Form and Function

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It is instructive to find that between Serres and Milne-Edwards there
existed the same antagonism as between von Baer and the German
transcendentalists. Milne-Edwards was a constant critic of the law of
parallelism which Serres continued to uphold with little modification
for over thirty years, just as von Baer was a critic of that form of the
doctrine which was current in the early part of the century. As early as
1833, Milne-Edwards, through his studies of crustacean development,[319]
had come to the conclusion, independently of von Baer, that development
always proceeded from the general to the special; that class characters
appeared before family characters, generic characters before specific.
In an interesting paper published in 1844,[320] he discussed the relation
of this law of development to the problems of classification, and
arrived at results almost identical with those set forth by von Baer in
his Fifth Scholion.

Like von Baer he rejected completely the theory of parallelism and the
doctrine of the scale of beings; like von Baer he held that the type of
organisation--of which there are several--is manifested in the very
earliest stages and becomes increasingly specialised throughout the
course of further development; like von Baer, too, he sketched a
classification based upon embryological characters.

These views were further developed in his volume of 1851, and also in
his _Rapport_ of 1867.

They brought him into conflict with his confrere in the Academy of
Sciences, Etienne Serres, who in a number of papers published in the
'thirties and 'forties,[321] and particularly in his comprehensive memoir
of 1860, still maintained the theory of parallelism and the doctrine of
the absolute unity of type. His memoir of 1860 shows how completely
Serres was under the domination of transcendental ideas. Much of it
indeed goes back to Oken. "The animal kingdom," he writes, "may be
considered in its entirety as a single ideal and complex being" (p.
141). His views have become a little more complicated since his first
exposition of them in 1827, and he has been forced to modify in some
respects the rigour of his doctrine. But he still holds fast to the main
thesis of transcendentalism--the absolute unity of plan of all animals,
vertebrate and invertebrate alike,[322] the gradual perfecting of
organisation from monad to man, the repetition in the embryogeny of the
higher animals of the "zoogeny" of the lower.

He recognised, however, that the idea of a simple scale of beings is
only an abstraction, and that the true repetition is of organs rather
than of organisms. He was willing even to admit, at least in the later
pages of his memoir, that there might be not one animal series but
several parallel series, as had been suggested by Isidore Geoffroy St
Hilaire (p. 749). In general, his views are now less dogmatic than they
were in his earlier writings, but they are not for all that changed in
any essential. For, in summing up his main results, he writes, "The
whole animal kingdom can in some measure be regarded ideally as a single
animal, which, in the course of formation and metamorphosis in its
diverse manifestations, here and there arrests its own development, and
thus determines at each point of interruption, by the very state it has
reached, the distinctive characters of the phyla, the classes, families,
genera, and species" (p. 833).[323]

To settle the dispute pending between two of its most illustrious
members, the Academy proposed in 1853, as the subject of one of its
prizes, "the positive determination of the resemblances and differences
in the comparative development of Vertebrates and Invertebrates." A
memoir was presented the next year by Lereboullet[324] which met with the
approval of the Academy in so far as its statements of fact were
concerned, but seemed to them to require amplification in its
theoretical part. But even in this memoir Lereboullet was able to show
that the balance of evidence was greatly in favour of Milne-Edwards'
views, and his general conclusions in 1854 were that "in the presence of
such fundamental differences, one is obliged to give up the idea of one
single plan in the formation of animals; while, on the contrary, the
existence of diverse plans or types is clearly demonstrated by all the
facts" (p. 79). To fulfil the Academy's requirements, Lereboullet
continued his work, and in 1861-63 he published a series of elaborate
monographs[325] on the embryology of the trout, the lizard and the
pond-snail _Lymnaea_, and rounded off his work with a full discussion[326]
of the theoretical questions involved. In this considered and
authoritative judgment he completely disposed of Serres' theories of the
unity of plan and the unity of genetic formation. Except in the very
earliest stages of oogenesis there is no real similarity between the
development of a Zoophyte, a Mollusc, an Articulate and a Vertebrate,
but each is stamped from the beginning with the characteristics of its
type. The lower animals are not, and cannot possibly be the permanent
embryos of the higher animals. "The results which I have obtained," he
writes, "are diametrically opposed to the theory of the zoological
series constituted by stages of increasing perfection, a theory which
tries to demonstrate in the embryonic phases of the higher animals a
repetition of the forms which characterise the lower animals, and which
has led to the assertion that the latter are permanent embryos of the
former. The embryo of a Vertebrate shows the vertebrate type from the
very beginning, and retains this type throughout the whole course of its
development; it never is, and never can be, either a Mollusc or an
Articulate" (xx., p. 54).

