a year are produced, they are produced under more or less unnatural
conditions. So that it takes almost a lifetime carefully to test and
record in a thoroughly scientific way the results of any extensive
experiments regarding variation and heredity.

In the case of mice or rats or rabbits or guinea pigs, many more
generations can be obtained in a few years; but in the case of the
larger kinds of animals the time taken for development to maturity and
for gestation is often much prolonged; and scientific observation of an
exact character has been in vogue for so short a time that there has
always been the chance for advocates of evolution to take refuge under
the plea that, if we only had longer and more carefully conducted
observations, we could really see species in the making, one form
becoming transformed into a distinct form, or perhaps giving rise to
another and distinct form as an offshoot.

But in the case of the bacteria and protozoa, we can have a new
generation every hour or so, sometimes every half hour. True, these
forms of minute life have been under observation for only a few years;
but their _effects_ have in many cases been observed for almost the
entire length of human history. No physician would tolerate the
suggestion that the bacillus of cholera can produce the symptoms of
diphtheria, or the tubercle bacillus produce the symptoms of leprosy.
Nor will any scientist deny that such diseases as the plague,
tuberculosis, or diphtheria are identical with diseases which ravaged
Rome or Greece or Egypt thousands of years ago. And as the symptoms of
these modern diseases are similar to those recorded by acute observers
in Greece or Egypt two thousand years or more ago, we must conclude that
the organisms causing these symptoms are doubtless identical. Similar
remarks might be made regarding fermentation and other forms of decay.