PANGENESIS.
A provisional hypothesis supplementing "Natural Selection."--Statement
of the hypothesis.--Difficulty as to multitude of gemmules.--As to
certain modes of reproduction.--As to formations without the requisite
gemmules.--Mr. Lewes and Professor Delpino.--Difficulty as to
developmental force of gemmules.--As to their spontaneous
fission.--Pangenesis and Vitalism.--Paradoxical reality.--Pangenesis
scarcely superior to anterior hypotheses.--Buffon.--Owen.--Herbert
Spencer.--"Gemmules" as mysterious as "physiological
units."--Conclusion.
In addition to the theory of "Natural Selection," by which it has been
attempted to account for the origin of species, Mr. Darwin has also put
forward what he modestly terms "a provisional hypothesis" (that of
_Pangenesis_), by which to account for the origin of each and every
individual form.
Now, though the hypothesis of Pangenesis is no necessary part of "Natural
Selection," still any treatise on specific origination would be incomplete
if it did not take into consideration this last speculation of Mr. Darwin.
The hypothesis in question may be stated as follows: That each living
organism is ultimately made up of an almost infinite number of minute
particles, or organic atoms, termed "gemmules," each of which has the power
of reproducing its kind. Moreover, that these particles circulate freely
about the organism which is made up of them, and are derived from all the
parts of all the organs of the less remote ancestors of each such {209}
organism during all the states and stages of such several ancestors'
existence; and therefore of the several states of each of such ancestors'
organs. That such a complete collection of gemmules is aggregated in each
ovum and spermatozoon in most animals, and in each part capable of
reproducing by gemmation (budding) in the lowest animals and in plants.
Therefore in many of such lower organisms such a congeries of ancestral
gemmules must exist in every part of their bodies, since in them every part
is capable of reproducing by gemmation. Mr. Darwin must evidently admit
this, since he says: "It has often been said by naturalists that each cell
of a plant has the actual or potential capacity of reproducing the whole
plant; but it has this power only in virtue of containing gemmules _derived
from every part_."[218]
Moreover, these gemmules are supposed to tend to aggregate themselves, and
to reproduce in certain definite relations to other gemmules. Thus, when
the foot of an eft is cut off, its reproduction is explained by Mr. Darwin
as resulting from the aggregation of those floating gemmules which come
next in order to those of the cut surface, and the successive aggregations
of the other kinds of gemmules which come after in regular order. Also, the
most ordinary processes of repair are similarly accounted for, and the
successive development of similar parts and organs in creatures in which
such complex evolutions occur is explained in the same way, by the
independent action of separate gemmules.
In order that each living creature may be thus furnished, the number of
such gemmules in each must be inconceivably great. Mr. Darwin says:[219]
"In a highly organized and complex animal, the gemmules thrown off from
each different cell or unit throughout the body must be inconceivably
numerous and minute. Each unit of each part, as it changes during
development--and we know that some insects undergo at least twenty {210}
metamorphoses--must throw off its gemmules. All organic beings, moreover,
include many dormant gemmules derived from their grandparents and more
remote progenitors, but not from all their progenitors. These _almost
infinitely numerous_ and minute gemmules must be included in each bud,
ovule, spermatozoon, and pollen grain." We have seen also that in certain
cases a similar multitude of gemmules must be included also in every
considerable part of the whole body of each organism, but where are we to
stop? There must be gemmules not only from every organ, but from every
component part of such organ, from every subdivision of such component
part, and from every cell, thread, or fibre entering into the composition
of such subdivision. Moreover, not only from all these, but from each and
every successive stage of the evolution and development of such
successively more and more elementary parts. At the first glance this new
atomic theory has charms from its apparent simplicity, but the attempt thus
to follow it out into its ultimate limits and extreme consequences seems to
indicate that it is at once insufficient and cumbrous.
Mr. Darwin himself is, of course, fully aware that there must be _some_
limit to this aggregation of gemmules. He says:[220] "Excessively minute
and numerous as they are believed to be, an infinite number derived, during
a long course of modification and descent, from each cell of each
progenitor, could not be supported and nourished by the organism."
But apart from these matters, which will be more fully considered further
on, the hypothesis not only does not appear to account for certain
phenomena which, in order to be a valid theory, it ought to account for;
but it seems absolutely to conflict with patent and notorious facts.
