arisen through spontaneous generation must be constituted, and on the
other hand, in regard to what kinds of living forms we can maintain
with certainty that they _could not_ have arisen thus, it is obvious
that we must place on the latter list all organisms which presuppose
the existence of others, from which they have been derived. But to
this category belong all the organisms which possess a germ-plasm,
an idioplasm that we conceive of as composed of primary constituents
(_Anlagen_) which have gradually been evolved and accumulated through
a long series of ancestors. Thus not only all multicellular animals
and plants which reproduce by means of germ-cells, buds, and so forth,
but also all unicellular organisms, must be placed in this class. For
these last--as we have seen--possess in their nucleus a substance made
up of primary constituents, without which the mutilated body is unable
to make good its loss, in short, an idioplasm. That this plays the same
rôle in unicellular as in multicellular organisms we can infer with
the greatest certainty from the process of amphimixis, which runs its
course in an analogous way in both cases.
Thus, even though we did not know what Ehrenberg demonstrated in the
third decade of last century, that Infusorians in an encapsuled state
can be blown about everywhere, and can even be carried across the
ocean in the dust of the trade-winds, to re-awaken to life wherever
they fall into fresh water, we should still not have remained at the
standpoint of Leuwenhoek, who regarded Infusorians as having arisen
through spontaneous generation. They cannot arise in this way, nor can
they have done so at any time, because they contain a substance made
up of primary constituents, which can only be of historic origin, and
cannot therefore have arisen suddenly after the manner of a chemical
combination.
The same is true of all the unicellular organisms, even of those
which are much more simple in structure than the Infusorians, whose
differentiation into cortical and medullary substances, oral and anal
openings, complex arrangements of cilia and much else, betokens a high
degree of differentiation in the cell. But even the Amœba is only
apparently simple, for otherwise it could not send out processes and
retract them again, creep in a particular direction, encyst itself,
and so on, for all this presupposes a differentiation of its particles
in different directions, and a definite arrangement of them; and
there is in addition the marvellous dividing-apparatus of the nucleus
which is not wanting even in the Amœba. All this again points to a
historic evolution, a gradual acquiring and an orderly arrangement of
differentiations, and such an organism cannot have arisen suddenly like
a crystal or a chemical combination.
Thus we are driven back to the lowest known organisms, and the question
now before us is whether these smallest living organisms, which are
only visible under the highest powers of the microscope, may be
referred to spontaneous generation. But here too the answer is, No; for
although there is no nucleus to be found, and no substance which we
can affirm with any certainty to be composed of primary constituents
or idioplasm, we do find distinct traces of a previous history, and
not the absolutely simple structure of homogeneous living particles,
unarranged in any orderly way, which is all that could be derived from
spontaneous generation. It has been shown quite recently that the
typhus bacillus possesses an extremely delicate much-branched tuft
of flagella, which gives it a tremulous motion, and in the cholera
bacillus cortical and medullary substances can be distinguished. Thus
even here there is differentiation according to the principle of
division of labour, and how numerous must be the minute vital particles
of which a substance consists when it can form such fine threads as the
flagella just mentioned! Nägeli, who elaborated an analogous train of
thought in regard to spontaneous generation, calculated the number of
these smallest vital particles (his 'micellæ') which must be contained
in a 'moneron' of 0.6 mm. diameter, if we take its dry substance at
10 per cent., and he arrived at the amazing figure of 100 billions of
vital particles. Even if we suppose the diameter of such an organism
to be 0.0006 mm., it would still be composed, according to this
calculation, of a million of these vital particles.
We have reached, in the course of these lectures, the conviction
that minute living units form the basis of all organisms, namely,
our 'life-bearers' or 'biophors.' These must be present in countless
multitudes, and in a great number of varieties in the different forms
of life, but all agree in this, that they are simple, that is, they are
not composed in their turn of living particles, but only of molecules,
whose chemical constitution, combination, and arrangement are such as
to give rise to the phenomena of life. But they may vary, and on this
power depends the possibility of their differentiation, which has
taken place in more and more diverse ways in the course of phylogeny.
