a “gastrula” has been produced by the permanent introversion of one
portion of the surface of the hollow sphere, it will suffice if we
consider what must thereafter have happened. That which continued to be
the outer surface was the part which from time to time touched quiescent
masses and occasionally received the collisions consequent on its own
motions or the motions of other things. It was the part to receive the
sound-vibrations occasionally propagated through the water; the part to
be affected more strongly than any other by those variations in the
amounts of light caused by the passing of small bodies close to it; and
the part which met those diffused molecules constituting odours. That is
to say, from the beginning the surface was the part on which there fell
the various influences pervading the environment, the part by which
there was received those impressions from the environment serving for
the guidance of actions, and the part which had to bear the mechanical
re-actions consequent upon such actions. Necessarily, therefore, the
surface was the part in which were initiated the various
instrumentalities for carrying on intercourse with the environment. To
suppose otherwise is to suppose that such instrumentalities arose
internally where they could neither be operated on by surrounding
agencies nor operate on them,—where the differentiating forces did not
come into play, and the differentiated structures had nothing to do; and
it is to suppose that meanwhile the parts directly exposed to the
differentiating forces remained unchanged. Clearly, then, organization
could not but begin on the surface; and having thus begun, its
subsequent course could not but be determined by its superficial origin.
And hence these remarkable facts showing us that individual evolution is
accomplished by successive in-foldings and in-growings. Doubtless
natural selection soon came into action, as, for example, in the removal
of the rudimentary nervous centres from the surface; since an individual
in which they were a little more deeply seated would be less likely to
be incapacitated by injury of them. And so in multitudinous other ways.
But nevertheless, as we here see, natural selection could operate only
under subjection. It could do no more than take advantage of those
structural changes which the medium and its contents initiated.
See, then, how large has been the part played by this primordial factor.
Had it done no more than give to _Protozoa_ and _Protophyta_ that
cell-form which characterizes them—had it done no more than entail the
cellular composition which is so remarkable a trait of _Metazoa_ and
_Metaphyta_—had it done no more than cause the repetition in all visible
animals and plants of that primary differentiation of outer from inner
which it first wrought in animals and plants invisible to the naked eye;
it would have done much towards giving to organisms of all kinds certain
leading traits. But it has done more than this. By causing the first
differentiations of those clusters of units out of which visible animals
in general arose, it fixed the starting place for organization, and
therefore determined the course of organization; and, doing this, gave
indelible traits to embryonic transformations and to adult structures.
* * * * *
Though mainly carried on after the inductive method, the argument at the
close of the foregoing section has passed into the deductive. Here let
us follow for a space the deductive method pure and simple. Doubtless in
biology _à priori_ reasoning is dangerous; but there can be no danger in
considering whether its results coincide with those reached by reasoning
_à posteriori_.
Biologists in general agree that in the present state of the world, no
such thing happens as the rise of a living creature out of non-living
matter. They do not deny, however, that at a remote period in the past,
when the temperature of the Earth's surface was much higher than at
present, and other physical conditions were unlike those we know,
inorganic matter, through successive complications, gave origin to
organic matter. So many substances once supposed to belong exclusively
to living bodies, have now been formed artificially, that men of science
scarcely question the conclusion that there are conditions under which,
by yet another step of composition, quaternary compounds of lower types
pass into those of highest types. That there once took place gradual
divergence of the organic from the inorganic, is, indeed, a necessary
implication of the hypothesis of Evolution, taken as a whole; and if we
accept it as a whole, we must put to ourselves the question—What were
the early stages of progress which followed, after the most complex form
of matter had arisen out of forms of matter a degree less complex?
At first, protoplasm could have had no proclivities to one or other
arrangement of parts; unless, indeed, a purely mechanical proclivity
towards a spherical form when suspended in a liquid. At the outset it
must have been passive. In respect of its passivity, primitive organic
matter must have been like inorganic matter. No such thing as
spontaneous variation could have occurred in it; for variation implies
some habitual course of change from which it is a divergence, and is
therefore excluded where there is no habitual course of change. In the
absence of that cyclical series of metamorphoses which even the simplest
living thing now shows us, as a result of its inherited constitution,
there could be no _point d'appui_ for natural selection. How, then, did
organic evolution begin?
If a primitive mass of organic matter was like a mass of inorganic
matter in respect of its passivity, and differed only in respect of its
greater changeableness; then we must infer that its first changes
conformed to the same general law as do the changes of an inorganic
mass. The instability of the homogeneous is a universal principle. In
all cases the homogeneous tends to pass into the heterogeneous, and the
less heterogeneous into the more heterogeneous. In the primordial units
of protoplasm, then, the step with which evolution commenced must have
been the passage from a state of complete likeness throughout the mass
to a state in which there existed some unlikeness. Further, the cause of
this step in one of these portions of organic matter, as in any portion
of inorganic matter, must have been the different exposure of its parts
to incident forces. What incident forces? Those of its medium or
environment. Which were the parts thus differently exposed? Necessarily
the outside and the inside. Inevitably, then, alike in the organic
aggregate and the inorganic aggregate (supposing it to have coherence
enough to maintain constant relative positions among its parts), the
first fall from homogeneity to heterogeneity must always have been the
differentiation of the external surface from the internal contents. No
matter whether the modification was physical or chemical, one of
composition or of decomposition, it comes within the same
generalization. The direct action of the medium was the primordial
factor of organic evolution.
