Form and Function

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It was a curious but very typical characteristic of evolutionary
morphology that its devotees paid very little attention to the positive
evidence accumulated by the palaeontologists,[543] but shut themselves up
in their tower of ivory and went on with their work of constructing
ideal genealogies. It was perhaps fortunate for their peace of mind that
they knew little of the advances made by palaeontology, for the evidence
acquired through the study of fossil remains was distinctly unfavourable
to the pretty schemes they evolved.

As Neumayr, Zittel, Deperet, Steinmann and others have pointed out, the
palaeontological record gives remarkably little support to the ideal
genealogies worked out by morphologists. There is, for instance, a
striking absence of transition forms between the great classificatory
groups. A few types are known which go a little way towards bridging
over the gaps--the famous _Archaeopteryx_, for example--but these do not
always represent the actual phylogenetic links. There is an almost
complete absence of the archetypal ancestral forms which are postulated
by evolutionary morphology. Amphibia do not demonstrably evolve from an
archetypal Proamphibian, nor do mammals derive from a single generalised
Promammalian type. Few of the hypothetical ancestral types imagined by
Haeckel have ever been found as fossils. The great classificatory groups
are almost as distinct in early fossiliferous strata as they are at the
present day. As Deperet says in his admirable book,[544] in the course of
a presentation of the matured views of the great Karl von Zittel, "We
cannot forget that there exist a vast number of organisms which are not
connected by any intermediate links, and that the relations between the
great divisions of the animal and vegetable kingdoms are much less close
than the theory [of evolution] demands. Even the Archaeopteryx, the
discovery of which made so much stir and appeared to establish a genetic
relation between classes so distinct as Birds and Reptiles, fills up the
gap only imperfectly, and does not indicate the point of bifurcation of
these two classes. Intermediate links are lacking between Amphibia and
Reptiles. Mammals, too, occupy an isolated position, and no zoologist
can deny that they are clearly demarcated from other Vertebrates;
indeed, no fossil mammal is certainly known which comes nearer to the
lower Vertebrates than does Ornithorhynchus at the present day" (p.
115).

To take a parallel from the Invertebrata, B. B. Woodward,[545] after
discussing the phylogeny of the Mollusca as worked out by the
morphologists and comparing it with the probable actual course of the
evolution of the group, as evidenced by fossil shells, sums up as
follows:--"The lacunae in our knowledge of the interrelationships of the
members of the various families and orders of Mollusca are slight
however, compared with the blank caused by the total absence from
palaeontological history of any hint of passage forms between the classes
themselves, or between the Mollusca and their nearest allies. Nor is
this hiatus confined to the Molluscan phylum; it is the same for all
branches of the animal kingdom. There is circumstantial evidence that
transitional forms must have existed, but of actual proof none whatever.
All the classes of Mollusca appear fully fledged, as it were. No form
has as yet been discovered of which it could be said that it in any way
approached the hypothecated prorhipidoglossate mollusc, still less one
linking all the classes" (p. 79).

Pointing in the same direction as the absence of transitional forms is
the undeniable fact that all the great groups of animals appear with all
their typical characters at a very early geological epoch. Thus, in the
Silurian age a very rich fauna has already developed, and
representatives are found of all the main Invertebrate groups--sponges,
corals, hydroid colonies, five types of Echinoderms, Bryozoa,
Brachiopods, Worms, many types of Mollusca and Arthropoda. Of
Vertebrates, at least two types of fish are present--Ganoids and
Elasmobranchs. In the very earliest fossiliferous rocks of all, the
Precambrian formation, there are remains of Molluscs, Trilobites and
Gigantostraca, similar to those which flourished in Cambrian and
Silurian times.

The contributions of palaeontology to the solution of the problems of
descent posed by morphology are, however, not all of this negative
character. The law of recapitulation is in some well-controlled cases
triumphantly vindicated by palaeontology. Thus Hyatt and others found
that in Ammonites the first formed coils of the shell often reproduce
the characters belonging to types known to be ancestral, and what is
more they have demonstrated the actual occurrence of the phenomenon
known as acceleration or tachygenesis, often postulated by speculative
morphologists.[546] This is the tendency universally shown by embryos to
reproduce the characters of their ancestors at earlier and earlier
stages in their development.

