University of Virginia Library

GENETIC CONTINUITY

All Life from Life of Its Own Kind. By genetic
continuity we mean not only that all life comes from
life (the “Law of Biogenesis”), but more particularly
that each organism comes from one or two parents
of its own species. It thus inherits its characteristics
in unbroken lineage from its ancestors, to the beginning
of its species on earth, and, if we accept the Theory
of Evolution, to the beginnings of all life on earth.

That living beings come from parents of their own
kind is an observation as old as man, but the conviction
that they can arise only in that manner was for ages
in dispute. The book of Genesis, in relating the Crea-
tion Story, says that each creature brought forth “ac-
cording to its kind.” Aristotle, writing in the fourth
century B.C., is more specific. In the Generation of
Animals (Loeb Classical Library, 747b 30-35), he
wrote: “In the normal course of nature the offspring
which a male and a female of the same species produce
is a male or female of that same species—for instance,
the offspring of a male dog and a female dog is a male
dog or a female dog.” Yet the Bible affords witness
of the common belief that the lower orders of life could
be generated otherwise, as when Samson found “bees”
in the carcass of a lion he had killed. Aristotle, too,
believed in the spontaneous generation of living things,
for in the History of Animals he says:

(trans. D'Arcy Thompson, Book
V, 539a 16-26).

Before men could accept the view that heredity
results from a biological mechanism of some sort, the
ghost of spontaneous generation had to be laid. For
Aristotle, the greatest biologist of ancient times, and
for all those who followed his ideas so unhesitatingly
until the year 1600 or later, the pattern of development
was accounted for by the Final Cause and the Formal
Cause, the former being the End for which the orga-
nism exists, and the latter being the logos, or essential
nature of the organism. Of the Greek philosopher's four
causes, the Material Cause was supplied by the female
parent and the Motive (Efficient) Cause was supplied
by the male parent. These two Causes supply substance
and energy; but there is no indication that the Formal
Cause is in any way transmitted from the parents. It
is more allied to the Final Cause, and exists in the very
nature of things. Hence, given the presence of the
proper Formal and Final Causes, a particular animal
might just as readily originate from slime or filth or
decaying matter as from the substance and energizing
force provided by parents of the same species.

The scientific disproof of the idea of spontaneous
generation required a series of investigations extending
over two centuries, beginning with the experiments of
Francesco Redi in 1668 and ending with those of Louis
Pasteur in 1860-64. Redi succeeded in showing that
blowflies lay the eggs from which maggots develop in
putrefying meat, and that in the absence of the eggs
no maggots, and subsequently no flies, make their ap-
pearance, even though the meat decays. The method
was simple, and affords a fine example of a scientific
experiment involving a control. Some vessels contain-
ing meat of various kinds were left open; others were
closed with paper and sealed. In the former the flies
laid eggs, and in due course the maggots made their
appearance; the sealed vessels remained free of
“worms.” Later, in order to answer the objection that
the sealing of the vessels might have prevented free
access of air, Redi performed other controlled experi-
ments in which some of the vessels were covered with
fine Naples netting, that would admit air but exclude
flies. In some experiments a double protection was
provided by adding a second shelter of net. Flies laid
eggs on the meshes of the cloth and the eggs developed
into maggots, but if the mesh was fine enough to keep
them from dropping through, not a single worm ap-
peared in the putrid meat.

Even so, Redi retained a belief that in certain other
cases—the origin of parasites inside the human or
animal body or of grubs inside of oak galls—there must
be spontaneous generation. Bit by bit the evidence
grew against such views. In 1670 Jan Swammerdam,
painstaking student of the insect's life cycle, suggested
that the grubs in galls were enclosed in them for the
sake of nourishment and must come from insects that
had inserted their semen or their eggs into the plants.
In 1687 Antony (Antonij) van Leeuwenhoek, in one
of his famous letters to the newly founded Royal Soci-
ety in London, described how a surgeon brought to
him some excised tissues from the leg of a patient. The
tissue had in it worms that the surgeon thought had
originated spontaneously. Leeuwenhoek readily rec-
ognized them as being insect larvae, removed them
to a piece of beef, found they grew and transformed
into pupae, and eventually hatched into flies. These,
having mated, produced fertile eggs from which mag-
gots like the original ones soon developed. Leeuwen-
hoek, although he performed no critical experiments
to test his belief, strongly denied that any of the micro-
scopic protozoans and bacteria he had discovered arose
spontaneously. “No creature takes birth without gen-
eration,” he wrote in 1694 (Letter 83).

Meanwhile (1700-11), Antonio Vallisneri, who was
a student of the great anatomist Marcello Malpighi
(1628-94), turned his attention to the nature of plant
galls, and proved that Swammerdam had been entirely
correct in his conjecture. Galls indeed arise from the
stinging of the plant tissues by the ovipositors of female
gall wasps, and the egg laid in the plant tissues develops
inside the gall into a grub, which eventually emerges
full-grown and transformed into a mature gall wasp.
Although the mystery of the generation of intestinal
worms and the muscle-embedded cysticerci of tape-
worms was not to be solved until 1832, it may fairly
be said that Redi, Leeuwenhoek, Swammerdam,
Malpighi, and Vallisneri wrought a revolution in bio-
logical thought hardly second to that of the nine-
teenth-century theory of organic evolution. The belief
in spontaneous generation, though still held by com-
mon folk and by some scientists, was disproved in the
main and was suspect in entirety. Genetic continuity
was established as the normal if not the only pattern
of life. Young developing organisms grow into adults
like their parents because they have the parents they
do. Presumably, then, they inherit some material basis
that holds them to the pattern of development that
is characteristic of their own species. A new question
began to arise: what might this material basis of genetic
continuity be?

By 1711, when Vallisneri was completing his studies
on the gall wasps, a French biologist, Louis Joblot, was
undertaking to test Leeuwenhoek's belief that even
protozoans and bacteria arise from parents of their own
kind. He prepared a boiled hay infusion, in which these
organisms commonly appear. Some vessels were cov-
ered with parchment, others were left uncovered. After
several days the microorganisms appeared in vast
numbers in the infusions left exposed, but not in the

closed ones. There was much talk of “vital forces” in
those days, so to avoid the criticism that by closing
the vessels the hay infusion in them had lost some vital
force, Joblot after a time removed the coverings from
the closed vessels. These were soon teeming with mi-
croorganisms.

