SPECIFIC FORM. LIVING BODIES AND CRYSTALS.
§ 1. Specific form and chemical constitution—The wide distribution
of crystalline forms—Organization of crystals—Law of relation
between specific form and chemical constitution—Value of form as a
characteristic of brute and living beings—Parentage, living beings and
mineral parentage—Iso-morphism and the faculty of cross-breeding—Other
analogies. § 2. Acquisition and re-establishment of the specific
form—Mutilation and regeneration of crystals—Mechanism of reparation.
§ 1. _Specific Form and Chemical Constitution._—In the enumeration
which we have made of the essential features of vitality there are
three that are, so to speak, of the highest value. They are, in
the order of their importance:—The possession of a specific form;
the faculty of growth or nutrition; and finally, the faculty of
reproduction by generation. By restricting our comparison between
brute bodies and living bodies to these truly fundamental characters
we sensibly restrict the field, but we shall see that it does not
disappear.
_Wide Distribution of Crystalline Forms._—The consideration of specific
forms shows us that in the mineral world we need only consider
crystallized bodies, as they are almost the only ones that possess
definite form. In restricting ourselves to this category we do not
limit our field as much as might be supposed. Crystalline forms are
very widely distributed. They are, in a measure, universal. Matter
has a decided tendency to assume these forms whenever the physical
forces which it obeys act with order and regularity, and when their
action is undisturbed by accidental occurrences. In the same way, too,
living forms are only possible in regulated environments, under normal
conditions, protected from cataclysms and convulsions of nature.
The possession of a specific form is the most significant feature of
an organized being. Its tendency, from the time it begins to develop
from the germ, is toward the acquirement of that form. The progressive
manner in which it seeks to realize its architectural plan in spite of
the obstacles and difficulties that arise—healing its wounds, repairing
its mutilations—all this, in the eyes of the philosophical biologist,
forms what is perhaps the most striking characteristic of a living
being, that which best shows its unity and its individuality. This
property of organogenesis seems pre-eminently the vital property. It is
not so, however, for crystalline bodies possess it in an almost equal
degree.
The parallel between the crystal and a living being has been often
drawn. I will not reproduce it here in detail. My sole desire, after
sketching its principal features, is to call attention to the new
information that has been brought out by recent investigations.
_Organization of Crystals. Views of Haüy, Delafosse, Bravais, and of
Wallerant._—In botany, zoology, and crystallography we understand by
form an assemblage of material constituents co-ordinated in a definite
system—_i.e._, the organization itself. The body of man, for example,
is an edifice in which sixty trillion cells ought each to find its own
predetermined place.
In crystallography also we understand by form the organization which
crystals present. The grouping of the elements of crystals is, perhaps,
more simple. They are none the less organized, in the same sense that
living bodies are.
Their organization, while more uniform than that of living bodies,
still shows a considerable amount of variation. It should not be
assumed that the area of a crystal is completely filled, with
contiguous parts applied one to the other by plane faces, as might
be supposed from the phenomenon of cleavage which dissociates the
parts of the crystalline body into solids of this kind. In reality,
the constituent parts are separated from each other by spaces. They
are arranged in a quincunx, as Haüy put it, or along the lines of a
network, to use the terms of Delafosse and Bravais. The intervals left
between them are incomparably larger than their diameters. So that in
the organization of a crystal it is necessary to take into account two
quite different things:—An element, the crystalline particle, which
is a certain aggregate of chemical molecules having a determinate
geometrical form; and a more or less regular, parallelopipedic network,
along the edges of which are arranged in a constant and definite manner
the aforesaid particles. The external form of the crystal indicates the
existence of the network. Its optical properties depend upon the action
of the particles, as Wallerant has shown: Thus we must distinguish in
a crystal between two kinds of geometrical figures—that of the network
and that of the particle—and their characters of symmetry may be either
concordant or discordant.