The most valuable contribution made by palaeontologists to morphology and
to the theory of evolution arose out of the careful and methodical study
of the actual succession of fossil forms as exemplified in limited but
richly represented groups. Classical examples were the researches of
Hilgendorf[547] on the evolution of _Planorbis multiformis_ in the
lacustrine deposits of Steinheim, those of Waagen[548] on the phylogeny of
_Ammonites subradiatus_, and the work of Neumayr and Paul[549] on
_Paludina_ (_Vivipara_).

These investigations demonstrated that it was possible to follow out
step by step in superjacent strata the actual evolution of fossil
species and to establish the actual "phyletic series."

To take an example from among the Vertebrates, Deperet has shown (_loc.
cit._, pp. 184-9), that the European Proboscidea, belonging to the three
different types of the Elephants, Mastodons and Dinotheria, have evolved
since the Oligocene epoch along five distinct but continuous lines. The
Dinotherian stock is represented at the beginning of the Miocene by the
relatively small form _D. cuvieri_; this changes progressively
throughout Miocene times into _D. laevius_, _D. giganteum_, and _D.
gigantissimum_. Among the Mastodons two quite distinct phyletic series
can be distinguished, the first commencing with _Palaeomastodon
beadnelli_ of the Oligocene, and evolving between the Miocene and
Pliocene into _Mastodon arvernensis_, after traversing the forms _M.
angustidens_ and _M. longirostris_, the second starting with the _M.
turicensis_ of the Lower Miocene and evolving through _M. borsoni_ into
the _M. americanus_ of the Quaternary. The phyletic series of the true
elephants in Europe are relatively short, and go back only to the
Quaternary, _Elephas antiquus_ giving origin to the Indian elephant, _E.
priscus_ to the African.

The careful study of phyletic series brought to light the significant
fact that these lines of filiation tend to run for long stretches of
time parallel to, and distinct from one another, without connecting
forms. This is clearly exemplified in the case of the Proboscidea, and
many other examples could be quoted. Almost all rich genera are
polyphyletic in the sense that their component species evolve along
separate and parallel lines of descent.[550] "Such great genera as the
genus _Hoplites_ among the Ammonites, the genus _Cerithium_ among the
Gastropoda, the genus _Pecten_ or the genus _Trigonia_ among the
Lamellibranchs, each comprise perhaps more than twenty independent
phyletic series" (Deperet, p. 200).

Variation along the phyletic lines is gradual[551] and determinate, and
appears to obey definite laws. The earliest members of a phyletic series
are usually small in size and undifferentiated in structure, while the
later members show a progressive increase in size and complexity. Rapid
extinction often supervenes soon after the line has reached the maximum
of its differentiation.

The general picture which palaeontology gives us of the evolution of the
animal kingdom is accordingly that of an immense number of phyletic
lines which evolve parallel to one another, and without coalescing,
throughout longer or shorter periods of geological times. "Each of these
lines culminates sooner or later in mutations of great size and highly
specialised characters, which become extinct and leave no descendants.
When one line disappears by extinction it hands the torch, so to speak,
to another line which has hitherto evolved more slowly, and this line in
its turn traverses the phases of maturity and old age which lead it
inevitably to its doom. The species and genera of the present day belong
to lines that have not reached the senile phase; but it may be surmised
that some of them, _e.g._ elephants, whales, and ostriches, are
approaching this final phase of their existence" (Deperet, p. 249).

It is one of the paradoxes of biological history that the
palaeontologists have always laid more stress upon the functional side of
living things than the morphologists, and have, as a consequence, shown
much more sympathy for the Lamarckian theory of evolution. The American
palaeontologists in particular--Cope, Hyatt, Ryder, Dall, Packard,
Osborn--have worked out a complete neo-Lamarckian theory based upon the
fossil record.

