not fail to take account of some among these effects directly produced
on organisms by surrounding inorganic agencies. Here are extracts from
the sixth edition of the _Origin of Species_ showing this.
“It is very difficult to decide how far changed conditions, such as
of climate, food, &c., have acted in a definite manner. There is
reason to believe that in the course of time the effects have been
greater than can be proved by clear evidence.... Mr. Gould believes
that birds of the same species are more brightly coloured under a
clear atmosphere, than when living near the coast or on islands; and
Wollaston is convinced that residence near the sea affects the
colours of insects. Moquin-Tandon gives a list of plants which, when
growing near the sea-shore, have their leaves in some degree fleshy,
though not elsewhere fleshy” (pp. 106-7). “Some observers are
convinced that a damp climate affects the growth of the hair, and
that with the hair the horns are correlated” (p. 159).
In his subsequent work, _Animals and Plants under Domestication_, Mr.
Darwin still more clearly recognizes these causes of change in
organization. A chapter is devoted to the subject. After premising that
“the direct action of the conditions of life, whether leading to
definite or indefinite results, is a totally distinct consideration from
the effects of natural selection;” he goes on to say that changed
conditions of life “have acted so definitely and powerfully on the
organisation of our domesticated productions, that they have sufficed to
form new sub-varieties or races, without the aid of selection by man or
of natural selection.” Of his examples here are two.
“I have given in detail in the ninth chapter the most remarkable
case known to me, namely, that in Germany several varieties of maize
brought from the hotter parts of America were transformed in the
course of only two or three generations.” (Vol. ii, p. 277.) [And in
this ninth chapter concerning these and other such instances he says
“some of the foregoing differences would certainly be considered of
specific value with plants in a state of nature.” (Vol. i, p. 321.)]
“Mr. Meehan, in a remarkable paper, compares twenty-nine kinds of
American trees, belonging to various orders, with their nearest
European allies, all grown in close proximity in the same garden and
under as nearly as possible the same conditions.” And then
enumerating six traits in which the American forms all of them
differ in like ways from their allied European forms, Mr. Darwin
thinks there is no choice but to conclude that these “have been
definitely caused by the long-continued action of the different
climate of the two continents on the trees.” (Vol. ii, pp. 281-2.)
But the fact we have to note is that while Mr. Darwin thus took account
of special effects due to special amounts and combinations of agencies
in the environment, he did not take account of the far more important
effects due to the general and constant operation of these agencies.[3]
If a difference between the quantities of a force which acts on two
organisms, otherwise alike and otherwise similarly conditioned, produces
some difference between them; then, by implication, this force produces
in both of them effects which they show in common. The inequality
between two things cannot have a value unless the things themselves have
values. Similarly if, in two cases, some unlikeness of proportion among
the surrounding inorganic agencies to which two plants or two animals
are exposed, is followed by some unlikeness in the changes wrought on
them; then it follows that these several agencies taken separately, work
changes in both of them. Hence we must infer that organisms have certain
structural characters in common, which are consequent on the action of
the medium in which they exist: using the word medium in a comprehensive
sense, as including all physical forces falling upon them as well as
matters bathing them. And we may conclude that from the primary
characters thus produced there must result secondary characters.
Before going on to observe those general traits of organisms due to the
general action of the inorganic environment upon them, I feel tempted to
enlarge on the effects produced by each of the several matters and
forces constituting the environment. I should like to do this not only
to give a clear preliminary conception of the ways in which all
organisms are affected by these universally-present agents, but also to
show that, in the first place, these agents modify inorganic bodies as
well as organic bodies, and that, in the second place, the organic are
far more modifiable by them than the inorganic. But to avoid undue
suspension of the argument, I content myself with saying that when the
respective effects of gravitation, heat, light, &c, are studied, as well
as the respective effects, physical and chemical, of the matters forming
the media, water and air, it will be found that while more or less
operative on all bodies, each modifies organic bodies to an extent
immensely greater than the extent to which it modifies inorganic bodies.
* * * * *
Here, not discriminating among the special effects which these various
forces and matters in the environment produce on both classes of bodies,
let us consider their combined effects, and ask—What is the most general
trait of such effects?