They, too, arise in the existing organism, like all vital units, only
by multiplication of the biophors already present, but they do not
necessarily presuppose a historic origin; it is conceivable of them,
at least as far as their first and simplest forms are concerned, that
they may have arisen some time or other through spontaneous generation.
In regard to them alone is the possibility of origin through purely
chemico-physical causes, without the co-operation of life already
existing, admissible. It is only in regard to them that spontaneous
generation is not inconceivable.
We must, therefore, assume that, at some time or other in the history
of the earth, the conditions necessary to the development of these
invisible little living particles must have existed, and that the whole
subsequent development of the organic world must have depended upon an
aggregation of these biophors into larger complexes, and upon their
differentiation within these complexes.
We shall never be able, then, directly to observe spontaneous
generation, for the simple reason that the smallest and lowest living
particles which could arise through it, the Biophoridæ, are so
extremely far below the limits of visibility, that there is no hope
of our ever being able to perceive them, even if we should succeed in
producing them by spontaneous generation.
I do not propose to discuss the chemical problem raised by the possible
occurrence of spontaneous generation. We have already seen that dead
protoplasm, in addition to water, salts, phosphorus, sulphur, and some
other elements, chiefly and invariably contains albumen; an albuminoid
substance must, therefore, have arisen from inorganic combinations.
No one will maintain that this is impossible, for we continually see
albuminoid substances produced in plants from inorganic substances,
compounds of carbon and nitrogen; but under what conditions this would
be possible in free nature, that is, outside of organisms, cannot as
yet be determined. Possibly we may some time succeed in procuring
albumen from inorganic substances in the laboratory, and if that
happens the theory of spontaneous generation will rest upon a firmer
basis, but it will not have been experimentally proved even then.
For while dead albumen is certainly nearly allied to living matter,
it is precisely _life_ that it lacks, and as yet we do not know what
kinds of chemical difference prevail between the dead proteid and the
living; indeed we must honestly confess that it is a mere assumption
when we take for granted that there are only chemico-physical
differences between the two. It cannot be proved, in the meantime,
that there is not another unknown power in the living protoplasm,
a 'vitalistic principle,' a 'life-force,' on the activity of which
these specific phenomena of life, and particularly the continually
repeated alternation of disruption and reconstruction of the living
substance, dissimilation and assimilation, growth and multiplication,
depend. It is just as difficult to prove the converse, that it is
impossible that chemico-physical forces alone should have called forth
life in a chemical substance of very special composition. Although
no one has ever succeeded, in spite of many attempts, in thinking
out a combination of chemical substances which--as this wonderful
living substance does--on the one hand undergoes combustion with
oxygen and, on the other hand, renews itself again with 'nutritive'
material, yet we cannot infer from this the impossibility of a purely
chemico-physical basis of life, but must rather hold fast to it until
it is shown that it is not sufficient to explain the facts, thus
following the fundamental rule that natural science must not assume
unknown forces until the known ones are _proved_ insufficient. If
we were to do otherwise we should have to renounce all hope of ever
penetrating deeper into the phenomena. And we have no need to do
this, for in a general way we can quite well believe that an organic
substance of exactly proportioned composition exists, in which the
fundamental phenomena of all life--combustion with simultaneous
renewal--must take place under certain conditions by virtue of its
composition.
How, and under what external conditions, such a substance first arose
upon the earth, from and of what materials it was formed, cannot
be answered with any certainty in the meantime. Who knows whether
the fantastic ideas of Empedocles in an altered form would not be
justified here? I mean that, at the time of the first origin of
life, the conditions necessary for many kinds of complex chemical
combinations may have been present simultaneously on the earth, and
that, out of a manifold variety of such substances, only those survived
which possessed that marvellous composition which conditioned their
continual combustion, but also their ceaseless reconstruction by
multiplication. According to Empedocles, there arose from chaos only
parts of animals--heads without bodies, arms without trunks, eyes
without faces, and so on--and these whirled about in wild confusion
and flew together as chance directed them. But those only survived
which had united rightly with others so as to form a whole, capable of
life. Translated into the language of our time, that would mean what I
have just said--that, of a large number of organic combinations which
arose, only a few, perhaps one, would possess the marvellously adjusted
composition which resulted in life, and with it self-maintenance and
multiplication; and that would be the first instance of selection!
