University of Virginia Library

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.