Later experimenters, especially John Turberville
Needham, were not satisfied. Needham repeated this
type of experiment many times (1748-50), using boiled
mutton gravy and infusions of boiled seeds. He used
corks to close his flasks. The results: bacteria appeared
in the corked vessels as well as in the open ones.
Spontaneous generation, at least for bacteria, thus
remained an unsettled question. Later in the century,
in 1765, the Abbé Lazzaro Spallanzani, perhaps the
greatest experimental biologist of his time, reinvesti-
gated Needham's results by more refined methods. He
found that infusions of seeds, even when most carefully
sealed, had to be boiled a long time (e.g., 45 minutes)
to remain free of microbial growth. Needham had used
corks sealed with mastic, and had merely set his flasks
by the fire at a temperature he thought sufficient to
kill all organisms. Spallanzani used glass flasks with
slender necks that could be fused in a flame and were
thus sealed hermetically beyond all doubt. The flasks
containing infusion were then immersed in boiling
water for 45 minutes. His sealed flasks remained clear
and free of organisms; the controls became turbid with
bacterial growth. Still the argument was not settled.
Needham maintained that the severe heating had de-
stroyed the capacity of the infusions to support life.
Spallanzani triumphantly broke the fused necks of the
flasks and showed that bacterial growth promptly oc-
curred in them. Then Needham maintained, and quite
correctly, that the heating led to the expansion of the
air in the flasks prior to the fusion of the necks, and
that after cooling there would consequently be a low
pressure or partial vacuum in the flasks. When one
broke the necks of the sealed flasks one could actually
hear the whistle of the entering air. Air is necessary
for the generation of life, claimed Needham, and Spal-
lanzani's experiments were therefore not conclusive.
There the matter rested for the time being.

When microscopes with achromatic lenses became
available in the 1830's, and good resolution at a mag-
nification of 400 diameters was possible, interest fo-
cused on the globules always to be seen in fermenting
liquors. The earliest conception of the nature of fer-
mentation, from Antoine Lavoisier through J. J. Berze-
lius to Justus von Liebig, was that it was a strictly
chemical process. Then, in 1835 to 1838, Charles
Cagniard de Latour and Theodor Schwann inde-
pendently reported that alcoholic fermentation is in-
variably associated with, and depends upon, the pres
ence of microscopic yeast cells. These were capable
of reproduction and were identified as plant cells. They
caused the fermentation of sugar only when they were
alive, for Schwann showed that boiling killed them and
that neither fermentation nor putrefaction occurred
after boiling, if all air admitted to the vessel was heated
prior to entry. In similar experiments F. F. Schulze
used sulfuric acid to purify the air entering the flasks;
and in 1854 H. G. F. Schröder and T. von Dusch
introduced the use of plugs of cotton wool, which
proved effective in excluding dust and bacteria by
mechanically filtering the air admitted to the sterile
flasks. The chemists J. J. Berzelius, Friedrich Wöhler,
and J. Liebig were not satisfied. Heat, strong chemicals,
or even mechanical filtration might in some way de-
nature the air. Liebig admitted that yeast played a role,
but he insisted that the fermentation was brought about
by some soluble substance formed through decomposi-
tion. Louis Pasteur, from 1857 to 1860, disputed with
Liebig the issue of a vital versus a purely chemical
character of fermentation.

At this time the bacteriologist F. A. Pouchet claimed
that he had actually demonstrated the spontaneous
origin of microorganisms during fermentation and
putrefaction. Pasteur set himself to reexamine the bases
of the ancient controversy. From 1861 to 1864 he
conducted his crucial experiments. He made micro-
scopic observations of particles trapped from the air
and showed that there were many bodies capable of
living growth floating in it. He confirmed Schwann's
experiments with heated air. Most convincingly, he
made flasks with long S-curved necks open to the air
at the tips, and demonstrated that liquid media capable
of supporting bacterial growth will remain sterile in
such flasks after boiling, unless even so little as a drop
flows into the final curve of the flask's neck, where
dust might have collected, and is then permitted to
flow back into the body of the flask. He examined the
air on a glacier high on Mont Blanc and found it to
be free of floating bacteria. Some of these flasks, with
their contents still sterile, are preserved to this day in
the Pasteur Institute in Paris. Similar flasks, exposed
to the air of the city, became heavily contaminated.
Even blood remained sterile when collected with suffi-
cient precautions to exclude bacterial pollution.

On the other hand, Pasteur's methods of sterilization
by means of a single exposure to boiling temperature
did not always prove effective; and Pouchet, who used
hay infusions rather than nutritive broth as a medium,
would have won his point—at least for a time—had
he not lost his courage or his conviction. John Tyndall
in 1877 studied the phenomenon just described, and
found that by boiling for intermittent periods of not
longer than a minute at intervals of 12 hours, sterili-

zation could be obtained even in cases where a single
boiling was ineffective. He was thus led to postulate
the existence of highly resistant “germs.” Ferdinand
Cohn, using similar methods, discovered the formation
of spores by Bacillus subtilis in hay infusions, and then
demonstrated that a single boiling will not kill the
spores but that, after these have once germinated, even
a very short exposure to a high temperature will kill
all the organisms present. Tyndall, who was a physicist,
also used optical methods to demonstrate that there
is dust in even the stillest air—and asserted that where
there is dust there are germs.

