TRANSMISSION BY INHERITANCE AND PROPAGATION.
Universality of Inheritance and Transmission by
Inheritance.—Special Evidences of the same.—Human
Beings with four, six, or seven Fingers and
Toes.—Porcupine Men.—Transmission of Diseases,
especially Diseases of the Mind.—Original
Sin.—Hereditary Monarchies.—Hereditary
Aristocracy.—Hereditary Talents and Mental
Qualities.—Material Causes of Transmission by
Inheritance.—Connection between Transmission by
Inheritance and Propagation.—Spontaneous Generation and
Propagation.—Non-sexual or Monogonous
Propagation.—Propagation by Self-Division.—Monera and
Amœbæ.—Propagation by the formation of Buds, by the
formation of Germ-Buds, by the formation of
Germ-Cells.—Sexual or Amphigonous
Propagation.—Formation of Hermaphrodites.—Distinction
of Sexes, or Gonochorism.—Virginal Breeding, or
Parthenogenesis.—Material Transmission of Peculiarities
of both Parents to the Child by Sexual
Propagation.—Difference between Transmission by
Inheritance in Sexual and in Asexual Propagation.
The reader has, in the last chapter, become acquainted with natural
selection according to Darwin’s theory, as the constructive force of
nature which produces the different forms of animal and vegetable
species. By natural selection we understand the interaction which takes
place in the struggle for life between the _transmission by inheritance_
and the _mutability_ of organisms, between two physiological functions
which are innate in all animals and plants, and which may be traced to
other processes of life—the functions of propagation and nutrition. All
the different forms of organisms, which people are usually inclined to
look upon as the products of a creative power, acting for a definite
purpose, we, according to the Theory of Selection, can conceive as the
necessary productions of natural selection, working without a
purpose,—as the unconscious interaction between the two properties of
Mutability and Hereditivity. Considering the importance which
accordingly belongs to these vital properties of organisms, we must
examine them a little more closely, and employ a chapter with the
consideration of Transmission by Inheritance. (Gen. Morph. ii. 170-191.)
Strictly speaking, we must distinguish between Hereditivity
(Transmissivity) and Inheritance (Transmission). Hereditivity is the
power of transmission, the capability of organisms to transfer their
peculiarities to their descendants by propagation. Transmission by
Inheritance, or Inheritance simply, on the other hand, denotes the
exercise of the capability, the actual transmission.
Hereditivity and Transmission by Inheritance are such universal,
everyday phenomena, that most people do not heed them, and but few are
inclined to reflect upon the operation and import of these phenomena of
life. It is generally thought quite natural and self-evident that every
organism should produce its like, and that children should more or less
resemble their parents. Heredity is usually only taken notice of and
discussed in cases relating to some special peculiarity, which appears
for the first time in a human individual without having been inherited,
and then is transmitted to his descendants. It shows itself in a
specially striking manner in the case of certain diseases, and in
unusual and irregular (monstrous) deviations from the usual formation of
the body.
Among these cases of the inheritance of monstrous deviations, those are
specially interesting which consist in an abnormal increase or decrease
of the number five in the fingers or toes of man. It is not unfrequently
observed in families through several generations, that individuals have
six fingers on each hand, or six toes on each foot. Less frequent is the
number of four or seven fingers or toes. The unusual formation arises at
first from a single individual who, from unknown causes, is born with an
excess of the usual number of fingers and toes, and transmits these, by
inheritance, to a portion of his descendants. In one and the same family
it has happened that, throughout three, four, or more generations,
individuals have possessed six fingers and toes. In a Spanish family
there were no less than forty individuals distinguished by this excess.
The transmission of the sixth finger or toe is not permanent or enduring
in all cases, because six-fingered people always intermarry again with
those possessing five fingers. If a six-fingered family were to
propagate by pure in-breeding, if six-fingered men were always to marry
six-fingered women, this characteristic would become permanent, and a
special six-fingered human race would arise. But as six-fingered men
usually marry five-fingered women, and _vice versâ_, their descendants
for the most part show a very mixed numerical relation, and finally,
after the course of some generations, revert again to the normal number
of five. Thus, for example, among eight children of a six-fingered
father and a five-fingered mother, two children may have on both hands
and feet six fingers and toes, four children may have a mixed number,
and two children may have the usual number of five on both hands and
feet. In a Spanish family, each child except the youngest had the number
six on both hands and feet; the youngest, only, had the usual number on
both hands and feet, and the six-fingered father of the child refused to
recognize the last one as his own.
