changes of an unfolding ovum of primitive type. The original fertilized
single cell, having by spontaneous fission multiplied into a cluster of
such cells, there begins to show itself a contrast between periphery and
centre; and presently there is formed a sphere consisting of a
superficial layer unlike its contents. The first change, then, is the
rise of a difference between that outer part which holds direct converse
with the surrounding medium, and that inclosed part which does not. This
primary differentiation in these compound embryos of higher animals,
parallels the primary differentiation undergone by the simplest living
things.
Leaving, for the present, succeeding changes of the compound embryo, the
significance of which we shall have to consider by-and-by, let us pass
now to the adult forms of visible plants and animals. In them we find
cardinal traits which, after what we have seen above, will further
impress us with the importance of the effects wrought on the organism by
its medium.
From the thallus of a sea-weed up to the leaf of a highly developed
phænogam, we find, at all stages, a contrast between the inner and outer
parts of these flattened masses of tissue. In the higher _Algæ_ “the
outermost layers consist of smaller and firmer cells, while the inner
cells are often very large, and sometimes extremely long;”[11] and in
the leaves of trees the epidermal layer, besides differing in the sizes
and shapes of its component cells from the parenchyma forming the inner
substance of the leaf, is itself differentiated by having a continuous
cuticle, and by having the outer walls of its cells unlike the inner
walls.[12] Especially significant is the structure of such intermediate
types as the Liverworts. Beyond the differentiation of the covering
cells from the contained cells, and the contrast between upper surface
and under surface, the frond of _Marchantia polymorpha_ clearly shows us
the direct effect of incident forces; and shows us, too, how it is
involved with the effect of inherited proclivities. The frond grows from
a flat disc-shaped gemma, the two sides of which are alike. Either side
may fall uppermost; and then of the developing shoot, the side exposed
to the light “is under all circumstances the upper side which forms
stomata, the dark side becomes the under side which produces root-hairs
and leafy processes.”[13] So that while we have undeniable proof that
the contrasted influences of the medium on the two sides, initiate the
differentiation, we have also proof that the completion of it is
determined by the transmitted structure of the type; since it is
impossible to ascribe the development of stomata to the direct action of
air and light. On turning from foliar expansions, to stems and roots,
facts of like meaning meet us. Speaking generally of epidermal tissue
and inner tissue, Sachs remarks that “the contrast of the two is the
plainer the more the part of the plant concerned is exposed to air and
light.”[14] Elsewhere, in correspondence with this, it is said that in
roots the cells of the epidermis, though distinguished by bearing hairs,
“are otherwise similar to those of the fundamental tissue” which they
clothe,[15] while the cuticular covering is relatively thin; whereas in
stems the epidermis (often further differentiated) is composed of layers
of cells which are smaller and thicker-walled: a stronger contrast of
structure corresponding to a stronger contrast of conditions. By way of
meeting the suggestion that these respective differences are wholly due
to the natural selection of favourable variations, it will suffice if I
draw attention to the unlikeness between imbedded roots and exposed
roots. While in darkness, and surrounded by moist earth, the outermost
protective coats, even of large roots, are comparatively thin; but when
the accidents of growth entail permanent exposure to light and air,
roots acquire coverings allied in character to the coverings of
branches. That the action of the medium causes these and converse
changes, cannot be doubted when we find, on the one hand, that “roots
can become directly transformed into leaf-bearing shoots,” and, on the
other hand, that in some plants certain “apparent roots are only
underground shoots,” and that nevertheless “they are similar to true
roots in function and in the formation of tissue, but have no root-cap,
and, when they come to the light above ground, continue to grow in the
manner of ordinary leaf-shoots.”[16] If, then, in highly developed
plants inheriting pronounced structures, this differentiating influence
of the medium is so marked, it must have been all-important at the
outset while types were undetermined.
As with plants so with animals, we find good reason for inferring that
while the specialities of the tegumentary parts must be ascribed to the
natural selection of favourable variations, their most general traits
are due to the direct action of surrounding agencies. Here we come upon
the border of those changes which are ascribable to use and disuse. But
from this class of changes we may fitly exclude those in which the parts
concerned are wholly or mainly passive. A corn and a blister will
conveniently serve to illustrate the way in which certain outer actions
initiate in the superficial tissues, effects of very marked kinds, which
are related neither to the needs of the organism nor to its normal
structure. They are neither adaptive changes nor changes towards
completion of the type. After noting them we may pass to allied, but
still more instructive, changes. Continuous pressure on any portion of
the surface causes absorption, while intermittent pressure causes
growth: the one impeding circulation and the passage of plasma from the
capillaries into the tissues, and the other aiding both. There are yet
further mechanically-produced effects. That the general character of the
ribbed skin on the under surfaces of the feet and insides of the hands
is directly due to friction and intermittent pressure, we have the
proofs:—first, that the tracts most exposed to rough usage are the most
ribbed; second, that the insides of hands subject to unusual amounts of
rough usage, as those of sailors, are strongly ribbed all over; and
third, that in hands which are very little used, the parts commonly
ribbed become quite smooth. These several kinds of evidence, however,
full of meaning as they are, I give simply to prepare the way for
evidence of a much more conclusive kind.
