certain manner, "after its kind," and another in still another manner,
"after its kind." At any rate, the _protoplasm_ of which they are all
alike composed _is_ identical wherever found, so far as any scientific
tests have yet been able to determine.

[Footnote 10: "Natural Law," Chapter X.]


II

There are many varieties of single cells known to science which maintain
an independent individual existence. Among the unicellular plants are
the bacteria, while the unicellular animals are known as the protozoa.
And although perhaps I ought to apologize to the reader for seeming to
anticipate here a part of the discussion of the problem of "species,"
yet it seems necessary to say a few words here regarding the
"persistence" of these unicellular forms.

Among the diseases which have been proved to be due to protozoa are
malaria, amoebic dysentery, and syphilis; while among the much larger
number which are due to bacteria, bacilli, or other vegetable parasites,
are cholera, typhoid fever, the plague, pneumonia, diphtheria,
tuberculosis, and leprosy.

One of the difficulties attending the study of "species" among the
higher forms of plants and animals has always been the length of time
required to obtain any large number of generations on which to make
observations. In the case of such plants as peas, wheat, corn, or indeed
almost any form of plant life, it is only with difficulty that more than
one generation a year can be obtained; and when two or more generations