The crystalline particle, the element of the crystal, is therefore
a certain molecular complex that repeats itself identically and is
identically placed at the nodes of the parallelopipedic network. It has
been given different names well calculated to produce confusion-the
crystallographic molecule of Mallard, the complex particle of other
authors. Some have separated this element into subordinate elements
(the fundamental particles of Wallerant and of Lapparent).
These very general outlines will suffice to show how complex and
adjustable is the organization of the crystalline individual, which in
spite of its geometric regularity and its rigidity, may be compared
with the still more flexible organization of the living element. The
mineral individual is more stable, more labile—_i.e._, less prone
to undergo change than is the living individual. We may say with
M. Lapparent that “crystallized matter presents the most perfect
and stable orderly arrangement of which the particles of bodies are
susceptible.”
_Law of Relation of Specific Form to Chemical
Constitution._—Crystallization is a method of acquiring specific form.
The geometrical architecture of the mineral individual is but little
less wonderful or characteristic than that of the living individual.
Its form is the result of the mutual reactions of its substances and
of the medium in which it is produced; it is the condition of material
equilibrium corresponding to a given situation. This idea of a specific
form belonging to a given substance under given conditions must be
borne in mind. We may consider it as a kind of principle of nature,
an elementary law, which may serve as a point of departure for the
explanation of phenomena. A particular substance under identical
conditions of environment, must always assume a certain form.
This close linking of substance and form, admitted as a postulate in
physical sciences, has been carried into biology by some philosophical
naturalists, by M. Le Dantec, for instance.
Let us imitate them for a moment. Let us cease to seek in the living
being for the prototype of the crystal; let us, on the contrary, seek
in the crystal the prototype of the living being. If we succeed in
this, we shall then have found the physical basis of life.
Let us say, then, with the biologists we have mentioned, that the
substance of each living being is peculiar to it; that it is specific,
and that its form—that is to say its organization—follows from it.
The morpholpgy of any being whatever, of an animal—of a setter, for
example—or even of a determinate being—of Peter, of Paul—is the
“crystalline form of their living matter.” It is the only form of
equilibrium that can be assumed under the given conditions by the
substance of the setter, of Peter, or of Paul, just as the cube is the
crystalline form of sea-salt. In this manner these biologists have
supposed that they could carry back the problem of living form to the
problem of living substance, and at the same time reduce the biological
mystery to the physical mystery. I have shown above (Chap. V. pp.
199-204) how far this idea is legitimate, and how far and with what
restrictions it may be welcomed and adopted.
_Value of Form as a Characteristic of Living and Brute Beings._—However
this may be, we may say, without fear of exaggeration, that the
crystalline form characterizes the mineral with no less precision than
the anatomical form characterizes the animal and the plant. In both
cases, form—regarded as a method of distribution of the parts—indicates
the individual and allows us to diagnose it with more or less facility.
_Parentage of Living Beings and Mineral Parentage._—Still another
analogy has been noted. In animals and plants similarity in form
indicates similarity in descent, community of origin, and proximity in
any scheme of classification. In the same way identity of crystalline
form indicates mineral relationship. Substances chemically analogous
show identical, geometrically superposable forms, and are thus arranged
in family or generic groups recognizable at a glance.
_Isomorphism and the Faculty of Cross-breeding._—And further, the
possibility in the case of isomorphous bodies, of their replacing each
other in the same crystal during the process of formation and of thus
mingling, so to speak, their congenital elements, may be compared
with the possibility of inter-breeding with living beings of the same
species. Isomorphism is thus a kind of faculty of crossing. And as the
impossibility of crossing is the touchstone of taxonomic relationship,
testing it, and separating stocks that ought to be separated, so the
operation of crystallization is also a means of separating from an
accidental mixture of mineral species the pure forms which are blended
therein. Crystallization is the touchstone of the specific purity of
minerals; it is the great process in chemical purification.
_Other Analogies._—The analogies between crystalline and living forms
have been pushed still further even to the verge of exaggeration.