The power of inheritance, moreover, shows itself very strikingly in the
formation and colour of the human skin and hair. It is well known how
exactly the nature of the complexion in many families—for instance, a
peculiar soft or rough skin, a peculiar luxuriance of the hair, a
peculiar colour and largeness of the eyes—is transmitted through many
generations. In like manner, peculiar local growths or spots on the
skin, the so-called moles, freckles, and other accumulations of pigment
which appear in certain places, are frequently transmitted through
several generations so exactly, that in the descendants they appear on
the same spots on which they existed in the parents. The porcupine men
of the Lambert family, who lived in London last century, are especially
celebrated. Edward Lambert, born in 1717, was remarkable for a most
unusual and monstrous formation of the skin. His whole body was covered
with a horny substance, about an inch thick, which rose in the form of
numerous thorn-shaped and scale-like processes, more than an inch long.
This monstrous formation of the outer skin, or epidermis, was
transmitted by Lambert to his sons and grandsons, but not to his
granddaughters. The transmission in this instance remained in the male
line, as is often the case. In like manner, an excessive development of
fat in certain parts of the body is often transmitted only in the
female line. I scarcely need call to mind how exactly the characteristic
formation of the face is transmitted by inheritance; sometimes it
remains within the male, sometimes within the female line; sometimes it
is blended in both.
The phenomena of transmission by inheritance of pathological conditions,
especially of the different forms of human diseases, are very
instructive and generally known. Diseases of the respiratory organs, the
glands, and of the nervous system, are specially liable to be
transmitted by inheritance. Very frequently there suddenly appears in an
otherwise healthy family a disease until then unknown among them; it is
produced by external causes, by conditions of life causing disease. This
disease, brought about in an individual by external cause, is propagated
and transmitted to his descendants, and some or all of them then suffer
from the same disease. In case of diseases of the lungs, for instance in
consumption, this sad transmission by inheritance is well known, and it
is the same with diseases of the liver, with syphilis, and diseases of
the mind. The latter are specially interesting. Just as peculiar
characteristic features of man—pride, ambition, frivolity, etc.—are
transmitted to the descendants strictly by inheritance, so too are the
peculiar abnormal manifestations of mental activity, which are usually
called fixed ideas, despondency, imbecility, and generally “diseases of
the mind.” This distinctly and irrefragably shows that the soul of man,
just as the soul of animals, is a purely mechanical activity, the sum of
the molecular phenomena of motion in the particles of the brain, and
that it is transmitted by inheritance, together with its substratum,
just as every other quality of the body is materially transmitted by
propagation.
When this exceedingly important and undeniable fact is mentioned, it
generally causes great offence, and yet in reality it is silently and
universally acknowledged. For upon what else do the ideas of “hereditary
sin,” “hereditary wisdom,” and “hereditary aristocracy,” etc., rest than
upon the conviction that the _quality of the human mind_ is transmitted
by propagation—that is, by a purely _material_ process—through the
body, from the parents to the descendants? The recognition of this great
importance of transmission by inheritance is shown in a number of human
institutions, as for example, among many nations in the division into
castes, such as the castes of warriors, castes of priests, and castes of
labourers, etc. It is evident that the institution of such castes
originally arose from the notion of the great importance of hereditary
distinctions possessed by certain families, which it was presumed would
always be transmitted by the parents to the children. The institution of
an hereditary aristocracy and an hereditary monarchy is to be traced to
the notion of such a transmission of special excellencies. However, it
is unfortunately not only virtues, but also vices that are transmitted
and accumulated by inheritance; and if, in the history of the world, we
compare the different individuals of the different dynasties, we shall
everywhere find a great number of proofs of the transmission of
qualities by inheritance, but fewer of transmissions of virtues than of
vices. Look only, for example, at the Roman emperors, at the Julii and
the Claudii, or at the Bourbons in France, Spain, and Italy!
In fact, scarcely anywhere could we find such a number of striking
examples of the remarkable transmission of bodily and mental features by
inheritance, as in the history of the reigning houses in hereditary
monarchies. This is specially true in regard to the diseases of the mind
previously mentioned. It is in reigning families that mental disorders
are hereditary in an unusual degree. Thus Esquirol, distinguished for
his knowledge of mental diseases, proved that the number of insane
individuals in the reigning houses was, in proportion to the number
among the ordinary population, as 60 to 1; that is, that disorders of
the brain occur 60 times more frequently in the privileged families of
the ruling houses than among ordinary people. If equally accurate
statistics were made of the hereditary nobility, the result would
probably be that here also we should find an incomparably larger
contingent of mental diseases than among the common, ignoble portion of
mankind. This phenomenon can scarcely astonish us if we consider what
injury these privileged castes inflict upon themselves by their
unnatural, one-sided education, and by their artificial separation from
the rest of mankind. By this means many dark sides of human nature are
specially developed and, as it were, artificially bred, and, according
to the laws of transmission by inheritance, are propagated through
series of generations with ever-increasing force and dominance.