Where a wide ulcer has eaten away the deep-seated layer out of which the
epidermis grows, or where this layer has been destroyed by an extensive
burn, the process of healing is very significant. From the subjacent
tissues, which in the normal order have no concern with outward growth,
there is produced a new skin, or rather a pro-skin; for this substituted
outward-growing layer contains no hair-follicles or other specialities
of the original one. Nevertheless, it is like the original one in so far
that it is a continually renewed protective covering. Doubtless it may
be contended that this make-shift skin results from the inherited
proclivity of the type—the tendency to complete afresh the structure of
the species when injured. We cannot, however, ignore the immediate
influence of the medium, on recalling the facts above named, or on
remembering the further fact that an inflamed surface of skin, when not
sheltered from the air, will throw out a film of coagulable lymph. But
that the direct action of the medium is a chief factor we are clearly
shown by another case. Accident or disease occasionally causes permanent
eversion, or protrusion, of mucous membrane. After a period of
irritability, great at first but decreasing as the change advances, this
membrane assumes the general character of ordinary skin. Nor is this
all: its microscopic structure changes. Where it is a mucous membrane of
the kind covered by cylinder-epithelium, the cylinders gradually
shorten, becoming finally flat, and there results a squamous epithelium:
there is a near approach in minute composition to epidermis. Here a
tendency towards completion of the type cannot be alleged; for there is,
contrariwise, divergence from the type. The effect of the medium is so
great that, in a short time, it overcomes the inherited proclivity and
produces a structure of opposite kind to the normal one.
With but little break we come here upon a significant analogy, parallel
to an analogy already described. As was pointed out, an inorganic body
that is modifiable by its medium, acquires, after a time, an outer coat
which has already undergone such change as surrounding agencies can
effect; has a contained mass which is as yet unchanged, because
unreached; and has a surface between the two where change is going on—a
region of activity. And we saw that alike in the vegetal cell and the
animal cell there exist analogous distributions: of course with the
difference that the innermost part is not inert. Now we have to note
that in those aggregates of cells constituting the _Metaphyta_ and
_Metazoa_, analogous distributions also exist. In plants they are of
course not to be looked for in leaves and other deciduous portions, but
only in portions of long duration—stems and branches. Naturally, too, we
need not expect them in plants having modes of growth which early
produce an outer practically dead part, that effectually shields the
inner actively living part of the stem from the influence of the
medium—long-lived acrogens such as tree-ferns and long-lived endogens
such as palms. But in the highest plants, exogens, which have the
actively living part of their stems within reach of environing agencies,
we find this part,—the cambium layer,—is one from which there is a
growth inwards forming wood, and a growth outwards forming bark: there
is an increasingly thick covering (where it does not scale off) of
tissue changed by the medium, and inside this a film of highest
vitality. In so far as concerns the present argument, it is the same
with the _Metazoa_, or at least all of them which have developed
organizations. The outer skin grows up from a limiting plane, or layer,
a little distance below the surface—a place of predominant vital
activity. Here perpetually arise new cells, which, as they develop, are
thrust outwards and form the epidermis: flattening and drying up as they
approach the surface, whence, having for a time served to shield the
parts below, they finally scale off and leave younger ones to take their
places. This still undifferentiated tissue forming the base of the
epidermis, and existing also as a source of renewal in internal organs,
is the essentially living substance; and facts above given imply that it
was the action of the medium on this essentially living substance,
which, during early stages in the organization of the _Metazoa_,
initiated that protective envelope which presently became an inherited
structure—a structure which, though now mainly inherited, still
continues to be modifiable by its initiator.