The establishment of the Germ Theory of Disease
is thus intimately related with the final establishment
of the fact of Genetic Continuity. But the fact that
there is genetic continuity only raises the question of
its mechanism. The eighteenth-century preformation-
ists, of whom Spallanzani was one and his friend
Charles Bonnet another, were the avowed mechanists
of their day. To them the idea that nutritive or heredi-
tary particles, derived either from the environment or
from an organism's parents, could of their own accord
become organized into all the complexity of a living
being was preposterous. Something preorganized must
itself be transmitted from parents (or parent) to off-
spring, to serve as a substructure and guide in the
course of development. The preformationists—who
were in the great majority among eighteenth-century
biologists—were thus convinced that either the ovum
or the sperm contains the germ of the future being,
just as one finds a small embryo plant within a seed.
To some preformationists this conviction meant the
presence of a little homunculus within the head of the
sperm, while the female parent would supply only
nutriment for the growth of the next generation. To
others, the ovists, the germ or embryo lay in the egg,
and the semen or sperm of the male merely activated
its development. The more sophisticated of the pre-
formationists, such as Bonnet, though at first charmed
by the idea of the infinite, or nearly infinite, array of
embryos within embryos going back to Mother Eve
or to the first female of every other species, never-
theless admitted in the face of the evidence of repro-
duction by budding that such a concept was too crude.

It was in particular the consideration of the forma-
tion of buds by Hydra, the little freshwater polyp
discovered by his cousin Abraham Trembley, that
forced Bonnet to a more general conclusion. The
hydra's bud can form anywhere on its body and it
clearly does not contain parts within it, like the bud
of a plant, all ready to expand and unfold. It is a mere
bump, an excrescence. Yet, as it grows in size, it puts
forth tentacles, develops a mouth between them, and
becomes a fully formed polyp of the same species as
the parent. There must then be something, reasoned
Bonnet, to make this happen, something that was pres-
ent from the beginning of the growth of the bud—
“certain particles which have been preorganized in
such a way that a little polyp results from their devel-
opment” (Palingénésie, “Tableau des Considérations,”
Art. XV). Since the polyp can regenerate itself from
any part of its body when cut into small pieces, the
preorganized particles must exist in every part of the
whole. The “germ,” then, is not necessarily a miniature
organism, it is “every preordination, every performa-
tion of parts capable by itself of determining the exist-
ence of a Plant or of an Animal” (ibid.). It is, in Bonnet's
further words,

(Palingénésie, Part VII, Ch. IV).

Evidently Bonnet's real opinions were far different
from the ludicrous view commonly attributed to him.
He clearly saw the need for a material pattern that
from the beginning of each life would control the
hereditary course of its development, and that would
of necessity be transmitted from the parent generation
to the offspring. Here, however, lay the unresolved
difficulty.

The dilemma was most clearly pointed out in 1745
by Pierre Louis Moreau de Maupertuis, Bonnet's con-
temporary. There is abundant evidence that in sexually
reproducing species the offspring inherit characteristics
from both their male and their female parents. In fact,
the very same characteristic can be transmitted in one
and the same family, at times through the female and
at others through the male line. Maupertuis studied
the inheritance of polydactyly in a Berlin family over
four generations and demonstrated this matter conclu-
sively. How, then, can a preformed embryo, or even
a preorganized particulate system, be involved? What-
ever is transmitted from parents to offspring, it must
be provided equally by both male and female parents.

The facts led Maupertuis to a daring speculation.
Let us suppose, he wrote, that particles corresponding
to every part of the offspring are provided by each
of the parents and that in the generation of the embryo
they find their way into the right places by reason of
chemical affinity between like particles. Then corre-
sponding particles will unite, and those that should be
next to each other to form a part properly will be
attracted together and by their union will exclude less
appropriate associations. The embryo will thus be built
up in the correct hereditary pattern of its species, but

since now the paternal and now the maternal particles
will be utilized, the hereditary character may resemble
the condition in either one of the parents.

Maupertuis' particulate theory of heredity was not
accepted in its time, because the very idea of chemical
attraction on the basis of affinity was too novel. And
to be sure, Maupertuis confused the hereditary parti-
cles with the effects they produce and with the parts
whose development they control. In those respects
Bonnet had clearer insight. But after all, the time was
nearly a century before the formulation of the Cell
Theory or any recognition of the microscopic elements
upon which heredity might depend. To see that at
bottom heredity must depend on a sort of organic,
chemical memory, and to attribute this capacity to
separable particles that maintain their intrinsic nature
when in combination was extraordinary enough. This
fundamental idea led Maupertuis further to suggest
that defective development—leading to the formation
of monsters—might arise from excesses in numbers or
deficiencies of the particles; that the particles might
undergo novel alterations giving rise to new hereditary
types; and even that the isolation of these forms in
different parts of the earth might lead to the origin
of new species.

Although Maupertuis' ideas of heredity were far in
advance of the more general notions of a blending of
parental characteristics and a loss of hereditary vari-
ability in the population through the mere action of
interbreeding and hybridization, they had little heuris-
tic value; that is, they stimulated few experiments. In
the absence of any chemical and cytological knowledge
of the physical basis of heredity they could not be
tested, and soon they were forgotten. Similarly, one
might say that Bonnet's views prefigure some of the
more important modern ideas of the relation of the
genetic pattern, or genotype, to the course of develop-
ment and the production of a phenotype, or assemblage
of final characteristics. Again, there was no way to test
such ideas until the eventual development of experi-
mental embryology. Yet it may fairly be said that had
Darwin and others of his generation had a proper
knowledge of the ideas of Maupertuis and Bonnet,
much fruitless theoretical speculation about heredity
might have been avoided.

Nothing has arisen to disturb the generality of this
principle. When plant, animal, and bacterial viruses
were discovered (1892-1918), the ghost of spontaneous
generation was evoked by some who were puzzled over
the release of viruses from healthy organisms. Further
investigations, however, disclosed that besides existing
in their typical virulent, infectious state, many viruses
are capable of adapting themselves so successfully to
their hosts that they may live within the host cells in
an avirulent, symbiotic or latent condition from which,
under appropriate conditions, they may be released
after long periods of time. In some cases the latent
viruses may even be transmitted from one generation
of host organisms to the next by being included in the
reproductive cells or buds from which the offspring
arise. Thus they become virtually an inherited trait of
the host species! Nevertheless, for viruses too, omniium
vivum ex vivo.