"We are led to establish ... as the general result of our researches,
the existence of several types, and, consequently, of different plans,
in the development of animals. These different types are manifested from
the very beginning of embryonic life; the characters distinguishing them
are therefore primordial, and we can say with M. Milne-Edwards that
_everything goes to prove that the distinction established by Nature
between animals belonging to different phyla is a primordial
distinction_" (p. 58).

In other directions also von Baer's work was confirmed and extended by
later observers--those parts of it particularly that had reference to
the germ-layer theory, and to the concept of histological
differentiation. His germ-layer theory was accepted in its main lines by
Rathke, Bischoff and Lereboullet, and applied by them to the multitude
of new facts they discovered. Rathke, in particular, was a firm upholder
of the doctrine, and made considerable use of it in his writings.[327]
Even before the publication of von Baer's book he had interpreted in
terms of the germ-layer theory sketched by his friend Pander the
splitting of the blastoderm which occurs in the early development of
_Astacus_, whereby there are formed a serous and a mucous layer, one
inside the other--like the coats of an onion, to use his own expressive
phrase.[328]

An ingenious application of the Pander-Baer theory was made by Huxley,
who compared the outer and inner cell-layers which form the groundwork
of the Coelentera with the serous and mucous layers of the vertebrate
germ.[329] He laid stress, it is true, rather on the physiological than on
the morphological resemblance. "A complete identity of structure," he
writes, "connects the 'foundation membranes' of the Medusae with the
corresponding organs in the rest of the series; and it is curious to
remark, that throughout, the outer and inner membranes appear to bear
the same physiological relation to one another as do the serous and
mucous layers of the germ; the outer becoming developed into the
muscular system, and giving rise to the organs of offence and defence;
the inner, on the other hand, appearing to be more closely subservient
to the purposes of nutrition and generation" (p. 24). Von Baer had
already hinted at this homology in the second volume of his
_Entwickelungsgeschichte_ (1837), where he says with reference to the
separation of the blastoderm of the chick into two layers. "Yet
originally there are not two distinct or even separable layers, it is
rather the two surfaces of the germ which show this differentiation,
just as polyps show the same contrast of an external surface and an
internal digestive surface. In between the two layers there is in our
germ as in the polyp an indifferent mass" (p. 67). The terms ectoderm
and entoderm were introduced by Allman[330] in 1853 for the two
cell-layers in the Hydrozoa.

Remak is the second great name in the history of the germ-layer theory.
He had the great advantage over von Baer of being able to make use of
the cell-theory in interpreting the formation of the germ-layers.
Microscopical technique also had been greatly improved since 1828.[331]

Remak's greatest service was that he put the germ-layer theory in direct
relation with the cell-theory by demonstrating the cellular continuity
from egg-cell to tissue, and by showing that each germ-layer possessed
distinctive histological characteristics. Hardly less important was his
clear marking-off of the "middle layer" as a separate and distinct layer
of the germ. He it was who introduced the modern conception of the
mesoderm, and cleared up the confusion in which Pander and von Baer had
left the organs formed between the serous and the mucous layer. Remak's
middle layer was a different thing from Pander's ill-defined
"vessel-layer"; it included and unified from a new point of view the
"vessel" and "muscle" layers of von Baer.