Obviously the most general trait is the greater amount of change wrought
on the outer surface than on the inner mass. In so far as the matters of
which the medium is composed come into play, the unavoidable implication
is that they act more on the parts directly exposed to them than on the
parts sheltered from them. And in so far as the forces pervading the
medium come into play, it is manifest that, excluding gravity, which
affects outer and inner parts indiscriminately, the outer parts have to
bear larger shares of their actions. If it is a question of heat, then
the exterior must lose it or gain it faster than the interior; and in a
medium which is now warmer and now colder, the two must habitually
differ in temperature to some extent—at least where the size is
considerable. If it is a question of light, then in all but absolutely
transparent masses, the outer parts must undergo more of any change
producible by it than the inner parts—supposing other things equal; by
which I mean, supposing the case is not complicated by any such
convexities of the outer surface as produce internal concentrations of
rays. Hence then, speaking generally, the necessity is that the primary
and almost universal effect of the converse between the body and its
medium, is to differentiate its outside from its inside. I say almost
universal, because where the body is both mechanically and chemically
stable, like, for instance, a quartz crystal, the medium may fail to
work either inner or outer change.
Of illustrations among inorganic bodies, a convenient one is supplied by
an old cannon-ball that has been long lying exposed. A coating of rust,
formed of flakes within flakes, incloses it; and this thickens year by
year, until, perhaps, it reaches a stage at which its exterior loses as
much by rain and wind as its interior gains by further oxidation of the
iron. Most mineral masses—pebbles, boulders, rocks—if they show any
effect of the environment at all, show it only by that disintegration of
surface which follows the freezing of absorbed water: an effect which,
though mechanical rather than chemical, equally illustrates the general
truth. Occasionally a “rocking-stone” is thus produced. There are formed
successive layers relatively friable in texture, each of which, thickest
at the most exposed parts, and being presently lost by weathering,
leaves the contained mass in a shape more rounded than before; until,
resting on its convex under-surface, it is easily moved. But of all
instances perhaps the most remarkable is one to be seen on the west bank
of the Nile at Philæ, where a ridge of granite 100 feet high, has had
its outer parts reduced in course of time to a collection of
boulder-shaped masses, varying from say a yard in diameter to six or
eight feet, each one of which shows in progress an exfoliation of
successively-formed shells of decomposed granite: most of the masses
having portions of such shells partially detached.
If, now, inorganic masses, relatively so stable in composition, thus
have their outer parts differentiated from their inner parts, what must
we say of organic masses, characterized by such extreme chemical
instability?—instability so great that their essential material is named
protein, to indicate the readiness with which it passes from one
isomeric form to another. Clearly the necessary inference is that this
effect of the medium must be wrought inevitably and promptly, wherever
the relation of outer and inner has become settled: a qualification for
which the need will be seen hereafter.
* * * * *
Beginning with the earliest and most minute kinds of living things, we
necessarily encounter difficulties in getting direct evidence; since, of
the countless species now existing, all have been subject during
millions upon millions of years to the evolutionary process, and have
had their primary traits complicated and obscured by those endless
secondary traits which the natural selection of favourable variations
has produced. Among protophytes it needs but to think of the
multitudinous varieties of diatoms and desmids, with their
elaborately-constructed coverings; or of the definite methods of growth
and multiplication among such simple _Algæ_ as the _Conjugatæ_; to see
that most of their distinctive characters are due to inherited
constitutions, which have been slowly moulded by survival of the fittest
to this or that mode of life. To disentangle such parts of their
developmental changes as are due to the action of the medium, is
therefore hardly possible. We can hope only to get a general conception
of it by contemplating the totality of the facts.
The first cardinal fact is that all protophytes are cellular—all show us
this contrast between outside and inside. Supposing the multitudinous
specialities of the envelope in different orders and genera of
protophytes to be set against one another, and mutually cancelled, there
remains as a trait common to them—an envelope unlike that which it
envelopes. The second cardinal fact is that this simple trait is the
earliest trait displayed in germs, or spores, or other parts from which
new individuals are to arise; and that, consequently, this trait must be
regarded as having been primordial. For it is an established truth of
organic evolution that embryos show us, in general ways, the forms of
remote ancestors; and that the first changes undergone, indicate, more
or less clearly, the first changes which took place in the series of
forms through which the existing form has been reached. Describing, in
successive groups of plants, the early transformations of these
primitive units, Sachs[4] says of the lowest Algæ that “the conjugated
protoplasmic body clothes itself with a cell-wall” (p. 10); that in “the
spores of Mosses and Vascular Cryptogams” and in “the pollen of
Phanerogams” ... “the protoplasmic body of the mother-cell breaks up
into four lumps, which quickly round themselves off and contract, and
become enveloped by a cell-membrane only after complete separation” (p.
13); that in the _Equisetaceæ_ “the young spores, when first separated,
are still naked, but they soon become surrounded by a cell-membrane” (p.
14); and that in higher plants, as in the pollen of many Dicotyledons,
“the contracting daughter-cells secrete cellulose even during their
separation” (p. 14). Here, then, in whatever way we interpret it, the
fact is that there quickly arises an outer layer different from the
contained matter. But the most significant evidence is furnished by “the
masses of protoplasm that escape into water from the injured sacs of
_Vaucheria_, which often instantly become rounded into globular bodies,”
and of which the “hyaline protoplasm envelopes the whole as a skin” (p.