Every Cell from a Cell. The early formulations of
the Cell Theory, especially in the classic form stated
by Matthias Schleiden and Theodor Schwann in 1839,
were not helpful to the development of the concept
of genetic continuity. At the turn of the century (1802)
K. Sprengel had thought that cells originate inside of
other cells in the form of granules or vesicles. He
probably mistook starch grains for newly forming cells.
Nevertheless, and in spite of criticism by others, this
mistaken idea was adopted by others and was accepted
even by Schleiden himself as late as 1849. As for
Schwann, he seems to have gotten his ideas of the
formation of new cells from the notions of Christian
Friedrich Wolff almost a century earlier (1759; 1768).
Schwann, in brief, thought that new cells might form
outside existing cells in the midst of a ground substance
supposed to exist between the cells, or alternatively
that they might form inside of the older cells by a kind
of crystallization from the mother liquor. Better ideas
of the genetic continuity of cells were to be based upon
the discovery made by Robert Brown, in 1831, that
a nucleus is a regular feature of each cell in a flowering
plant. (It is not true, though often so stated, that Robert
Brown discovered the nuclei of cells. They had been
seen many times before. What he actually did was to
develop a general concept of the essentiality of the
nucleus for the cell.) Schleiden and Schwann recog-
nized the importance of this concept, and Schwann's
work on animal cells, such as the cells of the notochord
and developing cartilage in embryos, made it possible
to extend the concept to the cells of animals as well
as of plants.

Cell division had already been observed carefully
and critically by a number of workers: by J. P. F.
Turpin (1826) and B. C. Dumortier (1823) in filamen-
tous algae, by Hugo von Mohl (1835-39) in filamentous
algae and in the club moss Anthoceros; by J. Meyen
(1830) in green algae, the mycelia of molds, and the
terminal buds and root tips of flowering plants; and
by C. G. Ehrenberg (1833) in the fission of various
protozoans. It was especially Meyen and von Mohl who
most vigorously opposed the views of cell formation
put forward by Schleiden and Schwann and who
maintained that on the contrary cells arise by self-
division. Over the two decades from 1840 to 1860,

these views were supported on the botanical side by
F. Unger and Carl Nägeli, and on the zoological side
by A. Kölliker, R. Remak, and Rudolf Virchow. These
men first succeeded in obtaining an admission that cells
do arise by division, and ultimately that they arise only
in that manner. Virchow's aphorism, so often quoted
—Omnis cellula e cellula—merely put a period to the
long dispute. It is very significant that both Remak and
Virchow opposed Schleiden's and Schwann's idea of
free cell formation because they regarded it as equiva-
lent to spontaneous generation.

Great changes in point of view rarely occur abruptly.
Although Virchow's and Remak's views eventually
carried the day and laid the foundation for the concept
of cellular continuity that is a basic corollary of overall
genetic continuity, the arguments continued for some
time after 1855. There were still many biologists who
believed that while cells might arise by division of
preexisting cells, they could also arise by free cell
formation. But slowly the increasing weight of evi-
dence and scientific opinion prevailed.

One of the most important early observations made
on the nature of cell divison was Nägeli's observation
that the nuclei of the two daughter cells are derived
from the division of the parent nucleus. (He saw this
in the stamen hairs of the spiderwort Tradescantia, still
a classic material for demonstrations of mitosis to biol-
ogy students of all ages.) Nägeli, however, thought that
division of the nucleus was exceptional. By laborious
and careful work Wilhelm Hofmeister (in 1848-49),
using the same material, detected the breakdown of
the nuclear membrane prior to divison of the cell, and
with remarkable clarity he figured the presence of a
cluster of what were later to be called chromosomes.
According to his observations these separated into two
groups, each of which became reconstituted into one
of the daughter nuclei. Considering that all of this was
observed without the benefit of staining and with the
imperfect microscopes of the time, it was a truly re-
markable achievement. But the fact that others were
unable to see nearly as much left them unconvinced
that Hofmeister was correct.

It was the zoologists, who were working largely with
separate dividing cells, such as blood cells in the chick
embryo or the dividing cells of newly fertilized eggs
of marine invertebrates, who seem first to have become
convinced that nuclear division is invariably a part of
cell division. Remak saw the chick's blood cells in late
stages of division, when connected by a narrow stalk,
and he observed that a fine thread connected the
daughter nuclei. He also figured the star-shaped asters
in some dividing cells. Some of the animal cytologists
became convinced that the original nucleus becomes
dissolved in the course of each cell division, and that
the daughter nuclei are reconstituted within each
daughter cell; but by 1852 Remak concluded that the
nuclear material does in fact persist from one cell
generation to the next. A most remarkable failure of
interpretation at this time was that of E. G. Balbiani,
who in 1861 was one of the very first biologists to apply
a fixative and then a stain, carmine, to produce a degree
of selective staining of different parts of the cell.
Observing ciliate protozoans during their conjugation,
he was misled into thinking of them as animals with
organ systems analogous to those of multicellular ani-
mals. Thus he interpreted the micronuclei as the
“testes” of the protozoan and completely missed the
significance of the beautiful examples of mitosis which
he actually saw and figured.

To sum up, by 1870 it was generally believed that
cells arise only from parent cells, but the origin of the
daughter nuclei from a parent nucleus remained in
some doubt because of the dissolution of the parent
nucleus at the commencement of cell division. What
was needed was a clear and unmistakable sign that the
principal bodies within the nucleus, namely, the chro-
mosomes, possess their own genetic continuity.

Every Chromosome from a Chromosome. In the
establishment of the concept of genetic continuity, the
decade following 1873 was a crucial period. During
these years the details of mitotic cell division were
worked out, step by step, by a considerable number
of cytologists, among whom Eduard Strasburger,
working on plant materials, and Walther Flemming,
working on animal materials, were leaders. Many of
these researches were closely connected with the study
of the events of gametogenesis and fertilization. Here
we shall look simply at the discovery of the sequence
of events in the division of the cell and its nucleus,
a towering achievement of nineteenth-century biology,
fully as important as the Cell Theory itself. One may
sharply contrast this remarkable development of bio-
logical science with the advent of Mendelian genetics,
or of Darwin's Theory of the Origin of Species by
means of Natural Selection, for both of those achieve-
ments were largely the creation of single men, whereas
in the unfolding of mitosis many individuals contrib-
uted essential parts. In that respect the discovery of
mitotic cell division was an advance more like those
of genetics in the twentieth century, when the Chro-
mosome Theory of Heredity and the elucidation of the
roles of DNA and the nature of the genetic code have
required the labors of many persons, even though some
individuals may stand out as leaders or originators.

