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.