41) which “is denser than the inner and more watery substance” (p. 42).
As in this case the protoplasm is but a fragment, and as it is removed
from the influence of the parent-cell, this differentiating process can
scarcely be regarded as anything more than the effect of
physico-chemical actions: a conclusion which is supported by the
statement of Sachs that “not only every vacuole in a solid protoplasmic
body, but also every thread of protoplasm which penetrates the
sap-cavity, and finally the inner side of the protoplasm-sac which
encloses the sap-cavity, is also bounded by a skin” (p. 42). If then
“every portion of a protoplasmic body immediately surrounds itself, when
it becomes isolated, with such a skin,” which is shown in all cases to
arise at the surface of contact with sap or water, this primary
differentiation of outer from inner must be ascribed to the direct
action of the medium. Whether the coating thus initiated is secreted by
the protoplasm, or whether, as seems more likely, it results from
transformation of it, matters not to the argument. Either way the action
of the medium causes its formation; and either way the many varied and
complex differentiations which developed cell-walls display, must be
considered as originating from those variations of this
physically-generated covering which natural selection has taken
advantage of.
The contained protoplasm of a vegetal cell, which has self-mobility and
when liberated sometimes performs amœba-like motions for a time, may be
regarded as an imprisoned amœba; and when we pass from it to a free
amœba, which is one of the simplest types of first animals, or
_Protozoa_, we naturally meet with kindred phenomena. The general trait
which here concerns us, is that while its plastic or semi-fluid sarcode
goes on protruding, in irregular ways, now this and now that part of its
periphery, and again withdrawing into its interior first one and then
another of these temporary processes, perhaps with some small portion of
food attached, there is but an indistinct differentiation of outer from
inner (a fact shown by the frequent coalescence of the pseudopodia in
Rhizopods); but that when it eventually becomes quiescent, the surface
becomes differentiated from the contents: the passing into an encysted
state, doubtless in large measure due to inherited proclivity, being
furthered, and having probably been once initiated, by the action of the
medium. The connexion between constancy of relative position among the
parts of the sarcode, and the rise of a contrast between superficial and
central parts, is perhaps best shown in the minutest and simplest
_Infusoria_, the _Monadinæ_. The genus _Monas_ is described by Kent as
“plastic and unstable in form, possessing no distinct cuticular
investment; ... the food-substances incepted at all parts of the
periphery”;[5] and the genus _Scytomonas_ he says “differs from _Monas_
only in its persistent shape and accompanying greater rigidity of the
peripheral or ectoplasmic layer.”[6] Describing generally such low
forms, some of which are said to have neither nucleus nor vacuole, he
remarks that in types somewhat higher “the outer or peripheral border of
the protoplasmic mass, while not assuming the character of a distinct
cell-wall or so-called cuticle, presents, as compared with the inner
substance of that mass, a slightly more solid type of composition.”[7]
And it is added that these forms having so slightly differentiated an
exterior, “while usually exhibiting a more or less characteristic normal
outline, can revert at will to a pseud-amœboid and repent state.”[8]
Here, then, we have several indications of the truth that the permanent
externality of a certain part of the substance, is followed by
transformation of it into a coating unlike the substance it contains.
Indefinite and structureless in the simplest of these forms, as instance
again the _Gregarina_,[9] the limiting membrane becomes, in higher
_Infusoria_, definite and often complex: showing that the selection of
favourable variations has had largely to do with its formation. In such
types as the _Foraminifera_, which, almost structureless internally
though they are, secrete calcareous shells, it is clear that the nature
of this outer layer is determined by inherited constitution. But
recognition of this consists with the belief that the action of the
medium initiated the outer layer, specialized though it now is; and that
even still, contact with the medium excites secretion of it.
A remarkable analogy remains to be named. When we study the action of
the medium in an inorganic mass, we are led to see that between the
outer changed layer and the inner unchanged mass, comes a surface where
active change is going on. Here we have to note that, alike in the
plant-cell and in the animal-cell, there is a similar relation of parts.
Immediately inside the envelope comes the primordial utricle in the one
case, and in the other case the layer of active sarcode. In either case
the living protoplasm, placed in the position of a lining to the cuticle
of the cell, is shielded from the direct action of the medium, and yet
is not beyond the reach of its influences.
* * * * *
Limited, as thus far drawn, to a certain common trait of those minute
organisms which are mostly below the reach of unaided vision, the
foregoing conclusion appears trivial enough. But it ceases to appear
trivial on passing into a wider field, and observing the implications,
direct and indirect, as they concern plants and animals of sensible
sizes.