It is sufficiently obvious from the history of nations how in successive
generations of many dynasties, for example, of the princes of Saxon
Thuringia and of the Medici, the noble solicitude for the most perfect
human accomplishments in science and art were retained and transmitted
from father to son; and how, on the other hand, in many other dynasties,
for centuries a special partiality for the profession of war, for the
oppression of human freedom, and for other rude acts of violence, have
been hereditary. In like manner talents for special mental activities
are transmitted in many families for generations, as, for instance,
talent for mathematics, poetry, music, sculpture, the investigation of
nature, philosophy, etc. In the family of Bach there have been no less
than twenty-two eminent musicians. Of course the transmission of such
peculiarities of mind depends upon the material process of reproduction,
as does the transmission of mental qualities in general. In this case
again, the vital phenomenon, the manifestation of force (as everywhere
in nature), is directly connected with definite relations in the
admixture of the material components of the organism. It is this
definite proportion and molecular motion of matter which is transmitted
by generation.
Now, before we examine the numerous, and in some cases most interesting
and important, laws of transmission by inheritance, let us make
ourselves acquainted with the actual nature of the process. The
phenomena of transmission by inheritance are generally looked upon as
something quite mysterious, as peculiar processes which cannot be
fathomed by natural science, and the causes and actual nature of which
cannot be understood. It is precisely in such a case that people very
generally assume supernatural influences. But even in the present state
of our physiology it can be proved with complete certainty that all the
phenomena of inheritance are entirely natural processes, that they are
produced by mechanical causes, and that they depend on the material
phenomena of motion in the bodies of organisms, which we may consider as
a part of the phenomena of propagation. All the phenomena of Heredity
and the laws of Transmission by Inheritance can be traced to the
material process of _Propagation_.
Every organism, every living individual, owes its existence _either_ to
an act of unparental or _Spontaneous Generation_ (Generatio Spontanea,
Archigonia), or to an act of Parental Generation or _Propagation_
(Generatio Parentalis, Tocogonia). In a future chapter we shall have to
consider Spontaneous Generation, or Archigony. At present we must occupy
ourselves with Propagation, or Tocogony, a closer examination of which
is of the utmost importance for understanding transmission by
inheritance. Most of my readers probably only know those phenomena of
Propagation which are seen universally in the higher plants and animals,
the processes of Sexual Propagation, or Amphigony. The processes of
Non-sexual Propagation, or Monogony, are much less generally known. The
latter, however, are far more suited to throw light upon the nature of
transmission by inheritance in connection with propagation.
For this reason, we shall first consider only the phenomena of
_non-sexual_ or _monogonic propagation_ (Monogonia). This appears in a
variety of different forms, as for example, self-division, formation of
buds, the formation of germ-cells or spores (Gen. Morph. ii. 36-58). It
will be most instructive, first, to examine the propagation of the
simplest organisms known to us, which we shall have to return to later,
when considering the question of spontaneous generation. These very
simplest of all organisms yet known, and which, at the same time, are
the simplest imaginable organisms, are the _Monera_ living in water;
they are very small living corpuscles, which, strictly speaking, do not
at all deserve the name of organism. For the designation “organism,”
applied to living creatures, rests upon the idea that every living
natural body is composed of organs, of various parts, which fit into
one another and work together (as do the different parts of an
artificial machine), in order to produce the action of the whole. During
late years we have become acquainted with _Monera_, organisms which are,
in fact, not composed of any organs at all, but consist entirely of
shapeless, simple, homogeneous matter. The entire body of one of these
Monera, during life, is nothing more than a shapeless, mobile, little
lump of mucus or slime, consisting of an albuminous combination of
carbon. Simpler or more imperfect organisms we cannot possibly conceive.
The first complete observations on the natural history of a Moneron
(Protogenes primordialis) were made by me at Nice, in 1864. Other very
remarkable Monera I examined later (1866) in Lanzarote, one of the
Canary Islands, and in 1867 in the Straits of Gibraltar. The complete
history of one of these Monera, the orange-red _Protomyxa aurantiaca_,
is represented in Plate I, and its explanation is given in the Appendix.
I have found some curious Monera also in the North Sea, off the
Norwegian coast, near Bergen. Cienkowski has described (1865) an
interesting Moneron from fresh waters, under the name of _Vampyrella_.
But perhaps the most remarkable of all Monera was discovered by Huxley,
the celebrated English zoologist, and called _Bathybius Hæckelii_.
“Bathybius” means, living in the deep. This wonderful organism lives in
immense depths of the ocean, which are over 12,000—indeed, in some
parts 24,000 feet below the surface, and which have become known to us
within the last ten years, through the laborious investigations made by
the English. There, among the numerous Polythalamia and Radiolaria
which inhabit the fine calcareous mud of these abysses, the Bathybius is
found in great quantities, sometimes in the shape of roundish, formless
lumps of mucus, sometimes in the form of a network of mucus, covering
fragments of stone and other objects. Small particles of chalk are
frequently embedded in these mucous gelatinous masses, and are, perhaps,
products of their secretion. The entire body of this remarkable
Bathybius consists solely of shapeless plasma, or protoplasm, as in the
case of the other Monera—that is, it consists of the same albuminous
combination of carbon, which in infinite modifications is found in all
organisms, as the essential and never-failing seat of the phenomena of
life. I have given a detailed description and drawing of the Bathybius
and other Monera in my “Monographie der Moneren,” 1870,(15) from which
the drawing in Fig. 9 is taken.
