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.