The internal and external symmetry of animals and plants has been
compared to that of crystals. Transitions or intergradations have been
sought between the rigid and faceted architecture of the latter and the
flexible structure and curved surface of the former; the utricular form
of flowers of sulphur on the one hand, and the geometrical structure
of the shells of radiolarians on the other, have shown an exchange of
typical forms between the two systems. An effort has even been made
to draw a parallel between six of the principal types of the animal
kingdom and the six crystalline systems. If carried as far as this, our
thesis becomes puerile. Real analogies will suffice. Among these the
curious facts of crystalline renewal come first.
§ 2. CICATRIZATION IN LIVING BEINGS AND IN CRYSTALS.
We know that living beings not only possess a typical architecture
which they have themselves constructed, but that they defend it against
destructive agencies, and that if need arise they repair it. The
living organism cicatrizes its wounds, repairs losses of substance,
regenerates more or less perfectly the parts that have been removed;
in other terms, when it has been mutilated it tends to reconstruct
itself according to the laws of its own morphology. This phenomenon of
reconstitution or reintegration, these more or less successful efforts
to re-establish its form and its integrity, at first appear to be a
characteristic feature of living beings. This is not the case.
_Mutilation and Re-integration of Crystals._—Crystals—let us say
crystalline individuals—show a similar aptitude for repairing their
mutilations. Pasteur, in an early work, discussed these curious facts.
Other experimenters, Gernez a little later and Rauber more recently,
took up the same subject, but could do no more than extend and confirm
his observations. Crystals are formed from a primitive nucleus, as the
animal is formed from an egg; their integral particles are disposed
according to efficient geometrical laws, so as to produce the typical
form by a constructive process that may be compared to the embryogenic
process which builds up the body of an animal. Now this operation
may be disturbed by accidents in the surrounding medium or by the
deliberate intervention of the experimenter. The crystal is then
mutilated. Pasteur saw that these mutilations repaired themselves.
“When,” said he, “a crystal from which a piece has been broken off
is replaced in the mother liquor, we see that while it increases in
every direction by a deposit of crystalline particles, activity occurs
at the place where it was broken off or deformed; and in a few hours
this suffices not only to build up the regular amount required for the
increase of all parts of the crystal, but to re-establish regularity
of form in the mutilated part.” In other words, the work of formation
of the crystal is carried on much more actively at the point of lesion
than it would have been had there been no lesion. The same thing would
have occurred with a living being.
_Mechanism of Reparation._—Gernez some years later made known the
mechanism of this reparation, or, at least, its immediate cause. He
showed that on the injured surface the crystal becomes less soluble
than on the other facets. This is not, however, an exceptional
phenomenon. It is, on the contrary, quite frequently observed that the
different faces of a crystal show marked differences in solubility.
This is what happens in every case for the mutilated face in comparison
with the others; the matter is less soluble there. The consequence
of this is clear; the growth must preponderate on that face, since
there the mother liquor will become super-saturated before being
super-saturated for the others. We may explain this result in another
way. Each face of the crystal in contact with the mother liquor is
exposed to two antagonistic actions: The matter deposited upon a
surface may be taken away and redissolved if, for any reason whatever,
such matter becomes more soluble than that of the liquid stratum in
contact with it; in the second place, the matter of this liquid stratum
may, under contrary conditions, be deposited, and thus increase the
body of the crystal. There is, then, for each point of the crystalline
facet, a positive operation of deposit which results in a gain, and a
negative operation of redissolution which results in a loss. One or
the other effect predominates according as the relative solubility is
greater or less for the matter of the facet under consideration. On the
mutilated surface it is diminished, deposition then prevails.
But this is only the immediate cause of the phenomenon; and if we wish
to know why the solubility has diminished on the mutilated surface
Ostwald explains it to us by showing that crystallization tends to form
a polyhedron in which the surface energy is a relative minimum.