* * * * *
And now, finally, let us look at the factors in their _ensemble_, and
consider the respective parts they play: observing, especially, the ways
in which, at successive stages, they severally give place one to another
in degree of importance.
Acting alone, the primordial factor must have initiated the primary
differentiation in all units of protoplasm alike. I say alike, but I
must forthwith qualify the word. For since surrounding influences,
physical and chemical, could not be absolutely the same in all places,
especially when the first rudiments of living things had spread over a
considerable area, there necessarily arose small contrasts between the
degrees and kinds of superficial differentiation effected. As soon as
these became decided, natural selection came into play; for inevitably
the unlikenesses produced among the units had effects on their lives:
there was survival of some among the modified forms rather than others.
Utterly in the dark though we are respecting the causes which set up
that process of fission everywhere occurring among the minutest forms of
life, we must infer that, when established, it furthered the spread of
those which were most favourably differentiated by the medium. Though
natural selection must have become increasingly active when once it had
got a start; yet the differentiating action of the medium never ceased
to be a co-operator in the development of these first animals and
plants. Again taking the lead as there arose the composite forms of
animals and plants, and again losing the lead with that advancing
differentiation of these higher types which gave more scope to natural
selection, it nevertheless continued, and must ever continue, to be a
cause, both direct and indirect, of modifications in structure.
Along with that remarkable process which, beginning in minute forms with
what is called conjugation, developed into sexual generation, there came
into play causes of frequent and marked fortuitous variations. The
mixtures of constitutional proclivities made more or less unlike by
unlikenesses of physical conditions, inevitably led to occasional
concurrences of forces producing deviations of structure. These were of
course mostly suppressed, but sometimes increased, by survival of the
fittest. When, along with the growing multiplication in forms of life,
conflict and competition became continually more active, fortuitous
variations of structure of no account in the converse with the medium,
became of much account in the struggle with enemies and competitors; and
natural selection of such variations became the predominant factor.
Especially throughout the plant-world its action appears to have been
immensely the most important; and throughout that large part of the
animal world characterized by relative inactivity, the survival of
individuals that had varied in favourable ways, must all along have been
the chief cause of the divergence of species and the occasional
production of higher ones.
But gradually with that increase of activity which we see on ascending
to successively higher grades of animals, and especially with that
increased complexity of life which we also see, there came more and more
into play as a factor, the inheritance of those modifications of
structure caused by modifications of function. Eventually, among
creatures of high organization, this factor became an important one; and
I think there is reason to conclude that, in the case of the highest of
creatures, civilized men, among whom the kinds of variation which affect
survival are too multitudinous to permit easy selection of any one, and
among whom survival of the fittest is greatly interfered with, it has
become the chief factor: such aid as survival of the fittest gives,
being usually limited to the preservation of those in whom the totality
of the faculties has been most favourably moulded by functional changes.
Of course this sketch of the relations among the factors must be taken
as in large measure a speculation. We are now too far removed from the
beginnings of life to obtain data for anything more than tentative
conclusions respecting its earliest stages; especially in the absence of
any clue to the mode in which multiplication, first agamogenetic and
then gamogenetic, was initiated. But it has seemed to me not amiss to
present this general conception, by way of showing how the deductive
interpretation harmonizes with the several inferences reached by
induction.
* * * * *
In his article on Evolution in the _Encyclopædia Britannica_, Professor
Huxley writes as follows:—
“How far 'natural selection' suffices for the production of species
remains to be seen. Few can doubt that, if not the whole cause, it
is a very important factor in that operation.... On the evidence of
palaeontology, the evolution of many existing forms of animal life
from their predecessors is no longer an hypothesis, but an
historical fact; it is only the nature of the physiological factors
to which that evolution is due which is still open to discussion.”
With these passages I may fitly join a remark made in the admirable
address Prof. Huxley delivered before unveiling the statue of Mr. Darwin
in the Museum at South Kensington. Deprecating the supposition that an
authoritative sanction was given by the ceremony to the current ideas
concerning organic evolution, he said that “science commits suicide when
it adopts a creed.”
Along with larger motives, one motive which has joined in prompting the
foregoing articles, has been the desire to point out that already among
biologists, the beliefs concerning the origin of species have assumed
too much the character of a creed; and that while becoming settled they
have been narrowed. So far from further broadening that broader view
which Mr. Darwin reached as he grew older, his followers appear to have
retrograded towards a more restricted view than he ever expressed. Thus
there seems occasion for recognizing the warning uttered by Prof.
Huxley, as not uncalled for.