[Illustration: Life history of a simplest organism. Pl. I.
E. Haeckel del. _Prototmyxa aurantiaca._ Lagesse sc.]
In a state of rest most Monera appear as small globules of mucus or
slime, invisible, or nearly so, to the naked eye; they are at most as
large as a pin’s head. When the Moneron moves itself, there are formed
on the upper surface of the little mucous globule, shapeless, fingerlike
processes, or very fine radiated threads; these are the so-called false
feet, or pseudopodia. The false feet are simple, direct continuations of
the shapeless albuminous mass, of which the whole body consists. We are
unable to perceive different parts in it, and we can give a direct proof
of the absolute simplicity of the semi-fluid mass of albumen, for with
the aid of the microscope we can follow the Moneron as it takes in
nourishment. When small particles suited for its nourishment—for
instance, small particles of decayed organic bodies or microscopic
plants and infusoria—accidentally come into contact with the Moneron,
they remain hanging to the sticky semi-fluid globule of mucus, and here
create an irritation, which is followed by a strong afflux of the mucous
substance, and, in consequence, they become finally completely inclosed
by it, or are drawn into the body of the Moneron by displacement of the
several albuminous particles, and are there digested, being absorbed by
simple diffusion (endosmosis).
[Illustration: FIG. 1.—Propagation of the simplest organism, a Moneron,
by self-division. _A._ The entire Moneron, a Protamœba. _B._ It falls
into two halves by a contraction in the middle. _C._ Each of the two
halves has separated from the other, and now represents an independent
individual.]
Just as simple as the process of nutrition is the _propagation_ of these
primitive creatures, which in reality we can neither call animals nor
plants. All Monera propagate themselves only in an asexual manner by
monogony; and in the simplest case, by that kind of monogony which we
place at the head of the different forms of propagation, that is, by
self-division. When such a little globule, for example a Protamœba or
a Protogenes, has attained a certain size by the assimilation of foreign
albuminous matter, it falls into two pieces; a pinching in takes place,
contracting the middle of the globule on all sides, and finally leads to
the separation of the two halves (compare Fig. 1). Each half then
becomes rounded off, and now appears as an independent individual, which
commences anew the simple course of the vital phenomena of nutrition and
propagation. In other Monera (Vampyrella), the body in the process of
propagation does not fall into two, but into four equal pieces, and in
others, again (Protomonas, Protomyxa, Myxastrum), at once into a number
of small globules of mucus, each of which again, by simple growth,
becomes like the parent body. Here it is evident that the process of
_propagation is nothing but a growth of the organism beyond its own
individual limit of size_.
The simple method of propagation of the Moneron by self-division is, in
reality, the most universal and most widely spread of all the different
modes of propagation; for by the same simple process of division,
_cells_ also propagate themselves. Cells are those simple organic
individuals, a large number of which constitute the bodies of most
organisms, the human body not excepted. With the exception of the
organisms of the lowest order, which have not even the perfect form of a
cell (Monera), or during life only represent a single cell (many
Protista and single-celled plants), the body of every organic individual
is composed of a great number of cells. Every organic cell is to a
certain degree an independent organism, a so-called “elementary
organism,” or an “individual of the first order.” Every higher organism
is, in a measure, a society or a state of such variously shaped
elementary individuals, variously developed by division of labour.(39)
Originally every organic cell is only a single globule of mucus, like a
Moneron, but differing from it in the fact that the homogeneous
albuminous substance has separated itself into two different parts, a
firmer albuminous body, the _cell-kernel_ (nucleus), and an external,
softer albuminous body, the _cell-substance_ or _body_ (protoplasma).
Besides this, many cells later on form a third (frequently absent)
distinct part, inasmuch as they cover themselves with a capsule, by
exuding an outer pellicle or _cell-membrane_ (membrana). All other forms
of cells, besides these, are of subordinate importance, and are of no
further interest to us here.
Every organism composed of many cells was originally a single cell, and
it becomes many-celled owing to the fact that the original cell
propagates itself by self-division, and that the new individual cells
originating in this manner remain together, and by division of labour
form a community or a state. The forms and vital phenomena of all
many-celled organisms are merely the effect or the expression of all the
forms and vital phenomena of all the individual cells of which they are
composed. The egg, from which most animals and plants are developed, is
a simple cell.
