The size of bacteria is in inverse ratio to their importance
in the primordial and present history of the earth. The largest
known are slightly above 1 / 20 of a millimetre in length and
1 / 200 of a millimetre in width. ¹ The smaller forms range
from 1 / 2000 of a millimetre to organisms on the very limit of
microscopic vision, 1 / 5000 of a millimetre in size, and to the
bacteria beyond the limits of microscopic vision, the existence
of which is inferred in certain diseases. The chemical consti-
tution of these microscopic and ultramicroscopic forms is
doubtless highly complex. The number of these organisms is
inconceivable. In the daily excretion of a normal adult human
being it is estimated that there are from 128,000,000,000 to
33,000,000,000,000 bacteria, which would weigh approximately
55 / 10 grams when dried, and that the nitrogen in this dried
mass would be about 0.6 gram, constituting nearly one-half
the total intestinal nitrogen. ²

The discovery of the chemical life of the lowliest bacteria
marks an advance toward the solution of the problem of the
origin of life as important as that attending the long-prior dis-
covery of the chemical action of chlorophyll in plants.

In their power of finding energy or food in a lifeless world
the bacteria known as prototrophic, or "primitive feeders," are
not only the simplest known organisms, but it is probable
that they represent the survival of a primordial stage of life
chemistry. These bacteria derive both their energy and their
nutrition directly from inorganic chemical compounds: such
types were thus capable of living and flourishing on the lifeless
earth even before the advent of continuous sunshine and long

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