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, however, the first propounders of
the cell theory (Schleiden and Schwann) had no clear or accurate idea of
the origin of cells, or of their essential characters and structure. As
to origin, they supposed that cells arose by a sort of crystallization
from a mother liquor; and as to structure, they looked upon the
cell-wall as the really important part, the fluid contents being quite