The functional point of view was well to the fore in the works of those
great palaeontologists, L. Ruetimeyer (1825-1895) and V. O. Kowalevsky
(1842-83), who seem to have carried on the splendid tradition of Cuvier.
Speaking of Kowalevsky's classical memoir, _Versuch einer natuerlichen
Classification der fossilen Hufthiere_, Osborn[552] writes:--"This work is
a model union of the detailed study of form and function with theory and
the working hypothesis. It regards the fossil not as a petrified
skeleton, but as having belonged to a moving and feeding animal; every
joint and facet has a meaning, each cusp a certain significance. Rising
to the philosophy of the matter, it brings the mechanical perfection and
adaptiveness of different types into relation with environment, with
changes of herbage, with the introduction of grass. In this survey of
competition it speculates upon the causes of the rise, spread, and
extinction of each animal group. In other words, the fossil quadrupeds
are treated _biologically_--so far as is possible in the obscurity of
the past" (p. 8). The same high praise might with justice be accorded to
the work of Cope on the functional evolution of the various types of
limb-skeleton in Vertebrates, and on the evolution of the teeth as well
as to the work of other American palaeontologists, including Osborn
himself.

Osborn's law of "adaptive radiation," which links on to Darwin's law of
divergence,[553] constitutes a brilliant vindication of the functional
point of view. "According to this law each isolated region, if large and
sufficiently varied in its topography, soil, climate, and vegetation,
will give rise to a diversified mammalian fauna. From primitive central
types branches will spring off in all directions, with teeth and
prehensile organs modified to take advantage of every possible
opportunity of securing food, and in adaptation of the body, limbs and
feet to habitats of every kind, as shown in the diagram [on p. 363]. The
larger the region and the more diverse the conditions, the greater the
variety of mammals which will result.

"The most primitive mammals were probably small insectivorous or
omnivorous forms, therefore with simple, short-crowned teeth, of
slow-moving, ambulatory, terrestrial, or arboreal habit, and with short
feet provided with claws. In seeking food and avoiding enemies in
different habitats the limbs and feet radiate in four diverse
directions; they either become _fossorial_ or adapted to digging habits,
_natatorial_ or adapted to _amphibious_ and finally to _aquatic_
habits, _cursorial_ or adapted to swift-moving, terrestrial progression,
_arboreal_ or adapted to tree life. Tree life leads, as its final stage,
into

LIMBS AND FEET.
Volant.
/
Fossorial. Arboreal.
\ /
Short-limbed, plantigrade, } Ambulatory
pentadactyl, unguiculate } or
Stem. } Terrestrial.
/ \
Natatorial. Cursorial
Amphibious. Digitigrade.
/ \
Aquatic Unguligrade.

TEETH.
Omnivorous.
{ Grass.
{ Fish. | { Herb.
Carnivorous { Flesh. | Herbivorous { Shrub.
\ { Carrion. | / { Fruit.
\ | / { Root.
\ | /
\ | / Myrmecophagous.
\ | / / Dentition reduced.
\ | / /
\ | / /
\ | / /
\ |/ /
Stem: Insectivorous.

the parachute types of the flying squirrels and phalangers, or into the
true flying types of the bats.... Similarly in the case of the teeth,
insectivorous and omnivorous types appear to be more central and ancient
than either the exclusively carnivorous or herbivorous types. Thus the
extremes of carnivorous adaptation, as in the case of the cats, of
omnivorous adaptation, as in the case of the bears, of herbivorous
adaptation, as in the case of the horses, or myrmecophagous adaptation,
as in the case of the anteaters, are all secondary" (_loc. cit._, pp.
23-4).

We have now reached the end of our historical survey of the problems of
form. What the future course of morphology will be no one can say. But
one may hazard the opinion that the present century will see a return to
a simpler and more humble attitude towards the great and unsolved
problems of animal form. Dogmatic materialism and dogmatic theories of
evolution have in the past tended to blind us to the complexity and
mysteriousness of vital phenomena. We need to look at living things with
new eyes and a truer sympathy. We shall then see them as active, living,
passionate beings like ourselves, and we shall seek in our morphology to
interpret as far as may be their form in terms of their activity.

This is what Aristotle tried to do, and a succession of master-minds
after him. We shall do well to get all the help from them we can.