There are in the unincubated blastoderm of the chick, according to
Remak,[332] two cell-layers, of which the undermost subsequently splits
into two. Three layers are thus formed--the upper, middle and lower. The
upper layer differentiates into a medullary plate and an epidermic plate
(Remak's _Hornblatt_), and gives origin to the medullary tube with all
its evaginations, and to the skin with all its derivatives and pockets.
It forms such diverse structures as the brain, the spinal cord, the eye,
the ear, the mouth, hairs, feathers, nails, sweat-glands, lacrymal
glands, and so forth. All these parts are connected directly or
indirectly with sensation, and the upper germ-layer may accordingly be
called the _sensory_ layer. The lower layer gives rise to the epithelium
and the proper tissue of the alimentary canal and its derivatives, as
the liver, lungs, pancreas, kidneys, thyroid, thymus, etc. These parts
are all concerned in the processes of assimilation and dissimilation,
and the lower layer may accordingly be called the _trophic_ layer. Now
between the upper or sensory layer and the lower or trophic layer there
exists, in spite of their very different functions, a close histological
likeness, for both are essentially epithelial layers. The resemblance is
particularly strong if we compare the lower layer with the _Hornblatt_
of the upper layer--both consist of epithelial tissue, and of its
derivative, glandular tissue, and form neither vessels nor nerves. The
middle layer, on the contrary, forms nerves and muscles, vessels and
connective tissue, and little or no epithelium. It does not form all the
blood-vessels without exception (and so cannot be called the
vessel-layer), for the blood-vessels of the central nervous system are
in all probability formed from the upper layer. So, too, it does not
form all the nerves and muscles--the optic and auditory nerves and the
nerves and muscles of the iris probably arise in the upper layer. But,
in spite of these exceptions, its general histological character is so
well defined that it may be contrasted with the other two as
preeminently the layer that forms muscular, nervous, vascular and
connective tissue. In view of its functional significance, it may be
called the _motory_ layer, or better, since it forms also the sexual
glands, the _motor-germinative_ layer. The middle layer, early in its
history, shows a division into dorsal plates (_Urwirbelplatten_) and
ventral plates (_Seitenplatten_). The former exhibit almost as soon as
they are formed the characteristic proto-vertebral segmentation, the
latter split to form the pleuro-peritoneal or body-cavity. Remak
describes the latter process as follows:--"In the region of the trunk,
where a greater independence of the fate of the alimentary canal and its
annexes becomes necessary for the voluntary executive organs, the
ventral plates undergo a process of splitting, leading to the formation
of the sensitive part of the integument (the _Hautplatten_), the
muscular part of the alimentary tube (the _Darmfaserplatten_), and the
mother-tissue of the generative organs (the _Mittelplatten_). From the
_Hautplatten_ there develops, without the dorsal plates seeming to take
any part in the process, the rudiment of the extremities" (p. 79).

[Illustration: FIG. 12.--Transverse Section of Chick Embryo. (After
Remak.)]

His _Darmfaserplatten_ form the nervous and muscular tissue of the
alimentary canal and its dependencies, and also the heart; the
_Hautplatten_ form the general body-wall (exclusive of the skin) and the
appendages. In the embryo they line the amniotic cavity. The skeleton
and peripheral nerves originate wholly within the middle layer.

Remak's conception of the relations of the three germ-layers to one
another and to the body-cavity is well illustrated in Fig. 12.

In his germ-layer theory Remak's standpoint is histological rather than
morphological. The distinction which he draws between the sensory and
trophic layers on the one hand, and the motor-germinative layer on the
other, is entirely a histological one. The greater part of his book,
indeed, is devoted to a study of the histogenesis of the different
organs of the body; he is bent chiefly upon unravelling the part which
each germ-layer takes in the formation of each tissue and organ.

His generalisation that two of the germ-layers give rise exclusively or
almost exclusively to one kind of tissue excited great interest at the
time, and gave the direction to histogenetic research for quite a number
of years, though in the end it turned out to be insufficiently founded.

Though Remak's germ-layer theory had thus principally a histological
orientation, it laid down the main lines of the modern morphological
treatment of the germ-layers.

[293] _Embryologie des Salmones_, 1842.

[294] _Die Cellularpathologie in ihrer Begruendung auf
physiologische und pathologische Gewebelehre_, Berlin,
2nd ed. 1859; Eng. trans., by Chance, 1860.

[295] _Arch. path. Anat. Phys_., vii., pp. 1-39 (1854).

[296] _Bericht ueber die Fortschritte der mikroskopischen
Anatomie im jahre 1854._ Mueller's _Archiv_, 1855. See
also 1856.

[297] _Hndb. d. Physiol._, i., 1835.

[298] See Leuckart's reply to Ludwig's criticism, in
_Zeit. f. wiss. Zool._, ii., p. 271, 1850.

[299] Leipzig, 1853.

[300] _Souvenirs d'un Naturaliste_, 2 vols., Paris, 1854.
Eng. Trans. as _Rambles of a Naturalist on the Coasts of
France, Spain, and Italy_, 2 vols., 1857.

[301] Milne-Edwards later published a classical textbook
on comparative anatomy and physiology--_Lecons sur la
Physiologie et l'Anatomie comparees_, 14 vols., Paris,
1857-80.

[302] Paris, 1834-40. Three volumes of the _Suites a
Buffon_.

[303] Paris, 1865. Two volumes of the _Suites a Buffon_.

[304] _U. d. Metamorphose der Ophiuren u. Seeigel._,
Berlin, 1848. _U. d. Metamorphose der Holothurien u.
Asterien._, Berlin, 1851.