How, for example, does it explain the peculiar reproduction which is {211}
found to take place in certain marine worms--certain annelids?
[Illustration: AN ANNELID DIVIDING SPONTANEOUSLY.
(A new head having been formed towards the hinder end of the body of the
parent.)]
In such creatures we see that, from time to time, one of the segments of
the body gradually becomes modified till it assumes the condition of a
head, and this remarkable phenomenon is repeated again and again, the body
of the worm thus multiplying serially into new individuals which
successively detach themselves from the older portion. The development of
such a mode of reproduction by "Natural Selection" seems not less
inexplicable than does its continued performance through the aid of {212}
"pangenesis." For how can gemmules attach themselves to others to which
they do not normally or generally succeed? Scarcely less difficult to
understand is the process of the stomach-carrying-off mode of metamorphosis
before spoken of as existing in the Echinoderms. Next, as to certain patent
and notorious facts: On the hypothesis of pangenesis, no creature can
develop an organ unless it possesses the component gemmules which serve for
its formation. No creature can possess such gemmules unless it inherits
them from its parents, grandparents, or its less remote ancestors. Now, the
Jews are remarkably scrupulous as to marriage, and rarely contract such a
union with individuals not of their own race. This practice has gone on for
thousands of years, and similarly also for thousands of years the rite of
circumcision has been unfailingly and carefully performed. If then the
hypothesis of pangenesis is well founded, that rite ought to be now
absolutely or nearly superfluous from the necessarily continuous absence of
certain gemmules through so many centuries and so many generations. Yet it
is not at all so, and this fact seems to amount almost to an experimental
demonstration that the hypothesis of pangenesis is an insufficient
explanation of individual evolution.
Two exceedingly good criticisms of Mr. Darwin's hypothesis have appeared.
One of these is by Mr. G. H. Lewes,[221] the other by Professor Delpino of
Florence.[222] The latter gentleman gives a report of an observation made
by him upon a certain plant, which observation adds force to what has just
been said about the Jewish race. He says:[223] "If we examine and compare
the numerous species of the genus _Salvia_, commencing with _Salvia
officinalis_, which may pass as the main state of the genus, and {213}
concluding with _Salvia verticillata_, which may be taken as the most
highly developed form, and as the most distant from the type, we observe a
singular phenomenon. The lower cell of each of the two fertile anthers,
which is much reduced and different from the superior even in _Salvia
officinalis_, is transmuted in other _salviæ_ into an organ (nectarotheca)
having a very different form and function, and finally disappears entirely
in _Salvia verticillata_.
"Now, on one occasion, in a flower belonging to an individual of _Salvia
verticillata_, and only on the left stamen, I observed a perfectly
developed and pollinigerous lower cell, perfectly homologous with that
which is normally developed in _Salvia officinalis_. This case of atavism
is truly singular. According to the theory of Pangenesis, it is necessary
to assume that all the gemmules of this anomalous formation, and therefore
the mother-gemmule of the cell, and the daughter-gemmules of the special
epidermic tissue, and of the very singular subjacent tissue of the
endothecium, have been perpetuated, and transmitted from parent to
offspring in a dormant state, and through a number of generations, such as
startles the imagination, and leads it to refuse its consent to the theory
of Pangenesis, however seductive it may be." This seems a strong
confirmation of what has been here advanced.
The main objection raised against Mr. Darwin's hypothesis is that it
(Pangenesis) requires so many subordinate hypotheses for its support, and
that some of these are not tenable.
Professor Delpino considers[224] that as many as eight of these subordinate
hypotheses are required, namely, that--
"1. The emission of the gemmules takes place, or may take place in all
states of the cell.
"2. The quantity of gemmules emitted from every cell is very great.
"3. The minuteness of the gemmules is extreme.
{214}
"4. The gemmules possess two sorts of affinity, one of which might be
called _propagative_, and the other _germinative_ affinity.
"5. By means of the propagative affinity all the gemmules emitted by all
the cells of the individual flow together and become condensed in the cells
which compose the sexual organs, whether male or female (embryonal vesicle,
cells of the embryo, pollen grains, fovilla, antherozoids, spermatozoids),
and likewise flow together and become condensed in the cells which
constitute the organs of a sexual or agamic reproduction (buds, spores,
bulbilli, portions of the body separated by scission, &c.).