In the year 1900 the American cytologist E. B.
Wilson, whose own work on genetic continuity was
to be so fruitful, wrote in the first edition of The Cell
in Development and Inheritance (p. 46) the following

words: “It was not until 1873 that the way was opened
for a better understanding of the matter. In this year
the discoveries by Anton Schneider, quickly followed
by others in the same direction by Otto Bütschli,
Hermann Fol, Eduard Strasburger, Eduard van
Beneden, Flemming, and Hertwig, showed cell-division
to be a far more elaborate process than had been
supposed...”—supposed, that is, by Remak and
others, who thought that nuclear and cell division
represented simply a pinching in two of the nucleus
and the body of the cell.

First it became evident that cell division is regularly
associated with the formation in the cell of an achro-
matic (nonstainable) figure called the spindle. Fol saw
the initiation and growth of two asters in each dividing
cell of the sea urchin egg, and Otto Bütschli observed
that a spindle-shaped structure, also achromatic, is
formed between the asters and is eventually cut
through by a deepening constriction or furrow around
the cell in the plane of the equator of the spindle. By
1875 Strasburger had shown that in the typical plant
cell things happen somewhat differently. A spindle is
indeed produced, but there are no asters at its poles,
and no furrow constricts the dividing cell. Instead, a
cell plate is formed across the equator of the spindle,
and gradually extends beyond the spindle until it meets
the old cell walls on all four sides. The new, rigid cell
wall separating the daughter cells is then deposited in
layers on either side of the cell plate.

As for the nuclear elements themselves, Fol showed
that they can be brought back into view after the
nuclear membrane has dissolved, and then in 1873 A.
Schneider, and shortly thereafter I. Tschistiakoff,
stained and observed the bodies later to be named
chromosomes. These structures of the cell were espe-
cially well observed in the studies of Strasburger on
dividing plant cells and of Balbiani on those of a grass-
hopper. The stained structures, rodlike in the grass-
hopper but often angled or V-shaped in the plant
material, were found to cluster on the center of the
spindle. They then divided—Strasburger thought it to
be transversely—and the two parts thus formed moved
to opposite poles of the spindle. Oscar Hertwig showed
that these two groups of chromosomes reconstitute the
nuclei of the daughter cells; and Strasburger showed
that in his plant cells, well before the spindle is formed,
the chromosomes are to be seen within the nucleus
as long, twisted double threads, which later shorten
and thicken.

Walther Flemming confirmed that this is also char-
acteristic of animal cells, and in 1879 he added a most
significant observation: the division of each chromo-
some to make two is longitudinal, not transverse. The
succession of the stages of mitosis deduced from fixed
and stained material was shown to be correct by Flem-
ming and W. Schleicher by observing cell division in
living material. By 1880 and 1882, when Strasburger's
third edition of Zellbildung und Zelltheilung (“Cell
Structure and Cell Division”) and Flemming's Zellsub-
stanz, Kern und Zelltheilung (“Cell Substance, Nu-
cleus, and Cell Division”) respectively appeared, the
story of mitosis was almost complete. The final proof
that the longitudinal halves of each split chromosome
separate and move to opposite poles was provided for
animal cells by van Beneden in 1883 and for plant cells
by F. Heuser in 1884.

It is a striking fact that the two greatest contributors
to the unfolding of the nature of the mitotic process,
Strasburger and Flemming, were each responsible for
a serious misconception that plagued later students for
many years. Strasburger's error, the conception of the
transverse division of the chromosomes, offered a seri-
ous block to recognition that the elements of heredity
might be linearly arranged within the chromosomes;
for of course, if a chromosome really divided trans-
versely, either its two parts would be genetically
different, or else each chromosome could contain only
a single genetic element to be duplicated and appor-
tioned to the daughter cells. Strasburger recognized
his error, however, in a few years. Flemming, on the
other hand, clung to his erroneous view that all the
chromosomes are at first united into one long continu-
ous thread, a “spireme,” which later breaks up into
the separate chromosomes. This conception, which was
based simply on inadequate observations of the number
of free chromosome ends in the early prophase nuclei,
was less in conflict with any of the principles of genet-
ics, and had a much longer life. Even in the middle
of the twentieth century, textbooks and teachers could
still be found perpetuating this error, in spite of the
fact that a careful look at Flemming's own figures of
nuclei in early prophase shows quite clearly that more
than two chromosome ends are apparent in various
prophase nuclei!

Every chromosome from a chromosome—how
sharply this continuity contrasts with the mass division
of the cytoplasm, which may be very unequal in
amount. The significance of this understanding was
quickly apparent. Wilhelm Roux in 1883 suggested that
the longitudinal splitting of the chromosomes implies
the existence of a linear array of different hereditary
“qualities” along the length of each chromosome.

In 1884 Carl Nägeli, a botanist noted for his work
in plant physiology and plant hybridization, and re-
ferred to already, proposed what he called a “mecha-
nistic-physiological theory of descent.” In part he was
undertaking to criticize Darwin's theory of natural
selection, but in part he was also attempting to supply

a conceptual scheme for a physical system to account
for heredity. Strangely and unaccountably, just as he
ignored Mendel's discoveries, he ignored entirely all
the contemporary developments in knowledge of the
roles of the nuclei of the germ cells during fertilization,
as well as the indications of the genetic significance
of the chromosomes that were to be drawn from
mitotic cell division. Instead, reasoning that the sperm
and the egg, in spite of their differences in size, have
an equal share in the determination of the hereditary
characteristics of the offspring (see Maupertuis), Nägeli
concluded that the hereditary material is not the entire
substance of the egg but only some special part of it.
This restricted hereditary substance he called the “idio-
plasma.” He supposed it to be dispersed in a sort of
network through the entire substance of the cell,
through nucleus and cytoplasm alike. By division of
the fertilized egg into cells, the idioplasm would be-
come distributed to every new cell and give to each
its hereditary character. Evolution was thought to take
place through changes in the idioplasm, changes going
on continuously and impelled by some inherent force
toward inevitable change. For a man who so insistently
proclaimed that he was a mechanistic biologist, this
inconsistency was truly remarkable, but Nägeli did not
seem to notice that it was in the least illogical.