Popular expositions of science have so far familiarized many readers
with a certain fundamental trait of living things around, that they have
ceased to perceive how marvellous a trait it is, and, until interpreted
by the Theory of Evolution, how utterly mysterious. In past times, the
conception of an ordinary plant or animal which prevailed, not
throughout the world at large only but among the most instructed, was
that it is a single continuous entity. One of these living things was
unhesitatingly regarded as being in all respects a unit. Parts it might
have, various in their sizes, forms, and compositions; but these were
components of a whole which had been from the beginning in its original
nature a whole. Even to naturalists fifty years ago, the assertion that
a cabbage or a cow, though in one sense a whole, is in another sense a
vast society of minute individuals, severally living in greater or less
degrees, and some of them maintaining their independent lives
unrestrained, would have seemed an absurdity. But this truth which, like
so many of the truths established by science, is contrary to that common
sense in which most people have so much confidence, has been gradually
growing clear since the days when Leeuwenhoek and his contemporaries
began to examine through lenses the minute structures of common plants
and animals. Each improvement in the microscope, while it has widened
our knowledge of those minute forms of life described above, has
revealed further evidence of the fact that all the larger forms of life
consist of units severally allied in their fundamental traits to these
minute forms of life. Though, as formulated by Schwann and Schleiden,
the cell-doctrine has undergone qualifications of statement; yet the
qualifications have not been such as to militate against the general
proposition that organisms visible to the naked eye, are severally
compounded of invisible organisms—using that word in its most
comprehensive sense. And then, when the development of any animal is
traced, it is found that having been primarily a nucleated cell, and
having afterwards become by spontaneous fission a cluster of nucleated
cells, it goes on through successive stages to form out of such cells,
ever multiplying and modifying in various ways, the several tissues and
organs composing the adult.
On the hypothesis of evolution this universal trait has to be accepted
not as a fact that is strange but unmeaning. It has to be accepted as
evidence that all the visible forms of life have arisen by union of the
invisible forms; which, instead of flying apart when they divided,
remained together. Various intermediate stages are known. Among plants,
those of the _Volvox_ type show us the component protophytes so feebly
combined that they severally carry on their lives with no appreciable
subordination to the life of the group. And among animals, a parallel
relation between the lives of the units and the life of the group is
shown us in _Uroglena_ and _Syncrypta_. From these first stages upwards,
may be traced through successively higher types, an increasing
subordination of the units to the aggregate; though still a
subordination leaving to them conspicuous amounts of individual
activity. Joining which facts with the phenomena presented by the
cell-multiplication and aggregation of every unfolding germ, naturalists
are now accepting the conclusion that by this process of composition
from _Protozoa_, were formed all classes of the _Metazoa_[10]—(as
animals formed by this compounding are now called); and that in a
similar way from _Protophyta_, were formed all classes of what I suppose
will be called _Metaphyta_, though the word does not yet seem to have
become current.
And now what is the general meaning of these truths, taken in connexion
with the conclusion reached in the last section. It is that this
universal trait of the _Metazoa_ and _Metaphyta_, must be ascribed to
the primitive action and re-action between the organism and its medium.
The operation of those forces which produced the primary differentiation
of outer from inner in early minute masses of protoplasm, pre-determined
this universal cell-structure of all embryos, plant and animal, and the
consequent cell-composition of adult forms arising from them. How
unavoidable is this implication, will be seen on carrying further an
illustration already used—that of the shingle-covered shore, the pebbles
on which, while being in some cases selected, have been in all cases
rounded and smoothed. Suppose a bed of such shingle to be, as we often
see it, solidified, along with interfused material, into a conglomerate.
What in such case must be considered as the chief trait of such
conglomerate; or rather—what must we regard as the chief cause of its
distinctive characters? Evidently the action of the sea. Without the
breakers, no pebbles; without the pebbles, no conglomerate. Similarly
then, in the absence of that action of the medium by which was effected
the differentiation of outer from inner in those microscopic portions of
protoplasm constituting the earliest and simplest animals and plants,
there could not have existed this cardinal trait of composition which
all the higher animals and plants show us.
So that, active as has been the part played by natural selection,
alike in modifying and moulding the original units—largely as
survival of the fittest has been instrumental in furthering and
controlling the combination of these units into visible organisms,
and eventually into large ones; yet we must ascribe to the direct
effect of the medium on the first forms of life, that character of
which this everywhere-operative factor has taken advantage.
* * * * *
Let us turn now to another and more obvious attribute of higher
organisms, for which also there is this same general cause. Let us
observe how, on a higher platform, there recurs this differentiation of
outer from inner—how this primary trait in the living units with which
life commences, re-appears as a primary trait in those aggregates of
such units which constitute visible organisms.