[519] See E. B. Wilson's masterly book, _The Cell in
Development and Inheritance_, New York and London, 1900.

[520] _Q.J.M.S._, xxvi. 1886.

[521] _Wood's Holl Biological Lectures_ for 1893.

[522] _Arch. f. Ent.-Mech._, i., pp. 380-90, 1895.

[523] _Beitraege zur Kritik der Darwinschen Lehre_,
Leipzig, 1898.

[524] See E. B. Wilson, "The Embryological Criterion of
Homology," _Wood's Holl Biological Lectures_, Boston,
pp. 101-24, 1895; Braem, _Biol. Centrblt._, xv., 1895;
T. H. Morgan, _Arch. f. Ent.-Mech._, xviii.; J. W.
Jenkinson, _Mem. Manchester Lit. Phil. Soc._, 1906, and
_Vertebrate Embryology_, Oxford, 1913; A. Sedgwick,
article "Embryology" in _Ency. Brit._, p. 318, vol. xi.,
11th Ed. (1910).

[525] For a detailed treatment of this important point see
the remarkable volume of E. Schulz (Petrograd),
_Prinzipien der rationellen vergleichenden Embryologie_,
Leipzig, 1910.

[526] "La Poecilogonie," _Bull. Sci. France et Belgique_,
xxxix., pp. 153-87, 1905.

[527] _Un probleme de l'evolution. La loi biogenetique
fondamentale_, Paris and Montpellier, 1908.

[528] _Vergleichung des Entwickelungsgrades der Organe zu
verschiedenen Entwickelungszeiten bei Wirbeltieren_,
Jena, 1891.

[529] Quoted by Keibel, _Ergebn. Anat. Entwick._, vii., p.
741.

[530] "Studien zur Entwickelungsgeschichte des Schweines,"
Schwalbe's _Morphol. Arbeiten_, iii., 1893, and v.,
1895.

_Normentafeln zur Entwickelungsgeschichte des
Schweines_, Jena, 1897.

"Das biogenetische Grundgesetz und die Cenogenese,"
_Ergebn. Anat. Entw._, vii., pp. 722-92, 1897.

"U. d. Entwickelungsgrad der Organe," _Handb. vergl.
exper. Entwick. der Wirbelthiere_, iii., 3, pp. 131-48,
1906.

[531] "Beitraege zur Embryologie der Wiederkaeuer," _Arch.
Anat. Entw._, 1889.

[532] "Die individ. Variation d. Wirbeltierembryo,"
_Morph. Arbeit._, v., 1895.

[533] "U. Variabilitaet u. Wachstum d. embryonalen
Koerpers," _Morph. Jahrb._, xxiv., 1896.

[534] "Gastrulation u. Keimblaetterbildung der _Emys
lutaria taurica_," _Morph. Arbeit._, i., 1891.
"Kainogenese," _Morph. Arbeit._, vii., pp. 1-156, 1897,
and also separately. _Biomechanik, erschlossen aus dem
Prinzipe der Organogenese_, Jena, 1898.

[535] This law was foreshadowed by Reichert in 1837, when he
wrote:--"We notice in our investigation of embryos of different
animal forms that it is those organs, those systems, which in the
fully developed individual are peculiarly perfect, that in their
earliest rudiments and also throughout the whole course of their
development appear with the most striking distinctness" (Mueller's
_Archiv_, p. 135, 1837). See also his _Entwick. Kopf. nackt.
Amphib._, p. 198, 1838. So, too, Rathke notes how the elongated
shape of the snake appears even in very early embryonic stages
(_Entwick. Natter._, p. 111, 1839).

[536] Quoted by Keibel (p. 790, 1897) from the
_Biomechanik_.

[537] _Die Zelle und die Gewebe_, Jena, 1898, and the
subsequent editions of this text-book, published under
the title of _Allgemeine Biologie. Die Entwickelung der
Biologie im neunzehnten Jahrhundert_, Jena, 1900, 2nd
ed., 1908. "Ueber die Stellung der vergl.
Entwickelungslehre zur vergl. Anatomie, zur Systematik
und Descendenztheorie," _Handb. vergl. exper.
Entwickelungslehre der Wirbeltiere_, iii., 3, pp.
149-80, Jena, 1906. (1906, b). Also in Pt. I. of Vol. I.
(1906, a).

