Gloucestershire, was the first person to think scientifically
on the fact that cowpox protected from smallpox. John Hunter
had said to him, "Jenner, don't think, try." Luckily, however,
he did both. Thinking alone avails little, experimentation
alone avails not much, but the one along with the other has
removed mountains. Just as Newton thought scientifically about
that falling apple and reduced our conceptions of the universe
to order, just as Watt thought scientifically about that
kettle-lid lifted by the steam and so introduced the modern era
of mechanical power brought under man's control, so Jenner
thought about and experimented with cowpox until he had
satisfied himself that he had discovered something which would
rid the human race forever of the incubus of an intolerable
pestilence.

It was in 1780 that Jenner set himself to study cowpox in a way
that had never before been attempted, for he was convinced that
in the having had an attack of the disease lay the secret of
the conquest of that world-scourge. He confided in his fried
Edward Gardner about "a most important matter . . . which I
firmly believe will prove of essential benefit to the human
race . . . should anything untoward turn up in my experiments,
I should be made, particularly by my medical brethren, the
subject of ridicule." Luckily he was quite prepared for both
ridicule and opposition; for has not everything new been
ridiculed and opposed? Galileo was opposed, Bruno was opposed,
Copernicus was opposed, Harvey was opposed, George Stevenson
was opposed, Pasteur was ridiculed and opposed, and so were
Darwin, Simpson and even Lister. The physiological inertia even
of the educated has too often blocked the path of advancement: