Consider Louis Pasteur, who gave
us a rabies vaccine in the 1890s. He
had a history of other major innovations
and was widely regarded as a
genius. Pasteur was a master chemist.
Two hundred years earlier, there was
no field of chemistry. Pasteur could
not have existed at that time. Similarly,
if he were born 50 years later, others in
the 1890s time period, who were also
working on vaccines, would in all likelihood
have discovered a rabies vaccine;
it would be a done deal in his new
time. The contribution we remember
him for was possible only at that moment
of time in France.