Heredity
6. With this fifth line of thought a sixth is associated and intertwined.
The problem of development is closely bound up with that of "heredity." A
developing organism follows the parental type. The acorn in its growth
follows the type of the parent oak, repeating all its morphological and
physiological characters down to the most intimate detail. And the animal
organism adds to this also the whole psychical equipment, the instinct
,
the capacities of will and consciousness which distinguish its parents.
The problems of the fifth and sixth order are closely inter-related, the
sixth problem being in reality the same as the fifth, only in greater
complexity.
A step towards the mechanical solution of this problem was indicated in
the "preformation theory" advanced by Leibnitz, and elaborated by Bonnet.
According to this theory the developing organism is enclosed in the
minutest possible form within the egg, and is thus included in the
parental organism, in miniature indeed, but quite complete. Thus the
problem of the "development of form" or of "heredity" was, so to speak,
ruled out of court; all that was assumed was continuous growth and
self-unfolding.
Opposed to this theory was one of later growth, the theory of epigenesis,
which maintained that the organism developed without preformation from the
still undifferentiated and homogeneous substance of the egg. The
supporters of the first theory considered themselves much more scientific
and exact than those of the second. And not without reason. For the theory
of epigenesis obviously required mysterious formative principles, and
equally mysterious powers of recollection and recapitulation, which
impelled the undifferentiated ovum substance into the final form,
precisely like that of its ancestors. Nor need the preformationists have
greatly feared the reproach, that the parental organism must have been
included within the grand-parental, and so on backwards to the first
parents in Paradise. For this "Chinese box" encapsulement theory only
requires that we should grant the idea of the infinitely little, and that
idea is already an integral part of our thinking.
Modern biologists ridicule the preformation hypothesis as altogether too
artificial. And undoubtedly it founders on the facts of embryology, which
disclose nothing to suggest the unfolding of a pre-existent miniature
model, but show us how the egg-cell divides into two, into four, and so
on, with continued multiplication followed by varied arrangements and
rearrangements of cells--in short, all the complex changes which constitute
development. But a preformation in some sense or other there must be;--some
peculiar material predisposition of the germ, which, as such, supplies the
directing principle for the development, and the sufficient reason for the
repetition of the parental form. This is of such obvious importance from
the mechanical point of view that the speculations of to-day tend to move
along the old preformationist lines. To these modern preformationists are
opposed the modern upholders of epigenesis or gradual differentiation, who
attempt to elaborate a mechanical theory of development. And with the
contrast between these two schools there is necessarily associated the
discussion as to the inheritance or non-inheritance of acquired
characters.
Darwin's contribution to the problem of the sixth order was his rather
vague theory of "Pangenesis." The living organism, according to him, forms
in its various organs, parts, and cells exceedingly minute particles of
living matter (gemmules), which, "in some way or other," bear within them
the special characteristics of the part in which they are produced. These
may wander through the organism and meet in the germ-plasm, and then, when
a child-organism is produced, they "swarm," so to speak, in it again "in
some way or other," and in some fashion control the development. This
gemmule-theory was too obviously a quid pro quo to hold its ground for
long. Various theories were elaborated, and the world of the invisibly
minute was flooded with speculations.
The most subtle of these, on the side of consistent Darwinism, is that of
Weismann, a pronounced preformation theory which has been increasingly
refined and elaborated in the course of years of reflection. According to
Weismann, the individual parts and characteristics of the organism are
represented in the germ-plasm, not in finished form, but as "determinants"
in a definite system which is itself the directing principle in the
building up of the bodily system, and with definite characteristics, which
determine the peculiarities of the individual organs and parts, down to
scales, hairs, skin-spots, and birth-marks. As the germ-cells have the
power of growth, and can increase endlessly by dividing and re-dividing,
and as each process of division takes place in such a way that each half
(each product of division) maintains the previous system, there arise
innumerable germ-cells corresponding to one another, from which,
therefore, corresponding bodies must arise (inheritance). It is not in
reality the newly developed bodies which give rise to new germ-cells and
transfer to them something of their own characters; the germ-cells of the
child-organism develop from that of the parent ("immortality" of the
germ-cells). Therefore there can be no inheritance of acquired characters,
and no modifications of type through external causes; and all variations
which appear in a series of generations are due solely to internal
variations in the germ-cells, whether brought about by the complication of
their system through the fusion of the male and female germ-cells, or
through differences in the growth of the individual determinants
themselves. The numerous subsidiary theses interwoven in Weismann's theory
are entirely coherent, and have been thought out to their conclusions with
praiseworthy determination.(67) To the theory as a whole, because of its
fundamental conception of preformation, and to its subsidiary hypotheses,
piece by piece, there has been energetic opposition on the part of the
upholders of the modern mechanical theory of epigenesis. This opposition
is most concretely and comprehensively expressed in Haacke's "Gestaltung
und Vererbung." The infinitely complex intricacy of Weismann's minute
microcosm within the germ-cell, indeed within every id in it, is justly
described as a mere duplication, a repetition in the infinitely little of
the essential difficulties to be explained. The complicated processes of
developing in the growing and inheriting organism cannot be explained,
they say, in terms of processes of the equally complex and likewise
developing germ-plasm. The complex, if it is to be explained at all, must
be explained by the simple--in this case by the functions of a homogeneous
uniform plasm.