[305] As I have been unable to obtain a copy of the
_Introduction_, the passages which follow are taken from
the _Rapport_ of 1867, where Milne-Edwards gives a
complete exposition of his doctrine, sometimes in the
words of the original.

[306] This principle was first developed by Milne-Edwards
in 1827, in the _Dictionnaire classique d'Hist.
naturelle_. It was probably suggested to him by his
studies on the Crustacea, among which the principle is
so beautifully exemplified in the concentration and
specialisation of the appendages and the ganglionic
chain.

[307] Studied by Isidore Geoffroy St Hilaire in his paper
_Classification parallelique des Mammiferes, C. R. Acad.
Sci._, xx., 1845. Remarked upon by Cuvier, _Regne
animal_., i., p. 171, 1817, also by de Blainville.

[308] Cuvier et Valenciennes, _Hist. nat. des Poissons_,
i., p. 550, 1828.

[309] _Myxinoiden_, Th. I. _Abh. k. Akad. Wiss. Berlin_
for 1834, pp. 100, 110, 179, etc.

[310] _Vergl. Entw. Kopf. nackt. Amphibien_, p. 101, 1838.

[311] I have not seen the companion volume on
palaeontological progression, _Unters. ue. d.
Entwickelungsgesetze der organischen Welt waehrend der
Bildungszeit unserer Erdoberflaeche_, Stuttgart, 1858.

[312] "Strobiloid" because of its spiral development. The
theory of the spiral growth of plants played an
important part in botanical morphology about this time.

[313] _Cf._ Meckel's Principle of progressive Evolution,
_supra_, p. 93.

[314] _System der thierischen Morphologie_, pp. 33, 457.
Also C. Bruch, _Die Wirbeltheorie des Schaedels, am
Skelette des Lachses geprueft_, Frankfort-on-Main, 1862.

[315] In France the vertebral theory was advocated by
Lavocat in his _Nouvelle Osteologie comparee de la tete
des animaux domestiques_, Toulouse, 1864. It seems also
that Lacaze-Duthiers held fast to it even in
1872--_Arch. zool. exp. gen._, i., p. 51, 1872.

[316] _An Essay on Classification_, Boston, 1857, London,
1859. He considered the classificatory categories to be
the categories of the Creator's thought, and hence
natural, and in no sense mere conventions.

[317] "Principes d'Embryogenie, de Zoogenie et de
Teratogenie," _Mem. Acad. Sci._, xxv., pp. 1-943, pls.
xxv., 1860.

[318] "On the Morphology of the Cephalous Mollusca,"
_Phil. Trans._, 1853, _Sci. Memoirs_, i., pp. 152-92.

[319] "Observations sur les changements de forme que les
divers Crustaces eprouvent," _Ann. Sci. nat._ (1) xxx.,
p. 360, 1833.

[320] "Considerations sur quelques principes relatifs a la
classification naturelle des animaux," _Ann. Sci. nat._
(3) i., p. 65, 1844.

[321] _Supra_, pp. 79-83. Also _Precis d'anatomie
transcendante, principes d'organogenie_, Paris, 1842.

[322] The inversion of the organs shown by Vertebrates as
compared with Invertebrates is due to the reversed
position of the embryo relatively to the yolk! (pp.
821-6).

[323] It is worth while recording that Serres enunciated a
"law of symmetry" according to which the embryo is
formed by the union of its two symmetrical halves--a law
which recalls the "concrescence theory" of His and some
modern embryologists.

[324] "Embryologie comparee du Brochet, de la Perche, et
de l'Ecrevisse," _Ann. Sci. nat._ (4), i., p. 237, 1854;
ii., p. 39, 1854. _Mem. Savans etrangers_, xvii.

[325] _Ann. Sci. nat._ (4) xvi., p. 113, 1861; xvii., p.
88, 1862; xviii., p. 5, 1862; xix., p. 5, 1863.

[326] xx., p. 5, 1863.

[327] Particularly in his _Blennius_ (1833) and _Natter_
(1839).

[328] In the "preliminary notice" of his Crayfish
paper--_Isis_, pp 1093-1100, 1825.

[329] "On the Anatomy and the Affinities of the Family of
the Medusae," _Phil. Trans._, 1849; _Sci. Memoirs_, i.,
pp. 9-32.

[330] _Phil. Trans._, cxliii., p. 368, 1853.