"6. By means of the germinative affinity, every gemmule (except in cases of
anomalies or monstrosities) can be developed only in cells homologous with
the mother-cells of the cell from which they originated. In other words,
the gemmules from any cell can only be developed in unison with the cell
preceding it in due order of succession, and whilst in a nascent state.
"7. Of each kind of gernmule a great number perishes; a great number
remains in a dormant state through many generations in the bodies of
descendants; the remainder germinate and reproduce the mother-cell.
"8. Every gemmule may multiply itself by a process of scission into any
number of equivalent gemmules."
Mr. Darwin has published a short notice in reply to Professor Delpino, in
_Scientific Opinion_ of October 20, 1869, p. 426. In this reply he admits
the justice of Professor Delpino's attack, but objects to the alleged
necessity of the first subordinate hypothesis, namely, that the emission of
gemmules takes place in all states of the cell. But if this is not the
case, then a great part of the utility and distinction of pangenesis is
destroyed, or as Mr. Lewes justly says,[225] "If gemmules produce whole
cells, we have the very power which was pronounced mysterious in larger
organisms."
{215}
Mr. Darwin also does not see the force of the objection to the power of
self-division which must be asserted of the gemmules themselves if
Pangenesis be true. The objection, however, appears to many to be
formidable. To admit the power of spontaneous division and multiplication
in such rudimentary structures, seems a complete contradiction. The
gemmules, by the hypothesis of Pangenesis, are the ultimate organized
components of the body, the absolute organic atoms of which each body is
composed; how then _can_ they be divisible? Any part of a gemmule would be
an impossible (because a _less_ than possible) quantity. If it is divisible
into still smaller organic wholes, as a germ-cell is, it must be made up as
the germ-cell is, of subordinate component atoms, which are then the _true_
gemmules. This process may be repeated _ad infinitum_, unless we get to
true organic atoms, the true gemmules, whatever they may be, and they
necessarily will be incapable of any process of spontaneous fission. It is
remarkable that Mr. Darwin brings forward in support of gemmule fission,
the observation that "Thuret has seen the zoospore of an alga divide
itself, and both halves germinate." Yet on the hypothesis of Pangenesis,
the zoospore of an alga must contain gemmules from all the cells of the
parent algæ, and from all the parts of all their less remote ancestors in
all their stages of existence. What wonder then that such an excessively
complex body should divide and multiply; and what parity is there between
such a body and a gemmule? A steam-engine and a steel-filing might equally
well be compared together.
Professor Delpino makes a further objection which, however, will only be of
weight in the eyes of Vitalists. He says,[226] Pangenesis is not to be
received because "it leads directly to the negation of a specific vital
principle, co-ordinating and regulating all the movements, acts, and
functions of the individuals in which it is incarnated. For Pangenesis of
the individual is a term without meaning. If, in contemplating an {216}
animal of high organization, we regard it purely as an aggregation of
developed gemmules, although these gemmules have been evolved successively
one after the other, and one within the other, notwithstanding they elude
the conception of the _real and true individual_, these problematical and
invisible gemmules must be regarded as so many individuals. Now, that real,
true, living individuals exist in nature, is a truth which is persistently
attested to us by our consciousness. But how, then, can we explain that a
great quantity of dissimilar elements, like the atoms of matter, can unite
to form those perfect unities which we call individuals, if we do not
suppose the existence of a specific principle, proper to the individual but
foreign to the component atoms, which aggregates these said atoms, groups
them into molecules, and then moulds the molecules into cells, the cells
into tissues, the tissues into organs, and the organs into apparatus?"
"But, it may be urged in opposition by the Pangenesists, your vital
principle is an unknown and irresolute _x_. This is true; but, on the other
hand, let us see whether Pangenesis produces a clearer formula, and one
free from unknown elements. The existence of the gemmules is a first
unknown element; the propagative affinity of the gemmules is a second;
their germinative affinity is a third; their multiplication by fission is a
fourth--and what an unknown element!"
"Thus, in Pangenesis, everything proceeds by force of unknown elements, and
we may ask whether it is more logical to prefer a system which assumes a
multitude of unknown elements to a system which assumes only a single one?"