At an earlier date Haeckel had made an attempt in this direction in his
theory of the "perigenesis of the plastidules." Peculiar states of
oscillation and rhythm in the molecules of the germ-substance, handed on
to it from the parent organism and transferable to all the assimilated
matter of the offspring, represent, according to this theory, the
principle which impels development to follow a particular course
corresponding to the type of the parents. This was a physical way of
interpreting the matter. Other investigators have given a chemical
expression to their theoretical schemes for explaining heredity.
Haacke declares both these to be unsatisfactory, and replaces them by
morphological formative principles. It is the structure of the otherwise
homogeneous living matter that explains morphogenesis and inheritance.
Minute "gemmae," homogeneous fundamental particles of living substance, not
to be compared to or confused with Darwin's "gemmules," are aggregated in
"Gemmaria," whose configuration, stability, symmetrical or asymmetrical
structure, and so on, are determined by the relative positions of the
gemmae to each other, and these in their turn control the organism and give
it a corresponding symmetrical or asymmetrical, a firmly or loosely
aggregated structure. The completed organism then forms a system in
organic equilibrium, which is constantly exposed to variations and
influences due to external causes (St. Hilaire), and to use and disuse of
organs (Lamarck). These influences affect the structure of the gemmaria,
and as the germ-cells consist of gemmaria, like those of the rest of the
organism, the possibility of the transmission of acquired new characters
is self-evident. The importance of correlated growth and orthogenesis is
explained on a similar basis, and the Darwinian conceptions of the
independent variation of individual parts, of the exclusive dominance of
utility, of the influence of the struggle for existence in regard to
individual selection, and of the omnipotence of natural selection, are
energetically denied.
Oscar Hertwig,(68) de Vries, Driesch(69) and others attempt to reconcile
the preformationist and the epigenetic standpoints, and "to extract what
is good and usable out of both." Hertwig and Driesch, however, can only be
mentioned with reservations in this connection.
We cannot better sum up the whole tendency of the construction of
mechanical theories on these last lines than in the words of Schwann:
"There is within the organism no fundamental force working according to a
definite idea; it arises in obedience to the blind laws of necessity."
So much for the different lines followed by the mechanical theories of
to-day. An idea of their general tenor can be gained from a series of much
quoted general treatises, of which we must mention at least the
"classics." In Wagner's "Handwoerterbuch der Physiologie," 1842, Vol. I.,
Lotze wrote a long introductory article to the whole work, on "Life and
Vital Force." It was the challenge of the newer views to the previously
vitalistic standpoint, and at the same time it was based on Lotze's
general principles and interspersed with philosophical criticism of the
concepts of force, cause, effect, law, &c.(70) A similar train of ideas to
Lotze's is followed to-day by O. Hertwig, especially in his "Mechanismus
und Biologie."(71) Lighter and more elegant was the polemic against vital
force, and the outline of a mechanical theory which Du Bois-Reymond
prefaced to his great work, "Untersuchungen ueber die tierische
Electricitaet" (1849). It did not go nearly so deep as Lotze's essay, but
perhaps for that very reason its phrases and epigrams soon became common
property. We may recall how he speaks of vital force as a "general servant
for everybody," of the iron atom which remains the same whether it be in
the meteorite in cosmic space, in the wheel of the railway carriage, or in
the blood of the thinker, and of analytic mechanics which may be applied
even to the problem of personal freedom.
The most comprehensive and detailed elaboration of the mechanical theory
of life is to be found in Herbert Spencer's "Principles of Biology."(72)
Friedrich Albert Lange's "History of Materialism" is a brilliant plea for
mechanical theories,(73) which he afterwards surpassed and neutralised by
his Kantian Criticism. Verworn, too, in his "Physiology"(74) gives a clear
example of the way in which the mechanical theory in its most consistent
form is sublimed, apparently in the idealism of Kant and Fichte, but in
reality in its opposite--the Berkeleyan psychology. A similar outcome is in
various ways indicated in the modern trend of things.