[538] _An Essay on Classification_, London, 1859.

[539] _Unsere Koerperform_, Leipzig, 1874.

[540] _Q.J.M.S._, xxxvi., pp. 35-52, 1894.

[541] Quoted by Hertwig. See also K. Goebel, "Die
Grundprobleme der heutigen Pflanzenmorphologie," _Biol.
Centrbl._, xxv., pp. 65-83, 1905.

[542] This is also emphasised by Fleischmann in his critical study of
evolutionary morphology entitled _Die Descendenztheorie_, Leipzig,
1901.

[543] The same remark applies to the bulk of speculation as to the
factors of evolution, with the exception of the contributions made
to evolution theory by the palaeontologists by profession, such as
Cope.

[544] _Les Transformations du Monde animal_, Paris, 1907.

[545] "Malacology _versus_ Palaeoconchology," _Proc.
Malacological Soc._, viii., pp. 66-83, 1908.

[546] Particularly by E. Perrier, "La Tachygenese," _Ann.
Sci. nat._ (_Zool._) (8), xvi., 1903.

[547] _Monatsber. k. Akad. Wiss._, Berlin, pp. 474-504,
1866.

[548] _Geognost. u. Palaeont. Beitraege_, ii., Heft 2, pp.
181-256, 1869.

[549] _Abhand. k.k. Geol. Reichsanstalt_, vii., Wien,
1875.

[550] The case for polyphyletism is very strongly put by
G. Steinmann in his book, _Die geologischen Grundlagen
der Abstammungslehre_, Leipzig, 1908.

[551] The steps in this chronological variation were
termed by Waagen "mutations."

[552] _The Age of Mammals in Europe, Asia, and North
America_, New York, 1910.

[553] _Origin of Species_, 6th ed., Chap. IV.

INDEX

ACTINOZOAN THEORY of Vertebrate Descent, 299-300

Adaptation as Conservative Principle--
Cuvier, 39, 76

Adaptation, Ecological--
Von Baer, 123
H. Milne-Edwards, 199
Lamarck, 221, 222, 223, 224, 227
Treviranus, 225 f.n.
C. Darwin, 231-2, 235, 239
Haeckel, 248, 263
Gegenbaur, 263
V. O. Kowalevsky, 362
Osborn, 362-4

Adaptation, Ecological, and Classification--
Bronn, 203

Adaptation of Parts. _See_ "Correlation, Functional," and "Conditions of
Existence"

Adaptive Radiation (Osborn), 362-4

Agassiz, A., 288 f.n., 295
On Coelom, 296

Agassiz, L.--
Criticism of Vertebral Theory of Skull, 157
Membrane and Cartilage Bones, 164
Transcendentalism, 203
Classification, 203 f.n.
Three-fold Parallelism, 230, 255
Influence on Darwin, 238
Specific Distinctness of Embryos, 353

Albertus Magnus, 17

Alcmaeon, 1

Aldrovandus, 18

Allman, 209

Analogy. _See also_ Homology.
Aristotle, 8-10
Owen, 108
Haeckel, 251
Gegenbaur, 266
Lankester, 267

Anaxagoras, 14

Anaximander, 14

Anaximenes, 1

Animal and Vegetative Lives--
Aristotle, 16, 32
Buffon, 26-7
Bergson, 26 f.n.
Cuvier, 26, 32
Bichat, 27-9
Oken, 94
K. G. Carus, 94
Von Baer, 116, 123, 131
Remak (Sensory and trophic layers), 210
Gegenbaur, 263

Annelid Theory of Vertebrate Descent, 274-85, 301

Archetype, Anatomical, 246, 302-3
E. Geoffroy, 54, 67
Owen, 104-7, 110
J. V. Carus, Huxley, 204
C. Darwin, 238 f.n.