Mr. Darwin appears, by "Natural Selection," to destroy the reality of
species, and by Pangenesis that of the individual. Mr. Lewes observes[227]
of the individual that "This whole is only a subjective conception which
summarizes the parts, and that in point of fact it is the parts which {217}
are reproduced." But the parts are also, from the same point of view,
merely subjective until we come to the absolute organic atoms. These atoms,
on the other hand, are utterly invisible, intangible; indeed, in the words
of Mr. Darwin, inconceivable. Thus, then, it results from the theories in
question, that the organic world is reduced to utter unreality as regards
all that can be perceived by the senses or distinctly imagined by the mind;
while the only reality consists of the invisible, the insensible, the
inconceivable; in other words, nothing is known that really is, and only
the nonexistent can be known. A somewhat paradoxical outcome of the
speculations of those who profess to rely exclusively on the testimony of
sense. "_Les extrêmes se touchent_," and extreme sensationalism shakes
hands with the "das seyn ist das nichts" of Hegel.
Altogether the hypothesis of Pangenesis seems to be little, if at all,
superior to anterior hypotheses of a more or less similar nature.
Apart from the atoms of Democritus, and apart also from the speculations of
mediæval writers, the molecules of Bonnet and of Buffon almost anticipated
the hypothesis of Pangenesis. According to the last-named author,[228]
organic particles from every part of the body assemble in the sexual
secretions, and by their union build up the embryo, each particle taking
its due place, and occupying in the offspring a similar position to that
which it occupied in the parents. In 1849 Professor Owen, in his treatise
on "Parthenogenesis," put forward another conception. According to this,
the cells resulting from the subdivision of the germ-cell preserve their
developmental force, unless employed in building up definite organic
structures. In certain creatures, and in certain parts of other creatures,
germ-cells unused are stored up, and by their agency lost limbs and {218}
other mutilations are repaired. Such unused products of the germ-cell are
also supposed to become located in the generative products.
According to Mr. Herbert Spencer, in his "Principles of Biology," each
living organism consists of certain so-called "physiological units." Each
of these units has an innate power and capacity, by which it tends to build
up and reproduce the entire organism of which it forms a part, unless in
the meantime its force is exhausted by its taking part in the production of
some distinct and definite tissue--a condition somewhat similar to that
conceived by Professor Owen.
Now, at first sight, Mr. Darwin's atomic theory appears to be more simple
than any of the others. It has been objected that while Mr. Spencer's
theory requires the assumption of an innate power and tendency in each
physiological unit, Mr. Darwin's, on the other hand, requires nothing of
the kind, but explains the evolution of each individual by purely
mechanical conceptions. In fact, however, it is not so. Each gemmule,
according to Mr. Darwin, is really the seat of powers, elective affinities,
and special tendencies as marked and mysterious as those possessed by the
physiological unit of Mr. Spencer, with the single exception that the
former has no tendency to build up the whole living, complex organism of
which it forms a part. Some may think this an important distinction, but it
can hardly be so, for Mr. Darwin considers that his gemmule has the innate
power and tendency to build up and transform itself into the whole living,
complex cell of which it forms a part; and the one tendency is, in
principle, fully as difficult to understand, fully as mysterious, as is the
other. The difference is but one of degree, not of kind. Moreover, the one
mystery in the case of the "physiological unit" explains all, while with
regard to the gemmule, as we have seen, it has to be supplemented by other
powers and tendencies, each distinct, and each in itself inexplicable and
profoundly mysterious. [Page 219]
That there should be physiological units possessed of the power attributed
to them, harmonizes with what has recently been put forward by Dr. H.
Charlton Bastian; who maintains that under fit conditions the simplest
organisms develop themselves into relatively large and complex ones. This
is not supposed by him to be due to any inheritance of ancestral gemmules,
but to direct growth and transformation of the most minute and the simplest
organisms, which themselves, by all reason and analogy, owe their existence
to immediate transformation from the inorganic world.
Thus, then, there are grave difficulties in the way of the reception of the
hypothesis of Pangenesis, which moreover, if established, would leave the
evolution of individual organisms, when thoroughly analysed, little if at
all less mysterious or really explicable than it is at present.
As was said at the beginning of this chapter, "Pangenesis" and "Natural
Selection" are quite separable and distinct hypotheses. The fall of one of
these by no means necessarily includes that of the other. Nevertheless, Mr.
Darwin has associated them closely together, and, therefore, the refutation
of Pangenesis may render it advisable for those who have hitherto accepted
"Natural Selection" to reconsider that theory. [Page 220]
* * * * *