[331] The principle of achromatism was discovered (by
Fraunhofer) and achromatic microscopes introduced in the
early part of the 19th century. The use of chemical
reagents, such as acetic acid, and various hardening
fluids, came into fashion not long after. J. Mueller
seems to have been one of the first to realise their
importance. Remak himself invented one or two fixing and
hardening mixtures (pp. 87, 127, 1855), which enabled
him to cut excellent hand sections. Section-cutting
machines were not invented till later (V. Hensen, 1866,
His, 1870).

[332] _Untersuchungen ueber die Entwickelung der
Wirbelthiere_, folio, pp. xxxvii + 195, 12 plates,
Berlin, 1850-1855.

CHAPTER XIII

THE RELATION OF LAMARCK AND DARWIN TO MORPHOLOGY.

It is a remarkable fact that morphology took but a very little part in
the formation of evolution-theory. When one remembers what powerful
arguments for evolution can be drawn from such facts as the unity of
plan and composition and the law of parallelism, one is astonished to
find that it was not the morphologists at all who founded the theory of
evolution.

It is true that the noticeable resemblances of animals to one another,
the possibility of arranging them in a system, the vague perception of
an all-pervading plan of structure, did suggest to many minds the
thought that systematic affinities might be due to blood-relationship.
Thus Leibniz considered that the cat tribe might possibly be descended
from a common ancestor,[333] and another great philosopher, Immanuel Kant,
was led by his perception of the unity of type to suggest as possible
the derivation of the whole organic realm from one parent form, or even
ultimately from inorganic matter. In the course of his masterly
discussion of mechanism and teleology,[334] he writes, "The agreement of
so many genera of animals in a certain common schema, which appears to
be fundamental not only in the structure of their bones, but also in the
disposition of their remaining parts--so that with an admirable
simplicity of original outline, a great variety of species has been
produced by the shortening of one member and the lengthening of another,
the involution of this part and the evolution of that--allows a ray of
hope, however faint, to penetrate into our minds, that here something
may be accomplished by the aid of the principle of the mechanism of
Nature (without which there can be no natural science in general). This
analogy of forms, which with all their differences seem to have been
produced according to a common original type, strengthens our suspicions
of an actual relationship between them in their production from a common
parent, through the gradual approximation of one animal-genus to
another--from those in which the principle of purposes seems to be best
authenticated, _i.e._, from man down to the polype, and again from this
down to mosses and lichens, and finally to the lowest stage of Nature
noticeable by us, viz., to crude matter."[335]

So, too, Buffon's evolutionism was suggested by his study of the
structural affinities of animals, and Erasmus Darwin in his _Zoonomia_
(1794) brought forward as one of the strongest proofs of evolution, "the
essential unity of plan in all warm-blooded animals."[336]

But, as a matter of historical fact, no morphologist, not even Geoffroy,
deduced from the facts of his science any comprehensive theory of
evolution. The pre-Darwinian morphologists were comparatively little
influenced by the evolution-theories current in their day, and it was in
the anatomist Cuvier and the embryologist von Baer that the early
evolutionists found their most uncompromising opponents.

Speaking generally, and excepting for the moment the theory of Lamarck,
we may say that the evolution-theories of the 18th and 19th centuries
arose in connection with the transcendental notion of the _Echelle des
etres_, or scale of perfection. This notion, which plays so great a part
in the philosophy of Leibniz, was very generally accepted about the
middle of the 18th century, and received complete and even exaggerated
expression from Bonnet and Robinet. Buffon also was influenced by it.
Towards the beginning of the 19th century the idea was taken up eagerly
by the transcendental school and by them given, in their theories of the
"one animal," a more morphological turn. Their recapitulation theory was
part and parcel of the same general idea.

One understands how easily the notion of evolution could arise in minds
filled with the thought of the ideal progression of the whole organic
kingdom towards its crown and microcosm, man. Their theory of
recapitulation led them to conceive evolution as the developmental
history of the one great organism.[337] Many of them wavered between the
conception of evolution as an ideal process, as a _Vorstellungsart_, and
the conception of it as an historical process. Bonnet, Oken, and the
majority of the transcendentalists seem to have chosen the former
alternative; Robinet, Treviranus, Tiedemann, Meckel, and a few others
held evolution to be a real process.

We have already in previous chapters[338] briefly noticed the relation of
one or two of the transcendental evolution-theories to morphology, and
there is little more to be said about them here. They had as good as no
influence upon morphological theory, nor indeed upon biology in
general.[339] It is different with the theory of Lamarck, which, although
it had little influence upon biological thought during and for long
after the lifetime of its author, is still at the present day a living
and developing doctrine.

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