, 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.

In the case of a form of bacteria which reaches maturity and redivides
in half an hour, the number of individual forms existing at the end of
two days would need about twenty-eight figures to represent it.
Doubtless these forms never multiply at this rate uninterruptedly for
any great length of time, or else they would occupy the whole world to
the exclusion of every other form of life. And doubtless instances arise
where the period of growth to maturity and division is prolonged to
several times the half-hour period mentioned above. But in any case, as
we contemplate the length of time during which such well marked diseases
as diphtheria, leprosy, or the plague have been known, we must
acknowledge that these unicellular forms seem to _breed true_ during a
most astonishingly long period. How can we deny that this "persistence"
of these unicellular forms constitutes a very strong argument in favor
of the "fixity" of these forms?

III

But we must proceed to examine the behavior of the various kinds of
cells of which the various multicellular organisms are composed.

Plants were known to be composed of cells, and their cells were studied
and described some years before it was understood that animals also are
composed of cells as units. Even then