Perhaps a word should be permitted to characterize
a long, voluminous record of analogies between hered-
ity and memory, best exemplified by a lecture given
by the physiologist Ewald Hering in 1870. The dia-
lectic progresses from the idea that memory must have
an unconscious organic, or material, basis to the analo-
gous idea that a material basis must be involved in
the transmission from one generation of living orga-
nisms to the next of the “memory” that guides its
development. The weakness is quickly apparent in the
purely speculative mechanism, which like Nägeli's was
conceived in total disregard of the superb cellular
discoveries that at the very time were laying a sound
basis for understanding the real nature of genetic con-
tinuity. The reason is readily found. Hering clearly
hoped to provide an organic basis for his Lamarckian
conviction that acquired characteristics can become
inherited. Among others, Ernst Haeckel in 1876,
Samuel Butler in 1878, H. B. Orr in 1893, and finally
R. Semon in 1904 all elaborated magnificent specula-
tions about heredity in the same amazing oblivion of
the developing knowledge of cell division, chromosome
individuality and persistence, and the Chromosome
Theory of Heredity. Like Darwin, in an effort to ac-
count for supposed heritable effects of the environ-
ment, they assumed the existence of “plastidules” or
other living units that could be modified in various
body parts, and were then transmitted through the
reproductive cells to members of the next generation.
Yet unlike Darwin, not one of them made an effort
to check his theory by further experiments. Not one
of them, in fact, reasoned as clearly or tested his system
as carefully against the known facts as Maupertuis had
done, over a century before. On the contrary, it seems
to have escaped these nature philosophers that memory
must at best be a poor analogy for heredity, since
memory exists demonstrably only in animals, whereas
heredity is just as characteristic of plants. Herbert
Spencer, in his Principles of Biology (1864), was equally
speculative and equally fallow. In postulating biologi-
cal units determinative of development, he clearly
revealed less breadth of knowledge and biological
perspicacity than Charles Bonnet had exhibited a cen-
tury earlier.

It was August Weismann, once a student of Nägeli,
who undertook the task of properly relating Nägeli's
concept of the idioplasm to the recent developments
of cytology. In his first famous paper on the subject
of heredity, in 1883, Weismann defined the germplasm
as the unbroken lineage of cells connecting the fertil-
ized egg from which an individual springs with that
individual's own gametes, which through their union
form the fertilized eggs of the next generation. “We
have an obvious means by which the inheritance of
all transmitted peculiarities takes place,” he said, “in
the continuity of the substance of the germ cells, or
germplasm.” Weismann stressed two principles about
the germplasm. The first principle was the Continuity
of the Germplasm. According to this concept, the
substance of the body (the somatoplasm) is in each
generation produced as an offshoot of the germplasm,
or germ-line, so that whatever characteristics are in-
herited must be transmitted from the germplasm to
the somatic part of the body. “Changes in the latter,”
Weismann stated, “only arise when they have been
preceded by corresponding changes in the former.” He
deduced also that characteristics acquired by the so-
matic cells cannot be transmitted to the next genera-
tion unless there is some physical mechanism to transfer
material substances or particles from the somatic cells
to the germplasm. Weismann believed that any such
transfer of particles was highly improbable, and in
subsequent years he set himself to test the inheritance
of acquired characteristics by experiment. All of his
later work confirmed the noninheritance of whatever
characteristics were acquired by the somatic cells, and
from this experience he derived his second major prin-
ciple, the Isolation of the Germplasm. By this he meant
that effects of the environment which are inherited
must be exerted directly on the germplasm and cannot
be produced in somatic tissues and thence be trans-
ferred to the germplasm.

Weismann, like Hertwig and Strasburger, identified
Nägeli's idioplasm with the chromosomes, but Weis-
mann extended the conception to the postulate that
each chromosome is made up of hereditary elements
he called “ids,” which in turn are composed of heredi-
tary determinants for each inherited characteristic.
During somatic development, the ids were supposed
to release their determinants and so to be used up. Only
in the germ cells would the undiminished quota of ids
be retained. Moreover, in Weismann's view, every
chromosome was like every other. In spite of growing
evidence of the individuality of the chromosomes, as
well as their longitudinal division, already noted,
Weismann resisted all objections to his schema. Here,
if ever, we have a supreme example of a scientist who
commences with great insight and who hardens, in
devotion to some favored conceptual model, into dog-
matic resistance to all evidence that would force him
to change his views!

A theory far more like our modern views was put
forward by Hugo de Vries in 1889, under the name
of “Intracellular Pangenesis.” De Vries wished to re-
strict the hereditary elements, or pangenes as he called
them, to the nucleus and the chromosomes, and also
to limit their activities to the particular cell within
which they might lie. That was what he meant by
“intracellular.” De Vries' pangenes differ little from
the conceptual genes of the twentieth century. In his
view they constituted the chromosomes, but could
migrate into the cytoplasm and become active there,
thus controlling the development of the cell. A repre-
sentative group of them, however, would always re-
main behind within the nucleus, to be handed on by
mitotic division to both body cells and gametes. Can
one fail to be struck by the profound similarity between
these pangenes supposed to remain in the chromosomes
and the current concept of genes composed of DNA
(deoxyribonucleic acid) and restricted to the chromo-
somes, or on the other hand between the pangenes
supposed to migrate into the cytoplasm in order to
regulate development and to control the hereditary
characteristics and the current views of messenger
RNA (ribonucleic acid)? Since the pangenes were lim-
ited to the cell and corresponded one to one with
particular hereditary characteristics, and since they
were always represented in full measure in the nucleus,
the conceptual model developed by de Vries was con-
sonant with the principle of the isolation of the germ-
plasm and the noninheritance of acquired charac-
teristics. Unfortunately, the use of the term “pangenes”
made everyone recall the speculative theory which
Darwin evoked to allow for some supposed inheritance
of acquired characteristics. Consequently de Vries is
often thought, by persons who have never read his
massive volume on Intracellular Pangenesis, to have
held views quite the opposite of his real ones.