Fully to perceive the way in which these evidences compel us to
recognize the influence of the medium as a primordial factor, we need
but conceive them as interpreted without it. Suppose, for instance, we
say that the structure of the epidermis is wholly determined by the
natural selection of favourable variations; what must be the position
taken in presence of the fact above named, that when mucous membrane is
exposed to the air its cell-structure changes into the cell-structure of
skin? The position taken must be this:—Though mucous membrane in a
highly-evolved individual organism, thus shows the powerful effect of
the medium on its surface; yet we must not suppose that the medium had
the effect of producing such a cell-structure on the surfaces of
primitive forms, undifferentiated though they were; or, if we suppose
that such an effect was produced on them, we must not suppose that it
was inheritable. Contrariwise, we must suppose that such effect of the
medium either was not wrought at all, or that it was evanescent: though
repeated through millions upon millions of generations it left no
traces. And we must conclude that this skin-structure arose only in
consequence of spontaneous variations not physically initiated (though
like those physically initiated) which natural selection laid hold of
and increased. Does any one think this a tenable position?
* * * * *
And now we approach the last and chief series of morphological phenomena
which must be ascribed to the direct action of environing matters and
forces. These are presented to us when we study the early stages in the
development of the embryos of the _Metazoa_ in general.
We will set out with the fact already noted in passing, that after
repeated spontaneous fissions have changed the original fertilized
germ-cell into that cluster of cells which forms a gemmule or a
primitive ovum, the first contrast which arises is between the
peripheral parts and the central parts. Where, as with lower creatures
which do not lay up large stores of nutriment with the germs of their
offspring, the inner mass is inconsiderable, the outer layer of cells,
which are presently made quite small by repeated subdivisions, forms a
membrane extending over the whole surface—the blastoderm. The next stage
of development, which ends in this covering layer becoming double, is
reached in two ways—by invagination and by delamination; but which is
the original way and which the abridged way, is not quite certain. Of
invagination, multitudinously exemplified in the lowest types, Mr.
Balfour says:—“On purely _à priori_ grounds there is in my opinion more
to be said for invagination than for any other view”;[17] and, for
present purposes, it will suffice if we limit ourselves to this: making
its nature clear to the general reader by a simple illustration.
Take a small india-rubber ball—not of the inflated kind, nor of the
solid kind, but of the kind about an inch or so in diameter with a small
hole through which, under pressure, the air escapes. Suppose that
instead of consisting of india-rubber its wall consists of small cells
made polyhedral in form by mutual pressure, and united together. This
will represent the blastoderm. Now with the finger, thrust in one side
of the ball until it touches the other: so making a cup. This action
will stand for the process of invagination. Imagine that by continuance
of it, the hemispherical cup becomes very much deepened and the opening
narrowed, until the cup becomes a sac, of which the introverted wall is
everywhere in contact with the outer wall. This will represent the
two-layered “gastrula”—the simplest ancestral form of the _Metazoa_: a
form which is permanently represented in some of the lowest types; for
it needs but tentacles round the mouth of the sac, to produce a common
hydra. Here the fact which it chiefly concerns us to remark, is that of
these two layers the outer, called in embryological language the
epiblast, continues to carry on direct converse with the forces and
matters in the environment; while the inner, called the hypoblast, comes
in contact with such only of these matters as are put into the
food-cavity which it lines. We have further to note that in the embryos
of _Metazoa_ at all advanced in organization, there arises between these
two layers a third—the mesoblast. The origin of this is seen in types
where the developmental process is not obscured by the presence of a
large food-yolk. While the above-described introversion is taking place,
and before the inner surfaces of the resulting epiblast and hypoblast
have come into contact, cells, or amœboid units equivalent to them, are
budded off from one or both of these inner surfaces, or some part of one
or other; and these form a layer which eventually lies between the other
two—a layer which, as this mode of formation implies, never has any
converse with the surrounding medium and its contents, or with the
nutritive bodies taken in from it. The striking facts to which this
description is a necessary introduction, may now be stated. From the
outer layer, or epiblast, are developed the permanent epidermis and its
out-growths, the nervous system, and the organs of sense. From the
introverted layer, or hypoblast, are developed the alimentary canal and
those parts of its appended organs, liver, pancreas, &c., which are
concerned in delivering their secretions into the alimentary canal, as
well as the linings of those ramifying tubes in the lungs which convey
air to the places where gaseous exchange is effected. And from the
mesoblast originate the bones, the muscles, the heart and blood-vessels,
and the lymphatics, together with such parts of various internal organs
as are most remotely concerned with the outer world. Minor
qualifications being admitted, there remain the broad general facts,
that out of that part of the external layer which remains permanently
external, are developed all the structures which carry on intercourse
with the medium and its contents, active and passive; out of the
introverted part of this external layer, are developed the structures
which carry on intercourse with the quasi-external substances that are
taken into the interior—solid food, water, and air; while out of the
mesoblast are developed structures which have never had, from first to
last, any intercourse with the environment. Let us contemplate these
general facts.
