the foetus of land
Vertebrates. Such "heritage characters" could only be explained on the
hypothesis that they had once had functional or adaptational meaning.
Why, for instance, should the blastopore so often appear as a long slit,
closing by concrescence, unless this had been the original method of its
formation in remote Coelenterate ancestors?
The point hardly requires elaboration, since it has become an integral
part of all our thinking on biological problems. It may be as well,
however, for the sake of continuity, to give one or two examples of the
historical interpretation of animal structures. The first may
conveniently be the phylogenetic interpretation of the contrast between
"membrane" and "cartilage" bones.
In his _Grundzuege_ of 1870, Gegenbaur made the suggestion that the
investing or membrane bones were derived phylogenetically from
integumentary ossifications, and this was worked out in detail a few
years later by O. Hertwig.[458]
Many years before, several observers--J. Mueller, Williamson, and
Steenstrup--had been struck with the resemblance existing between the
placoid scales and the teeth of Elasmobranch fishes. Hertwig followed up
this clue, and came to the conclusion not only that placoid scales and
teeth were strictly homologous, but also that all membrane bones were
derived phylogenetically from ossifications present in the skin or in
the mucous membrane of the mouth, just as cartilage bones were derived
from the cartilaginous skeletons of the primitive Vertebrates. In some
cases this manner of derivation could even be observed in ontogeny, as
Reichert had seen in the Newt, where certain bones in the roof of the
mouth are actually formed by the concrescence of little teeth, (_supra_,
p. 163). Hertwig considered that the following bones were originally
formed by coalescence of teeth--parasphenoid, vomer, palatine,
pterygoid, the tooth-bearing part of the pre-maxillary, the maxillary,
the dentary and certain bones of the hyo-mandibular skeleton of
Teleosts. All the investing bones (_Deckknochen_) of the skull were of
common origin, and could be traced back to integumentary skeletal
plates, which in the ancestral fish formed a dense carapace.
These conclusions were accepted by Koelliker himself, who wrote in his
_Entwickelungsgeschichte_ (1879)--"The distinction between the primary
or primordial, and the investing or secondary bones is from the
morphological standpoint sharp and definite. The former are
ossifications of the (cartilaginous) primordial skeleton, the latter are
formed outside this skeleton, and are probably all ossifications of the
skin or the mucous membrane" (p. 464).
Gegenbaur[459] consistently upheld the phylogenetic derivation of
investing bones from dermal ossifications, and even went further and
derived substitutionary bones as well from the integument, thus
establishing a direct comparison between the skeletal formations of
Vertebrates and Invertebrates. Investing bones were actual integumentary
ossifications which had gradually sunk beneath the skin to become part
of the internal skeleton; substitutionary bones were produced by cells
(osteoblasts) which were ultimately derived from the integument.[460]
A further instance of the historical interpretation of animal structure,
taken from quite a different field, is afforded by the speculations of
Dollo[461] on the ancestral history of the Marsupials. In a brilliant
paper of 1880[462] Huxley made the suggestion that the ancestors of
Marsupials were arboreal forms. "I think it probable," he wrote, "from
the character of the pes, that the primitive forms, whence the existing
Marsupialia have been derived, were arboreal animals; and it is not
difficult, I conceive, to see that, with such habits, it may have been
highly advantageous to an animal to get rid of its young from the
interior of its body at as early a period of development as possible,
and to supply it with nourishment during the later periods through the
lacteal glands, rather than through an imperfect form of placenta" (p.
655). Dollo followed up this suggestion, which had in the meantime been
strengthened by Hill's discovery of a true allantoic placenta in
_Perameles_, by demonstrating in the foot of present-day Marsupials
certain features which could only be interpreted as inherited from a
time when the ancestors of Marsupials were tree-living animals. These
were the occurrence of an opposable big toe (when this was present at
all), the great development of the fourth toe, the reduction and partial
syndactylism of the second and third toes, and in some cases the
regression of the nails. These characters were shown to be typical of
arboreal Vertebrates, and their occurrence in forms not arboreal
indicated that these were descended from tree-living ancestors. Traces
of an arboreal ancestry could be demonstrated even in the marsupial mole
_Notoryctes_.
These are only two examples out of hundreds that might be given. Present
day structure was interpreted in the light of past history; the common
element in organic form was seen to be due to common descent; the
existence of vestigial and non-functional organs was no longer a riddle.