But let us leave these imaginings, and wait to see whether the chemists
will not possibly be able to furnish us with a starting-point for a
more concrete picture of the first origin of life. In the meantime, we
must confess that we find ourselves confronted with deep darkness.
The question as to the 'Where' of spontaneous generation must also be
left without any definite answer. Some have supposed that life began
in the depths of the sea, others on the shore, and others in the air.
But who is to divine this, when we cannot even name theoretically the
conditions and the materials out of which albuminoid-like substances
might be built up in the laboratory? Nägeli's hypothesis still seems
to me to have the greatest probability. According to his theory, the
first living particles originated not in a free mass of water, but in
the reticulated superficial layer of a fine porous substance (clay or
sand), where the molecular forces of solid, fluid, and gaseous bodies
were able to co-operate.
Only so much is certain, that wherever life may first have arisen upon
this earth, it can have done so only in the form of the very simple
and very minute vital units, which even now we only infer to be parts
of the living body, but which must first have arisen as independent
organisms, the 'Biophoridæ.' As these, according to our theory,
possessed the character of life, they must have possessed above all the
capacity of assimilating in the sense in which the plants assimilate,
that is, of renewing their bodily substance continually from inorganic
substances, of growing, and of reproducing. They need not on that
account have possessed the chemical constitution of chlorophyll,
although the capacity of assimilation in green plants depends upon this
substance, for we know colourless fungi, which, notwithstanding the
absence of chlorophyll, are able to build up the substance of their
body from compounds of carbon and nitrogen.
The first advance to a higher stage of life must have been brought
about by multiplication, since accumulations of Biophoridæ,
unintegrated but connected masses, would be formed.
In this way the threshold of microscopical visibility would gradually
be reached and crossed, but--to argue from the modern Baccilli--long
before that time a differentiation of the biophors on the principle
of division of labour would have taken place within a colony of
Biophoridæ. This first step towards higher organization must probably
have taken enormous periods of time, for before any differentiation
could occur and bring any advantage the unintegrated aggregates of
Biophors must first have become orderly, and have formed themselves
into a stable association with definite form and definite structure,
somewhat analogous to the spherical cell-colonies of _Magosphæra_ or
_Pandorina_. Only then was the further step made of a differentiation
of the individual biophors forming the colony, and this is comparable
to the species of _Volvox_ among the lower Algæ. The gradual ascent of
these colonies of biophors must, then, be referred to the principles
to which we attribute the ascent of the higher forms of life to
ever-higher and ever-new differentiations; the principles of division
of labour and selection.
These differentiated colonies of biophors have brought us nearer to the
lowest known organisms, among which there are some whose existence we
can only infer from their pathological effects, since we have not been
able to make them visible. The bacillus of measles has never yet been
seen, but we cannot doubt its existence, and we must assume that there
are bacilli of such exceeding smallness that we shall never be able
to see them, even with the most improved methods of staining and the
strongest lenses.
These non-nucleated Monera lead on to the stage of nucleus-formation,
and this at once implies the cell. As, on our view, the nucleus is
primarily a storehouse of 'primary constituents' (_Anlagen_), its
origin must have begun at the moment at which the differentiation of
the cell-body reached such a degree of differentiation of its parts
that a mechanical division into two halves was no longer possible, and
that the two products of division, if they were each to develop to
a new and intact whole, required a reserve of primordia (_Anlagen_)
to give rise to the missing parts. As this higher differentiation
would bring about a superiority over the lower forms of life, in that
they would make possible the utilization of new conditions of life,
but on the other hand could only survive if the differentiation of a
reserve of primary constituents, that is, a nucleus, were introduced
at the same time, the development of the nucleus can be ranged under
the principle of utility to which we traced back the evolution of all
higher and more differentiated forms of life. But it would scarcely
be profitable to try to follow out in detail the first steps in the
progress of organization under the control of selective processes,
since we know far too little about the life of the simplest organisms
to be able to judge how far their differentiations are of use in
improving their capacity for life.