By the turn of the century, when the rediscovery
of Gregor Mendel's work really gave birth to modern
genetics, the cytological basis of genetic continuity had
been established. Omnis chromosoma e chromosoma:
every chromosome from a chromosome.

Every Gene from a Gene. The line of thought about
genetic continuity developed thus far has described an
ever-increasing degree of precision in the generation
of living forms. Biogenesis becomes reproduction; re-
production becomes cellular; cell division becomes
mitotic; chromosomes split longitudinally, or put more
accurately, they replicate themselves, since each new
chromosome is no half-chromosome but a chromosome
entire; and finally, the substituent elements of the
chromosomes, whether visible chromatids or invisible
genes, are held likewise to replicate themselves. During
the lengthy period from about 1883 to 1953, a span
of 70 years, little was added to this particular line of
development of the concept. True, the development
of genetics made it clear that one is entitled to say:
“Every gene from a gene.” But that deduction was
made on the basis of evidence that genetic continuity
is not interrupted when cells divide, or when gametes
are formed, unite, and generate a new individual. One
could say where a gene resided in a particular chromo-
some, but not what it was. The gene and its replication
remained total abstractions.

Every DNA Molecule from a DNA Molecule. All of
this changed in the decade following 1944, when it
became evident that the physical material of heredity
is not, as had been generally supposed, protein but
instead is deoxyribonucleic acid (DNA). The problem
of replication again became real when J. D. Watson
and F. H. C. Crick in 1953 proposed, on the basis of
chemical considerations and X-ray diffraction data, that
the DNA molecule is a double helix. Its two strands
have -sugar-phosphate-sugar-phosphate- backbones
from which paired purine and pyrimidine bases extend
inward toward the axis of the helix and are held to-
gether by hydrogen bonds that regularly match adenine
with thymine and guanine with cytosine. The basic
problem of genetic continuity was at once recognized
to be the nature of the mechanism whereby the DNA
molecule replicates itself (Figure 1).

Several aspects of the model of DNA and its replica-
tion need some emphasis. First, the two strands of the
double helix are in every detail complementary. They
are not identical. A portion of the sequence that in
one strand might run -CATCATCAT- in the other
would run -GTAGTAGTA- and read in either direction
would be quite different from the first. The equivalence
in amount of adenine with thymine and of guanine

with cytosine, so characteristic of all DNA's no matter
what their AT:GC ratios may be, is a property of the
double helix and not of single-stranded DNA. Similarly,
the equality of purine bases to pyrimidine bases is a
property of the double helix, not of the single strand.
Replication itself is not a process that can be performed
in a single step by single-stranded DNA. The single
strand makes, or is a template for, a complementary
strand, with a polarity that runs in the opposite direc-
tion along the molecule, since the 3′-5′ phosphate ester
linkages are reversed in direction in the two strands.
Properly considered, replication is thus performed only
by the double helix, which upon separation and forma-
tion of two complementary strands generates two dou-
ble helices.

Watson and Crick found that certain evidence ex
cluded the possibility that the two polynucleotide
chains of a DNA molecule are paranemically coiled,
that is, are so coiled that they can simply slip into and
out of each other. Instead, they were plectonemically
coiled, like strands of a twisted rope, and therefore,
in order to come apart, they must in fact untwist. Since
they could not very well be conceived to replicate
while bound in the double helix, when every base is
paired and held by hydrogen bonds to its partner, it
seemed that replication must require a prior untwisting
and separation of the strands. As a most important part
of their theory of DNA structure, Watson and Crick
therefore postulated that replication is preceded by
uncoiling of the strands, after which each strand could
attract free nucleotides from the metabolic pool. These
nucleotides could then become united by phosphate

ester linkages so as to form a new strand that would,
with the original strand, twist into the double helix
again. Thus each of the separated strands of the original
double helix would serve as a template, and two double
helices would be produced from one.

Step by step, evidence has been found to support
the validity of this hypothesis. An important early
piece of evidence was Arthur Kornberg's discovery
(1956) that for the synthesis of DNA in vitro one must
supply a pool of nucleotide triphosphates, that is,
nucleotides which are already provided with the
high-energy phosphate bonds that enable formation of
the phosphate ester linkages to proceed. Calculations
by Cyrus Levinthal and H. R. Crane, and by others,
showed that the energy required to spin a very long
DNA molecule so as to untwist it is not inordinately
great but is only a small part of the available nucleotide
triphosphate energy of the cell, while the time required
would be brief. A model was proposed that envisaged
the progressive uncoiling of the double helix from one
end and the beginning of replication of the separated
strands while the remainder of the double helix was
still intact. It was found independently by Paul Doty
and J. Marmur that exposure of DNA to critical high
temperatures would lead to dissociation of the strands
of the double helix and that if cooling thereafter was
sufficiently gradual, the strands would in fact reassoci-
ate, or “anneal.” In this way it was possible to produce
certain kinds of hybrid DNA artificially, by bringing
about the association, while cooling, of single strands
from different sources.

A celebrated experiment of M. Meselson and F. W.
Stahl in 1958 provided very convincing evidence of
the correctness of the Watson-Crick hypothesis. A
culture of the bacterium Escherichia coli was first
grown in a medium containing heavy nitrogen (N15),
until all the DNA was labeled with this isotope. The
cells were then transferred to a medium containing
ordinary nitrogen (N14) for periods equal to one cell
generation and two cell generations. DNA was ex-
tracted from a sample of the original N15-labeled cells
and subjected to ultracentrifugation in a cesium chlo-
ride density gradient, which differentiates molecules
by weight. The DNA formed a single band at a charac-
teristic place. DNA from the sample taken after one
cell division formed a single band at a different place,
while DNA from the sample taken after two divisions
revealed two bands, one of them at the same place
as in the DNA from cells after the first replication,
the other a new band still further displaced from the
band characteristic of N15. The interpretation seems
clear. When the N15-labeled double helical DNA mol-
ecule replicates in medium containing only N14, each
new duplex will contain one strand labeled with heavy
and one labeled with ordinary nitrogen. There will
therefore be only a single band, but it should lie—as
indeed it does—just midway between the positions
occupied by pure N15-labeled and pure N14-labeled
DNA. After a second division, the separation of the
double helices will provide in each case one heavy and
one ordinary strand as templates. Hence, after replica-
tion, duplexes will be formed half of which will contain
one heavy and one ordinary strand and half of which
will contain two ordinary strands. The latter will form
a band at the position characteristic of DNA in which
all replication has occurred in medium with ordinary
nitrogen.