[Illustration: FIG. 2.—Propagation of a single-celled organism,
Amœba sphærococcus, by self-division. _A._ The enclosed Amœba, a
simple globular cell consisting of a lump of protoplasm (_c_), which
contains a kernel (_b_) and a kernel speck (_a_), and is surrounded by a
cell-membrane or capsule. _B._ The free Amœba, which has burst and
left the cyst or cell-membrane. _C._ It begins to divide by its kernel
forming two kernels, and by the cell-substance between the two becoming
contracted. _D._ The division is completed by the cell-substance
likewise falling into two halves (_Da_ and _Db_).]
The single-celled organisms, that is, those which during life retain the
form of a single cell, for example the Amœbæ, as a rule propagate
themselves in the simplest way by self-division. This process differs
from the previously described self-division of the Moneron only in the
fact that at the commencement the firmer cell-kernel (nucleus) falls
into two halves, by a pinching in at its middle. The two young kernels
separate from each other and act now as two distinct centres of
attraction upon the surrounding softer albuminous matter, that is, the
cell-substance (protoplasma). By this process finally the latter also
divides into two halves, and there now exist two new cells, which are
like the mother cell. If the cell was surrounded by a membrane, this
either does not divide at all, as in the case of egg-cleavage (Fig. 3,
4), or it passively follows the active pinching in of the protoplasm;
or, lastly, every new cell exudes a new membrane for itself.
[Illustration: FIG. 3.—Egg of a mammal (a simple cell). _a._ The small
kernel speck or nucleolus (the so-called germ-spot of the egg). _b._
Kernel or nucleus (the so-called germ-bladder of the egg). _c._
Cell-substance or protoplasm (the so-called yolk of the egg). _d._
Cell-capsule or membrane (membrane of the yolk) of the egg; called in
mammals, on account of its transparency, Membrana pellucida.]
The non-independent cells which remain united in communities or states,
and thus constitute the body of higher organisms, are propagated in the
same manner as are independent single-celled organisms, for example,
Amœba (Fig. 2). Just as in that case, the cell with which most
animals and plants commence their individual existence, namely, the egg,
multiplies itself by simple division. When an animal, for instance a
mammal (Fig. 3, 4), develops out of an egg, this process of development
always begins by the simple egg-cell (Fig. 3) forming an accumulation of
cells (Fig. 4) by continued self-division. The outer covering, or cell
membrane, of the globular egg remains undivided. First, the cell-kernel
of the egg (the so-called germinal vesicle) divides itself into two
kernels, then follows the cell-substance (the yolk of the egg) (Fig. 4
_A_). In like manner, the two cells, by continued self-division,
separate into four (Fig. 4 _B_), these into eight (Fig. 4 _C_), into
sixteen, thirty-two, etc., and finally there is produced a globular mass
of very numerous little cells (Fig. 4 _D_). These now, by further
increase and heterogeneous development (division of labour), gradually
build up the compound many-celled organism. Every one of us, at the
commencement of our individual development, has undergone the very same
process as that represented in Fig. 4. The egg of a mammal—represented
in Fig. 3, and its development in Fig. 4—might as well be that of a
man, as of an ape, dog, horse, or any other placental mammal.
[Illustration: FIG. 4.—First commencement of the development of a
mammal’s egg, the so-called “cleavage of the egg” (propagation of the
egg-cell by repeated self-division). _A._ The egg, by the formation of
the first furrow, falls into two cells. _B._ These separate by division
into four cells. _C._ The latter have divided into eight cells. _D._ By
repeated division a globular accumulation of numerous cells has
arisen.]
Now, when one examines this simplest form of propagation, this
self-division, it surely cannot be considered wonderful that the
products of the division of the original organism should possess the
same qualities as the parental individual. For they are parts or halves
of the parental organism, and the matter or substance in both halves is
the same, and as both the young individuals have received an equal
amount and the same quality of matter from the parent individual, one
can but consider it natural that the vital phenomena, the physiological
qualities should be the same in both children. In fact, in regard to
their form and substance, as well as to their vital phenomena, the two
produced cells can in no respect be distinguished from one another, or
from the mother cell. They have _inherited_ from her the same nature.
But this same simple propagation by self-division is not only confined
to simple cells—it is the same also in the higher many-celled
organisms; for example, in the coral zoophytes. Many of them which
exhibit a high complexity of composition and organization, nevertheless,
propagate themselves by simple division. In this case the whole
organism, with all its organs, falls into two equal halves as soon as by
growth it has attained a certain size. Each half again develops itself,
by growth, into a complete individual. Here, again, it is surely
self-evident that the two products of division will share the qualities
of the parental organism, as they themselves are in fact halves of that
parent.
Next to propagation by division we come to propagation by the _formation
of buds_. This kind of monogony is exceedingly widely spread. It occurs
both in the case of simple cells (though not frequently) and in the
higher organisms composed of many cells. The formation of buds is
universal in the vegetable kingdom, less frequent in the animal kingdom.
However, here also it occurs in the tribe of Plant-like Animals,
especially among the Coral Zoophytes, and among the greater portion of
the Hydroid Polyps very frequently, further also among some worms
(Planarian Worms, Ring-Worms, Moss Animals, Tunicates). Most branching
animal-trees or colonies, which are exceedingly like branching plants,
arise like those plants, by the formation of buds.