There was even a tendency to concentrate attention upon the historical
side of structure, upon what the animal passively inherited rather than
upon what it personally achieved. Homologies were considered more
interesting than analogies, vestigial organs more interesting than
foetal and larval adaptations. Convergence was anathema. The dead-weight
of the past was appreciated at its full and more than its full value;
and the essential vital activity of the living thing, so clearly shown
in development and regeneration, was ignored or forgotten.
But evolutionary morphology for all practical purposes was a development
of pure or idealistic morphology, and was powerless to bring to fruit
the new conception with which evolution-theory had enriched it. The
reason is not far to seek. Pure morphology is essentially a science of
comparison which seeks to disentangle the unity hidden beneath the
diversity of organic form. It is not immediately concerned with the
causes of organic diversity--that is rather the task of the sciences of
the individual, heredity and development. To take an example--the
recapitulation theory may legitimately be used as a law of pure
morphology, as stating the abstract relation of ontogeny to phylogeny,
and the probable line of descent of any organism may be deduced from it,
as a mere matter of the ideal derivation of one form from another; but
an explanation of the reason for the recapitulation of ancestral history
during development can clearly not be given by pure morphology unaided.
From the fact that the common starfish shows in the course of its
development distinct traces of a stalk[463] it is possible to infer,
taking other evidence also into consideration, that the ancestors of the
starfish were at one stage of their existence stalked and sessile
organisms. But this leaves unanswered the question as to how and why the
starfish does still repeat after so many millions of years part of the
organisation of one of its remote ancestors. Why is this feature
retained, and by what means has it been conserved through countless
generations? It is clear that the answer can be given only by a science
of the causes of the production and retention of form, by a causal
morphology, based upon a study of heredity and development.
From the point of view of the pure morphologist the recapitulation
theory is an instrument of research enabling him to reconstruct probable
lines of descent; from the standpoint of the student of development and
heredity the fact of recapitulation is a difficult problem whose
solution would perhaps give the key to a true understanding of the real
nature of heredity.
To make full use of the conception of the organism as an historical
being it is necessary then to understand the causal nexus between
ontogeny and phylogeny.
We shall see in the next chapter that the transformation of morphology
from a comparative to a causal science did take place towards the end of
the century, and that some progress was made towards an understanding of
the relation between individual development and ancestral history,
particularly by Roux and Samuel Butler, working with the fruitful
Lamarckian conception of the transforming power of function.
[456] The importance of convergence came to be realised
after the vogue of phylogenetic speculation had
passed--see Friedmann, _Die Konvergenz der Organismen_,
Berlin, 1904, and A. Willey, _Convergence in Evolution_,
London, 1911. Also L. Vialleton, _Elements de
morphologie des Vertebres_, Paris, 1912.
[457] From this point of view there is a very profound
analogy between artificial and natural selection. Upon
the theory of natural selection organisms are lifeless
constructs which are mechanically perfected by external
agency, just as machines are improved by a process of
conscious selection of the most successful among a
number of competing models. (_Cf._ passage quoted below,
on p. 308.)
[458] _Arch. f. mikr. Anat._, xi. (suppl.), 1874; _Morph.
Jahrb._, ii., 1876, v. 1879, and vii., 1882.
[459] _Vergleich. Anat. d. Wirbelthiere_, i., pp. 200-1,
1898.
[460] For a full historical account of work on membrane
and cartilage bones (as well as on the theory of the
skull) see E. Gaupp, "Altere und neuere Arbeiten ueber
den Wirbelthierschaedel," _Ergeb. Anat. Entw._, x., 1901,
and "Die Entwickelung des Kopfskelettes," in Hertwig's
"_Handbuch vergl. exper. Entwickelungslehre d.
Wirbelthiere_," iii., 2, pp. 573-874, 1905.
[461] "Les Ancetres des Marsupiaux etaient-ils
arboricoles?" _Trav. Stat. zool. Wimereux_, vii., pp.
188-203, pls. xi.-xii., 1899. See also Bensley, _Trans.
Linn. Soc._ (2) ix., pp. 83-214, 1903.
[462] _Proc. Zool. Soc._, pp. 649-62, 1880. _Sci. Mem._,
iv., pp. 457-72.
[463] J. F. Gemmill, _Phil. Trans. B_, ccv., p. 255, 1914.