Who would have imagined that the nervous system is a modified portion of
the primitive epidermis? In the absence of proofs furnished by the
concurrent testimony of embryologists during the last thirty or forty
years, who would have believed that the brain arises from an unfolded
tract of the outer skin, which, sinking down beneath the surface,
becomes imbedded in other tissues and eventually surrounded by a bony
case? Yet the human nervous system in common with the nervous systems of
lower animals is thus originated. In the words of Mr. Balfour, early
embryological changes imply that—
“the functions of the central nervous system, which were originally
taken by the whole skin, became gradually concentrated in a special
part of the skin which was step by step removed from the surface,
and has finally become in the higher types a well-defined organ
imbedded in the subdermal tissues.... The embryological evidence
shows that the ganglion-cells of the central part of the nervous
system are originally derived from the simple undifferentiated
epithelial cells of the surface of the body.”[18]
Less startling perhaps, though still startling enough, is the fact that
the eye is evolved out of a portion of the skin; and that while the
crystalline lens and its surroundings thus originate, the “percipient
portions of the organs of special sense, especially of optic organs, are
often formed from the same part of the primitive epidermis” which forms
the central nervous system.[19] Similarly is it with the organs for
smelling and hearing. These, too, begin as sacs formed by in-foldings of
the epidermis; and while their parts are developing they are joined from
within by nervous structures which were themselves epidermic in origin.
How are we to interpret these strange transformations? Observing, as we
pass, how absurd from the point of view of the special-creationist,
would appear such a filiation of structures, and such a round-about mode
of embryonic development, we have here to remark that the process is not
one to have been anticipated as a result of natural selection. After
numbers of spontaneous variations had occurred, as the hypothesis
implies, in useless ways, the variation which primarily initiated a
nervous centre might reasonably have been expected to occur in some
internal part where it would be fitly located. Its initiation in a
dangerous place and subsequent migration to a safe place, would be
incomprehensible. Not so if we bear in mind the cardinal truth above set
forth, that the structures for holding converse with the medium and its
contents, arise in that completely superficial part which is directly
affected by the medium and its contents; and if we draw the inference
that the external actions themselves initiate the structures. These once
commenced, and furthered by natural selection where favourable to life,
would form the first term of a series ending in developed sense organs
and a developed nervous system.[20]
Though it would enforce the argument, I must, for brevity's sake, pass
over the analogous evolution of that introverted layer, or hypoblast,
out of which the alimentary canal and attached organs arise. It will
suffice to emphasize the fact that having been originally external, this
layer continues in its developed form to have a quasi-externality, alike
in its digesting part and in its respiratory part; since it continues to
deal with matters alien to the organism. I must also refrain from
dwelling at length on the fact already adverted to, that the
intermediate derived layer, or mesoblast, which was at the outset
completely internal, originates those structures which ever remain
completely internal, and have no communication with the environment save
through the structures developed from the other two: an antithesis which
has great significance.
Here, instead of dwelling on these details, it will be better to draw
attention to the most general aspect of the facts. Whatever may be the
course of subsequent changes, the first change is the formation of a
superficial layer or blastoderm; and by whatever series of
transformations the adult structure is reached, it is from the
blastoderm that all the organs forming the adult originate. Why this
marvellous fact?
Meaning is given to it if we go back to the first stage in which
_Protozoa_, having by repeated fissions formed a cluster, then arranged
themselves into a hollow sphere, as do the protophytes forming a
_Volvox_. Originally alike all over its surface, the hollow sphere of
ciliated units thus formed, would, if not quite spherical, assume a
constant attitude when moving through the water; and hence one part of
the spheroid would more frequently than the rest come in contact with
nutritive matters to be taken in. A division of labour resulting from
such a variation being advantageous, and tending therefore to increase
in descendants, would end in a differentiation like that shown in the
gemmules of various low types of _Metazoa_, which, ovate in shape, are
ciliated over one part of the surface only. There would arise a form in
which the cilium-bearing units effected locomotion and aeration; while
on the others, assuming an amœba-like character, devolved the function
of absorbing food: a primordial specialization variously indicated by
evidence.[21] Just noting that an ancestral origin of this kind is
implied by the fact that in low types of _Metazoa_ a hollow sphere of
cells is the form first assumed by the unfolding embryo, I draw
attention to the point here of chief interest; namely that the primary
differentiation of this hollow sphere is in such case determined by a
difference in the converse of its parts with the medium and its
contents; and that the subsequent invagination arises by a continuance
of this differential converse.