This experiment not only neatly confirms the Wat-
son-Crick hypothesis of replication, but shows that the
process is “semi-conservative,” which may be defined
in the following way. Once replication has taken place
in heavy nitrogen, there will always be some double
helices containing one heavy and one ordinary strand
when replicating in ordinary medium, but the propor-
tion should decline from one hundred per cent in the
first daughter generation to one-half in the next, one-
fourth in the third, one-eighth in the fourth, etc. If
this is so, the original strand that serves as a template
remains intact. Yet we know from Herbert Taylor's
studies that the original chromosome does not always
remain intact. It may undergo exchange at one or more
points with the new sister-chromatid that is formed.
There is clearly a discrepancy here; but it serves mainly
to emphasize the tremendous shift in dimensions when
a DNA double helix is compared with a chromosome.
The DNA double helix has a diameter of 20 Å, the
completely uncoiled chromosome one of at least 0.2
microns (or 2000 Å), one hundred times greater. Until
we know much more about the internal construction
of the chromosomes and the exact arrangement of the
DNA molecules in them, this hundred-fold difference
in dimensions (two orders of magnitude) leaves plenty
of scope both for molecules and for imagination. Nu-
merous models have been proposed to explain how the
replication of DNA can be semi-conservative while
that of the chromosome is not.

In this essay we have sought for the meaning of
reproduction in its broadest terms. We began with the
idea: all life from life of its own species. We have ended
with the replication of the DNA molecule. Reversing
the direction of our discourse, we see that the whole
of genetic continuity really lies here. Because each
DNA molecule can replicate itself, each gene and
chromosome undergoes replication. Because, whenever
the chromosomes divide, the sister chromatids separate
and move by means of the spindle mechanism into the
daughter cells, it follows that every cell comes from
a cell and contains within it the same genetic heritage.

Because cells arise mitotically from parent cells and
because each individual must originate as a single cell
or a cluster of cells derived either from two parent
organisms or from one, all life comes from life of its
own kind.

It remains for us to place this concept, genetic con-
tinuity, within a social context. Scientific ideas may
lead to technological applications which increase
human power and alter the course of civilization. In
time, the concept that genetic continuity resides ulti-
mately in the replicating strands of the DNA double
helix may assist in the techniques of genetic surgery
and manipulation whereby man will some day acquire
total control over the evolutionary process and alter
hereditary characteristics in selected directions. That
time is not yet. On the other hand, the greatest influ-
ence of scientific concepts may lie not in the field of
technological applications, but rather in the profound
alterations of man's philosophical views of nature, life,
and man himself. The ultimate concern of man finds
voice in the age-old cry: Whence? And whither?

In man's construction of his world view, the refine-
ment of the idea of genetic continuity to a point where
it is shown clearly to reside in the replications of a
remarkable sort of molecule represents the final step
in the validation of J. O. de La Mettrie's “L'Homme
Machine.” It is the ultimate reduction of life, symbo-
lized by its most unique and characteristic property,
reproduction, to the physical and chemical behavior
of molecules. All the genetically transmitted charac-
teristics and potentialities, both those defining the spe-
cies and those distinguishing the individual, are coded
in the sequence of nucleotides in the DNA molecule
and are produced during development through its
chemical control over the synthesis of a thousand—ten
thousand—proteins in the cells of the growing body.
Like the Theory of Organic Evolution, like the Theory
of Natural Selection, the full explication of Genetic
Continuity is destined to affect most profoundly man's
view of man, man's view of life.

Yet the mystery is not quite destroyed, not fully
replaced by “L'Homme Machine.” One must remem-
ber that the DNA double helix cannot replicate outside
of its most complex living surroundings. It cannot
replicate outside a system that includes not only neces-
sary components such as trinucleotides and necessary
sources of energy, such as adenosine triphosphate
(ATP); it also cannot replicate without the assistance
of a specific enzyme, itself a protein synthesized under
the directions of some part of the DNA molecules
present in the cell. Life, reproduction, the replicating
molecules are after all parts of an integral, complex
system; and it is the system that lives, reproduces itself,
and replicates its genetic code. There is mystery
enough here to satisfy anyone who persists in asking:
Whence? And whither?

The Principle of Genetic Continuity, in its final
refinement, would of itself produce a world of species
inalterable, of populations composed only of identical
individuals, because the DNA double helix replicates
itself so precisely. Yet the actual world is full of differ-
ent species, and populations belonging to the same
species exhibit incessant variety. The representatives
of the species Man differ almost as much as they re-
semble one another. Our world view must therefore
accommodate the existence of novelty and change in
hereditary characteristics, and the mutations of genes
and chromosomes which can be observed must have
their final locus in some change of a component of
the DNA, some error occurring in the process of exact
replication. Admit these alterations of the code, and
at once natural selection is supplied the material to
play upon. Thus, in our final view, Genetic Continuity
and Evolution are the two great themes of life, and
are linked through mutation and natural selection.
Genetic continuity implies the replication of chance
errors as well as the persistence in the main of the
old, tried and tested, reasonably successful attributes.
Genetic continuity is both the stable element in the
nature of man (and all other living species), and also
the basis for our hope that change may continue, that
new adaptations may be realized, and that a more
prescient creature may succeed us in the end.

BIBLIOGRAPHY

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York and London, 1965), for papers by J. D. Watson and
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York, 1900).

BENTLEY GLASS

[See also Biological Conceptions, Homologies; Inheritance;
Man-Machine; Spontaneous Generation.]