Propagation by the _formation of buds_ (Gemmatio) is essentially
distinguished from propagation by division, in the fact that the two
organisms thus produced by budding are not of equal age, and therefore
at first are not of equal value, as they are in the case of division. In
division we cannot clearly distinguish either of the two newly produced
individuals as the parental, that is as the producer, because, in fact,
both have an equal share in the composition of the original parental
individual. If, on the other hand, an organism sends out a bud, then the
latter is the child of the former. The two individuals are of unequal
size and of unequal form. If, for instance, a cell propagates itself by
the formation of buds, we do not see the cell fall into two equal
halves, but there appears at one point of it a protuberance, which
becomes larger and larger, more or less separates itself from the
parental cell, and then grows independently. In like manner we observe
in the budding of a plant or animal, that a small local growth arises on
a part of the mature individual, which growth becomes larger and larger,
and likewise more or less separates itself from the parental organism by
an independence in its growth. The bud, after it has attained a certain
size, may either completely separate itself from the parental
individual, or it may remain connected with it and form a stock or
colony, whilst at the same time its life may be quite independent of
that of its parent. While the growth which starts the propagation, in
the case of self-division, is a total one affecting the whole body, it
is in the formation of buds only partial, affecting merely a portion of
the parental organism. But here, also, the bud—the newly-produced
individual which remains so long most directly connected with the
parental organism, and which proceeds from it—retains the essential
qualities and the original tendency of development of its parent.
A third mode of non-sexual propagation, that of the _formation of
germ-buds_ (Polysporogonia), is intimately connected with the formation
of buds. In the case of the lower, imperfect organisms, among animals,
especially in the case of the Plant-like animals and Worms, we very
frequently find that in the interior of an individual composed of many
cells, a small group of cells separates itself from those surrounding
it, and that this small isolated group gradually develops itself into
an individual, which, becomes like the parent, and sooner or later comes
out of it. Thus, for example, in the body of the Fluke-worms
(Trematodes) there often arise numerous little bodies consisting of many
cells, that is _germ-buds_, or _polyspores_, which, at an early stage
separate themselves completely from the parent body, and leave it when
they have attained a certain stage of development.
The formation of germ-buds is evidently but little different from real
budding. But, on the other hand, it is connected with a fourth kind of
non-sexual propagation, which almost forms a transition to sexual
reproduction, namely, the _formation of germ-cells_ (Monosporogonia),
which is often briefly called formation of spores (sporogonia). In this
case it is no longer a group of cells, but a single cell, which
separates itself from the surrounding cells in the interior of the
producing organism, and which only becomes further developed after it
has come out of its parent. After this _germ-cell_, or monospore (or,
briefly, spore), has left the parental individual, it multiplies by
division, and thus forms a many-celled organism, which by growth and
gradual development attains the hereditary qualities of the parental
organism. This occurs very generally among lower plants (Cryptogama).
Although the formation of germ-cells very much resembles the formation
of germ buds, it evidently and very essentially differs from the latter,
and also from the other forms of non-sexual propagation which have
previously been mentioned, by the fact that only a very small portion of
the producing organism takes part in the propagation and, accordingly,
in the transmission by inheritance. In the case of self-division, where
the whole organism falls into two halves, in the formation of buds,
where a considerable portion of the whole body, already more or less
developed, separates from the producing individual, we easily understand
that the forms and vital phenomena should be the same in the producing
and produced organism. It is much more difficult to understand in the
formation of germ-buds, and more difficult still in the formation of
germ-cells, how this very small, quite undeveloped portion of the body,
this group of cells, or this single cell, not only directly takes with
it certain parental qualities into its independent existence, but also
after its separation from the parental individual develops into a
many-celled body, and in this repeats the forms and vital phenomena of
the original producing organism. This last form of monogonic
propagation—that of the germ cells, or spore-formation—leads us
directly to a form of propagation which is the most difficult of all to
explain, namely, sexual propagation.
_Sexual or amphigonic propagation_ (Amphigonia) is the usual method of
propagation among all higher animals and plants. It is evident that it
has only developed, at a very late period of the earth’s history, from
non-sexual propagation, and apparently in the first instance from the
method of propagation by germ-cells. In the earliest periods of the
organic history of the earth, all organisms propagated themselves in a
non-sexual manner, as numerous lower organisms still do, especially all
those which are at the lowest stage of organization, and which, strictly
speaking, can be considered neither as animals nor as plants, and which
therefore, as primary creatures, or Protista, are best excluded from
both the animal and vegetable kingdoms. In the case of the higher
animals and plants, the increase of individuals, as a rule, is at
present brought about in the majority of cases by sexual propagation.