Archetype, Anatomical, as Ancestral--
C. Darwin, 235, 247
Haeckel, 251
Gegenbaur, 265
Sedgwick, 300
Criticism of this idea--
O. Hertwig, 355-7

Archetype, Embryological, 168, 246, 302-3
Von Baer, 126, 132
Reichert, 139, 147, 149
Rathke, 151, 153
Huxley, 159-61

Archetype, Embryological, as Ancestral--
C. Darwin, 233, 236-7
Haeckel, 254, 289-91
Gegenbaur, 266
O. and R. Hertwig, 298
Sedgwick, 300
A. Kowalevsky, 300

Arendt, 162

Aristotle, 2-16, 17, 345, 364
_Historia Animalium_, 2
_De Partibus Animalium_, 2, 9
Knowledge of Animals, 3, 4
Comparative Embryology, 4
Classification of Animals, 4-6
Unity of Plan, 6-7, 10
Homology and Analogy, 7-10
Teleology and Correlation, 10-12
Law of Compensation, 11
Division of Labour, 12
Degrees of Composition--homogeneous and heterogeneous parts, 12-14, 169
Law of Development (Von Baer), 14
Scale of Beings, 14-16
Functional attitude, 15-16, 197
Animal and Vegetative Lives, 16, 32

Ascidian Theory of Vertebrate Descent, 269-73, 304

Atomists, 16

Atomists, "Biological," 192-4

Audouin, V.--
Unity of plan in Arthropods, 85-6
Law of Compensation, 86
Marine Zoology, 195

Autenrieth, 90, 96

Avicenna, 17

BABAK, E., 333

Baer, K. E. von, 113-32, 133, 251, 304, 345, 356
Founder of Embryology, 113
_Entwickelungsgeschichte der Thiere_, 114
Regulation of Development, 114, 350
Development as Differentiation, 115, 128
Germ-Layer Theory, 115-6, 118-119, 208-9, 296
Morphological Differentiation, 116-7
Histological Differentiation, 117-8
Tissues and Germ-Layers, 118
Double symmetrical Development, 118, 279
Criticism of Meckel-Serres Law, 120-3, 304
Theory of Types, 123-4, 289, 291
Law of Development, 124-6
Embryological Criterion, 126-8, 132, 138
Embryological Archetype, 126, 132
Types of Development, 127-8
Von Baer and Cuvier, 128-30
Functional attitude, 129
Relation to Transcendentalists, 129, 131
Criticism of Scale of Beings, 130
Vertebral Theory of Skull, 131, 142
Serial Homology, 131-2
Gill-slits, Gill-arches and Aortic arches, 135-6, 146
Membrane and Cartilage Bones, 162-3
Degrees of Composition, 172
Ova of Mammals, 175-6
Segmentation of Ovum, 186
Criticism of Evolution Theory, 229, 242
Influence on Darwin, 236, 238
Criticism of Darwinism, 242
Teleology and Correlation, 242
On Ascidians, 271

Baer's Law. _See_ "Development, Von Baer's Law"

Bagge, 187

_Balanoglossus_ Theory of Vertebrate Descent, 285-7

Balbiani, 330

Balfour, F. M., 247, 299
Annelid Theory, 282-4
Gastrulation and Gastraea Theory, 295
Mesoderm, 296 f.n.
Coelom, 297

Barfurth, D., 330

Barry, M., 186, 188

Bateson, W.--
Metamerism, Vegetative Repetition, 286
_Balanoglossus_ Theory, 286-7
On Phylogenetic Speculation, 302

Beard, J., 285

Belon, 18

Beneden van, and Julin, 271, 285, 346

Bensley, A. B., 311 f.n.

Bergmann, 187

Bergson, H., 26 f.n., 341, 345

Bernard, Claude, 195, 314

Bert, P., 315

Bichat, X., 27-30, 118, 132, 169, 178, 263
Animal and Vegetative Lives, 27-9
"General Anatomy," 29-30
_Vie propre_ of Tissues, 30

Biogenetic Law. _See_"Development, Haeckel's Law"

Bischoff, 138
Segmentation, 186, 188

Blainville, de, 96, 128, 141, 199 f.n.

Bojanus, 96, 97

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