That would be a bold undertaking even in regard to unicellular
organisms, and it is only in the case of multicellular organisms that
we can speak with greater certainty and really recognize the changing
of the external conditions, in the most general and comprehensive
sense, as the fundamental cause of the lasting variations of organic
forms. We can here distinguish with certainty between the direct and
the indirect effect of external influences, and we see how these
sources of variation interact upon each other. The lowest and deepest
root of variation is without doubt the direct effect of changed
conditions. Without this the indirect effect would have had no lever
with which to work, for the primitive beginnings of variation would be
absent, and an accumulation of these through personal selection could
not take place. It is a primitive character of living substance to be
variable, that is, to be able to respond to some extent to changed
external conditions, and to vary in accordance with them, or--as we
might also say--to be able to exist in many very similar but not
identical combinations of substances, and we must imagine that even
the first biophors which arose through spontaneous generation were
different according to the conditions under which, and the substances
from which, they originated. And from each of these slightly different
beginnings there must, in the course of multiplication by fission,
have been produced a whole genealogical tree of divergent variations
of the primitive Biophoridæ, since it is inconceivable that all the
descendants would remain constantly under the same conditions of
life under which they originated. For every persistent change in
the conditions of existence, and especially of nutrition, must have
involved a variation in the constitution of the organism, whose vital
processes, and especially the repair of its body, depended on these
conditions.
But the external influences to which the descendants of a particular
form of life were subject never remained permanently the same. Not
only did the surface of the earth and its climatic conditions change
in the course of time with the cooling of the earth, but mountains
arose and were levelled again, old land-surfaces sank out of sight or
emerged again, and so on; all that, of course, played its part in the
transformation of the forms of life, but did so to any considerable
extent only at a later stage, when there were already highly
differentiated organisms. These unknown primitive beginnings of life
must have been forced to diverge into different variations through the
different conditions of the same place in which they lived.
Let us think of the simplest microscopic Monera on the mud of the
sea-coast, equipped with the faculty of plant-like assimilation, and we
shall see that their unlimited multiplication would cause differences
in nutrition, for those lying uppermost would be in a stronger light
than those below, and would, therefore, be better nourished, and,
consequently, would transmit the variations thus induced to their
progeny which arose by fission. Thus it is conceivable that even the
more or less favourable position as regards light would bring about the
origin of two different races from the same parent form, and as it is
conceivable in the case of light, so is it also in regard to all the
influences which cause variation in the organism.
We have already seen that variations in the lowest (non-nucleated)
forms of life caused by the direct influence of the vital processes
may be directly transmitted to the descendants, but that in all
those whose bodies have already differentiated into a germ- or
idioplasmic-substance, in contrast to a somatic substance in the more
restricted sense, this hereditary transmission is only possible in the
case of the variations of the germ-plasm, and _hereditary_ variations
of the species can only arise by the circuitous route of influencing
the germ-plasm. The body (soma) can be caused to change by external
influences, by the use or disuse of an organ, but variations of this
kind are not transmitted; they do not become a lasting possession of
the species, but cease with the individual; they are transient changes.
Thus it was only through those external influences--including
those from the soma of the organism itself--which affected the
germ-substance, either as a whole or in certain of its primary
constituents, that hereditarily transmissible variations of the
organism arose, and we have already discussed in detail how particular
variational tendencies may arise through the struggle of the parts
within the germ-plasm, which may give an advantage to certain groups of
primary constituents. And these tendencies are of themselves sufficient
to cause the specific type to vary further and further in given
directions.
Nevertheless, the infinite diversity of the forms of life could never
have been brought about in this way alone, if there had not been
another--the _indirect_--effect of the changeful external influences.