In all the chief forms of non-sexual propagation mentioned above—in
fission, in the formation of buds, germ buds, and germ cells—the
separated cell or group of cells was able by itself to develop into a
new individual, but in the case of sexual propagation the cell must
first be fructified by another generative substance. The fructifying
male sperm must first mix with the female germ-cell (the egg) before the
latter can develop into a new individual. These two different
generative substances, the male sperm and the female egg, are either
produced by one and the same individual hermaphrodite
(Hermaphroditismus), or by two different individuals (sexual separation,
Gonochorismus) (Gen. Morph. ii. 58, 59).
The simpler and more ancient form of sexual propagation is through
double-sexed individuals (Hermaphroditismus). It occurs in the great
majority of plants, but only in a minority of animals, for example, in
the garden snails, leeches, earth-worms, and many other worms. Every
single individual among hermaphrodites produces within itself materials
of both sexes—eggs and sperm. In most of the higher plants every
blossom contains both the male organ (stamens and anther) and the female
organs (style and germ). Every garden snail produces in one part of its
sexual gland eggs, and in another part sperm. Many hermaphrodites can
fructify themselves; in others, however, copulation and reciprocal
fructification of both hermaphrodites is necessary for causing the
development of the eggs. This latter case is evidently a transition to
sexual separation.
_Sexual separation_ (Gonochorismus,) which characterizes the more
complicated of the two kinds of sexual reproduction, has evidently been
developed from the condition of hermaphroditism at a late period of the
organic history of the world. It is at present the universal method of
propagation of the higher animals, and occurs, on the other hand, only
in the minority of plants (for example, in many aquatic plants, _e.g._
Hydrocharis, Vallisneria; and in trees, _e.g._ Willows, Poplars). Every
organic individual, as a non-hermaphrodite (Gonochoristus), produces
within itself only one of two generative substances, either the male or
the female. The female individuals, both in animals and plants, produce
eggs or egg-cells. The eggs of plants in the case of flowering plants
(Phanerogama), are commonly called “embryo sacs”; in the case of
flowerless plants (Cryptogama), “fruit spores.” In animals, the male
individual secretes the fructifying sperm (sperma); in plants, the
corpuscles, which correspond to the sperm. In the Phanerogama, these are
the pollen grains, or flower-dust; in the Cryptogama, a sperm, which,
like that of most animals, consists of floating vibratile cells actively
moving in a fluid—the zoosperms, spermatozoa, or sperm-cells.
The so-called _virginal reproduction_ (Parthenogenesis) offers an
interesting form of transition from sexual reproduction to the
non-sexual formation of germ-cells (which most resembles it); it has
been demonstrated to occur in many cases among Insects, especially by
Siebold’s excellent investigations. In this case germ-cells, which
otherwise appear and are formed exactly like egg-cells, become capable
of developing themselves into new individuals without requiring the
fructifying seed. The most remarkable and most instructive of the
different partheno-genetic phenomena are furnished by those cases in
which the same germ-cells, according as they are fructified or not,
produce different kinds of individuals. Among our common honey bees, a
male individual (a drone) arises out of the eggs of the queen, if the
egg has not been fructified; a female (a queen, or working bee), if the
egg has been fructified. It is evident from this, that in reality there
exists no wide chasm between sexual and non-sexual reproduction, but
that both modes of reproduction are directly connected. The
parthenogenesis of Insects must probably be regarded as a _relapse_ from
the sexual mode of propagation (possessed by the original parents of the
insects) to the earlier condition of non-sexual propagation. (Gen.
Morph. ii. 86.) In any case, however, sexual reproduction, both in
plants and animals, which seems such a wonderful process, has only
arisen at a later date out of the more ancient process of non-sexual
reproduction. In both cases heredity is a necessary part of the
phenomenon.
In all the different modes of propagation the essential point of the
process is invariably a detachment of a portion of the parental organism
possessing the capability of leading an individual, independent
existence. We may, therefore, in all cases expect, _à priori_, that the
produced individuals—which are, in fact, as is commonly said, “the
flesh and blood” of the parents—will receive the vital characteristics
and qualities of form which the parental individuals possess. It is
simply a larger or smaller quantity of the parental material, in fact of
its albuminous protoplasm, or cell-substance, which passes to the
produced individual. But together with the material, its vital
properties—that is, the molecular motions of the plasma—are
transmitted, which then manifest themselves in its form. Inheritance by
sexual breeding loses very much of the mysterious and wonderful
character which it at first sight possesses for the uninitiated, if we
consider the above-mentioned series of the different modes of
propagation, and their connection one with another. It at first appears
exceedingly wonderful that in the sexual propagation of man, and of all
higher animals, the small egg, the minute cell, often invisible to the
naked eye, is able to transfer to the produced organism all the
qualities of the maternal organism, and, no less mysterious, that at
the same time the essential qualities of the paternal organism are
transferred to the offspring by means of the male sperm, which
fructifies the egg-cell by means of a viscid substance in which minute
thread-like cells or zoosperms move about. But as soon as we compare the
connected stages of the different kinds of propagation, in which the
produced organism separates itself more and more as a distinct growth
from the parental individual, and more or less early enters upon its
independent career; as soon as we consider, at the same time, that the
growth and development of every higher organism only depends upon the
increase of the cells composing it—that is, upon their simple
propagation by division—it becomes quite evident that all these
remarkable processes belong to one series.
