se
wonderful protoplasmic units from which all living things originate seem
identical. But it is equally possible that they _are really identical_
in structure and in chemical composition, and that only the ever present
watchcare of the great Author of nature directs the one to develop in a
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
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