Whatever may be thought of the arguments and conclusions set forth in
this article and the preceding one, they will perhaps serve to show that
it is as yet far too soon to close the inquiry concerning the causes of
organic evolution.
-----
Footnote 2:
Though Mr. Darwin approved of this expression and occasionally
employed it, he did not adopt it for general use; contending, very
truly, that the expression Natural Selection is in some cases more
convenient. See _Animals and Plants under Domestication_ (first
edition) Vol. i, p. 6; and _Origin of Species_ (sixth edition) p. 49.
Footnote 3:
It is true that while not deliberately admitted by Mr. Darwin, these
effects are not denied by him. In his _Animals and Plants under
Domestication_ (vol. ii, 281), he refers to certain chapters in the
_Principles of Biology_, in which I have discussed this general
inter-action of the medium and the organism, and ascribed certain most
general traits to it. But though, by his expressions, he implies a
sympathetic attention to the argument, he does not in such way adopt
the conclusion as to assign to this factor any share in the genesis of
organic structures—much less that large share which I believe it has
had. I did not myself at that time, nor indeed until quite recently,
see how extensive and profound have been the influences on
organization which, as we shall presently see, are traceable to the
early results of this fundamental relation between organism and
medium. I may add that it is in an essay on “Transcendental
Physiology,” first published in 1857, that the line of thought here
followed out in its wider bearings, was first entered upon.
Footnote 4:
_Text-Book of Botany_, &c. by Julius Sachs. Translated by A. W.
Bennett and W. T. T. Dyer.
Footnote 5:
_A Manual of the Infusoria_, by W. Saville Kent. Vol. i, p. 232.
Footnote 6:
_Ib._ Vol. i, p. 241.
Footnote 7:
Kent, Vol. i, p. 56.
Footnote 8:
_Ib._ Vol. i, p. 57.
Footnote 9:
_The Elements of Comparative Anatomy_, by T. H. Huxley, pp. 7-9.
Footnote 10:
_A Treatise on Comparative Embryology_, by F. M. Balfour, Vol. ii,
chap. xiii.
Footnote 11:
Sachs, p. 210.
Footnote 12:
_Ibid._ pp. 83-4.
Footnote 13:
_Ibid._ p. 185.
Footnote 14:
_Ibid._ p. 80.
Footnote 15:
Sachs, p. 83.
Footnote 16:
_Ibid._ p. 147.
Footnote 17:
_A Treatise on Comparative Embryology._ By Francis M. Balfour, LL.D.,
F.R.S. Vol. ii, p. 343 (second edition).
Footnote 18:
Balfour, l.c. Vol. ii, 400-1.
Footnote 19:
Balfour, l.c. Vol. ii, p. 401.
Footnote 20:
For a general delineation of the changes by which the development is
effected, see Balfour, l.c. Vol. ii, pp. 401-4.
Footnote 21:
_See_ Balfour, Vol. i, 149 and Vol. ii, 313-4.
NOTE.
After the above articles were published, I received from Dr. Downes a
copy of a paper “On the Influence of Light on Protoplasm,” written by
himself and Mr. T. P. Blunt, M.A., which was communicated to the Royal
Society in 1878. It was a continuation of a preceding paper which,
referring chiefly to _Bacteria_, contended that—
“Light is inimical to, and under favourable conditions may wholly
prevent, the development of these organisms.”
This supplementary paper goes on to show that the injurious effect of
light upon protoplasm results only in presence of oxygen. Taking first a
comparatively simple type of molecule which enters into the composition
of organic matter, the authors say, after detailing experiments:—
“It was evident, therefore, that oxygen was the agent of destruction
under the influence of sunlight.”
And accounts of experiments upon minute organisms are followed by the
sentence—
“It seemed, therefore, that in absence of an atmosphere, light
failed entirely to produce any effect on such organisms as were able
to appear.”
They sum up the results of their experiments in the paragraph—
“We conclude, therefore, both from analogy and from direct
experiment, that the observed action on these organisms is not
dependent on light per se, but that the presence of free oxygen is
necessary; light and oxygen together accomplishing what neither can
do alone: and the inference seems irresistible that the effect
produced is a gradual oxidation of the constituent protoplasm of
these organisms, and that, in this respect, protoplasm, although
living, is not exempt from laws which appear to govern the relations
of light and oxygen to forms of matter less highly endowed. A force
which is indirectly absolutely essential to life as we know it, and
matter in the absence of which life has not yet been proved to
exist, here unite for its destruction.”
What is the obvious implication? If oxygen in presence of light destroys
one of these minutest portions of protoplasm, what will be its effect on
a larger portion of protoplasm? It will work an effect on the surface
instead of on the whole mass. Not like the minutest mass made inert all
through, the larger mass will be made inert only on its outside; and,
indeed, the like will happen with the minutest mass if the light or the
oxygen is very small in quantity. Hence there will result an envelope of
changed matter, inclosing and protecting the unchanged protoplasm—there
will result a rudimentary cell-wall.
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TRANSCRIBER'S NOTES
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2. Silently corrected simple spelling, grammar, and typographical
errors.
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