The life of every organic individual is nothing but a connected chain of
very complicated material phenomena of motion. These motions must be
considered as changes in the position and combination of the molecules,
that is, of the smallest particles of animated matter (of atoms placed
together in the most varied manner). The specific, definite tendency of
these orderly, continuous, and inherent motions of life depends, in
every organism, upon the chemical mingling of the albuminous generative
matter to which it owes its origin. In man, as in the case of the higher
animals which propagate themselves in a sexual manner, the individual
vital motion commences at the moment in which the egg-cell is fructified
by the spermatic filaments of the seed, in which process both generative
substances actually mix; and here the tendency of the vital motion is
determined by the specific, or more accurately, by the individual
nature of the sperm as well as of the egg. There can be no doubt as to
the purely mechanical material nature of this process. But here we stand
full of wonder and astonishment before the infinite and inconceivable
delicacy of this albuminous matter. We are amazed at the undeniable fact
that the simple egg-cell of the maternal organism, and a single paternal
sperm-thread, transfer the molecular individual vital motion of these
two individuals to the child so accurately, that afterwards the minutest
bodily and mental peculiarities of both parents reappear in it.
Here we stand before a mechanical phenomenon of nature of which Virchow,
whose genius founded the “cellular pathology,” says with full justice:
“If the naturalist cared to follow the custom of historians and
preachers, and to clothe phenomena, which are in their way unique, with
the hollow pomp of ponderous and sounding words, this would be the
opportunity for him; for we have now approached one of those great
mysteries of animal nature, which encircle the region of animal life as
opposed to all the rest of the world of phenomena. The question of the
formation of cells, the question of the excitation of a continuous and
equable motion, and, finally, the questions of the independence of the
nervous system and of the soul—these are the great problems on which
the human mind can measure its strength.” To comprehend the relation of
the male and female to the egg-cell is almost as much as to solve all
those mysteries. The origin and development of the egg-cell in the
mother’s body, the transmission of the bodily and mental peculiarities
of the father to it by his seed, touch upon all the questions which the
human mind has ever raised about man’s existence. And, we add, these
most important questions are solved, by means of the Theory of Descent,
in a purely mechanical and purely monistic sense!
There can then be no further doubt that, in the sexual propagation of
man and all higher organisms, inheritance, which is a purely mechanical
process, is directly dependent upon the material continuity of the
producing and produced organism, just as is the case in the simplest
non-sexual propagation of the lower organisms. However, I must at once
take this opportunity of drawing attention to an important difference
which inheritance presents in sexual and non-sexual propagation. It is a
fact long since acknowledged, that the individual peculiarities of the
producing organism are much more accurately transmitted to the produced
organism by non-sexual than by sexual propagation. Gardeners have for a
long time made use of this fact in many ways. When, for instance, a
single individual of a species of tree with stiff, upright branches
accidentally produces down-hanging branches, a gardener, as a rule,
cannot transmit this peculiarity by sexual, but only by non-sexual
propagation. The twigs cut off such a weeping tree and planted as
cuttings or slips, afterwards produce trees having likewise hanging
branches, as, for example, the weeping willows and beeches. Seedlings,
on the other hand, which have been reared out of the seed of such a
weeping tree, generally have the original stiff and upright form of
branches possessed by their ancestors. The same may be observed in a
very striking manner in the so-called “copper-coloured trees,” that is,
varieties of trees which are characterized by a red or reddish brown
colour of the leaves. Off-shoots from such copper-coloured trees (for
example, the copper beech), which have been propagated by cuttings in a
non-sexual manner, show the peculiar colour and nature of the leaves
which distinguished the parental individual, while others reared from
seeds of such a copper-coloured tree return to the green-coloured
condition of leaf.
This difference in inheritance will seem very natural when we consider
that the material connection between the producing and produced
individuals is much closer and lasts much longer in non-sexual than in
sexual propagation. The special tendency of the molecular motion of life
can therefore fix itself much longer and more thoroughly in the filial
organism, and be more strictly transmitted by non-sexual than by sexual
propagation. All these phenomena, considered in connection, clearly
prove that the transmission of bodily and mental peculiarities is a
purely material and mechanical process. By propagation a greater or
lesser quantity of albuminous particles, and together with them the
individual form of motion inherent in these molecules of protoplasm, are
transmitted from the parental organism to the offspring. As this form of
motion remains continuous, the more delicate peculiarities inherent in
the parental organism must sooner or later reappear in the filial
organism.