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, 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?