The Science Book

157

See also: Robert Hooke 54 ■ Antonie van Leeuwenhoek 56–57 ■ Thomas Henry Huxley 172–73 ■
Harold Urey and Stanley Miller 274–75

underwear and some wheat grain
left in a jar in the open would
spawn adult mice. Spontaneous
generation had its advocates until
well into the 19th century. In 1859,
however, a French microbiologist
named Louis Pasteur devised a
clever experiment that disproved it.
In the course of his studies, he also
proved that infections were caused
by living microbes—germs.
Before Pasteur, the link between
disease or decay and organisms
had been suspected but never
substantiated. Until microscopes
could prove otherwise, the notion
that there were such things as tiny
living entities that were invisible to
the naked eye seemed fanciful. In

1546, Italian physician Girolamo

Fracastoro described “seeds of

contagion,” and came close to the

truth of the matter. But he fell short

of explicitly stating that they were

living, reproducible things, and his

theory made little impact. Instead,

people believed that infectious

disease was caused by “miasma”—

or noxious air—that came from

rotting matter. Without a clear idea

of the nature of germs as microbes,

no one could properly appreciate

that the transmission of infection

and the propagation of life were in

effect two sides of the same coin.

First scientific observations

In the 17th century, scientists

attempted to trace the origins

of larger creatures by studying

reproduction. In 1661, English

physician William Harvey (known

for his discovery of the circulation

of blood) dissected a pregnant deer

in an effort to discover the origin

of a fetus, and proclaimed “Omne

vivum ex ovo”—all life from eggs.

He failed to find the deer’s egg in

question, but it was at least a hint

of things to come.

Italian physician Francesco Redi

was the first to offer experimental

evidence for the impossibility of

A CENTURY OF PROGRESS

In the field of experimentation,

chance favours only the

prepared mind.

Louis Pasteur

spontaneous generation—at least

in so far as creatures visible to the

human eye were concerned. In

1668, he studied the process by

which meat becomes riddled with

maggots. He covered one piece of

meat with parchment and left

another exposed. Only the exposed

meat became infected with

maggots, because it attracted flies,

which deposited their eggs on it.

Redi repeated the experiment with

cheesecloth—which absorbed the

meat’s odor and attracted flies—

and showed that flies’ eggs taken

from the cheesecloth could then

be used to “seed” uninfected meat

with maggots. Redi argued that

maggots could only arise from ❯❯

This drawing by Francesco Redi

shows maggots turning into flies.

His work showed not only that flies

come from maggots, but also that

maggots come from flies.

Many living organisms

are microscopic, and are

suspended in the air

around us.

Some of these microbes

cause spoilage of food

or infectious disease.

Microbes cannot arise by spontaneous

generation. All life comes from life.

Spoilage or infection

do not occur if microbes

are prevented from

contaminating and

reproducing.