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Updated: May 15, 2025
He showed that they always reacted in a certain definite way in response to particular stimuli, and he was led to believe that he could see "what must be considered the beginnings of intelligence and of many other qualities found in the higher animals." A species of Vorticella was probably the first Protozoan that was ever observed.
A few years ago certain observers recorded finding Protozoan parasites in the blood of those suffering from the disease, and although more recent investigations have failed to confirm these particular observations it is now quite generally believed that the disease is caused by some such parasite and that the organism is transferred from one host to another by certain species of ticks that live on wild mammals of the region where the disease exists.
Amoebae were demonstrated in one form of dysentery, and in other tropical diseases protozoa were discovered, so that we were really prepared for the announcement in 1905, by Schaudinn, of the discovery of a protozoan parasite in syphilis.
Recent observations, however, have shown that it is probably caused by a certain Protozoan parasite that is found in the blood of dengue patients and several experiments have been conducted by Dr. Graham which seem to indicate that it is transmitted by mosquitoes. In these experiments, Culex fatigans, a common tropical or subtropical mosquito, was used.
In a later chapter it will be shown how the tsetse-fly, which is somewhat like the stable-fly, is responsible for the spread of the disease known as the sleeping sickness. This disease is caused by a Protozoan parasite, a trypanosome, which is transmitted from one host to another by the tsetse-fly.
This is a very fatal infectious disease of many tropical and subtropical regions, spreading terror among the natives wherever it occurs. It is caused by the presence in the system of Protozoan parasites, the so-called Leishman-Donovan bodies, that have recently been studied by several observers. Dr. He has found the parasite in all stages of development in the bedbug.
There are a number of "suspects" against which a great deal of circumstantial evidence exists: a streptococcus in scarlet fever, a bacillus in whooping-cough, and a protozoan in measles; but none of these have been definitely convicted.
We might almost claim that if the evolution of man from the single cell be granted, no other order of their occurrence is possible. The protozoa are mostly, though not purely, nutritive and reproductive. These functions are essential to the existence of the species. Naturally in the early protozoan colonies, and in forms like hydra, these functions predominated.
It was difficult perhaps to believe that these successive grades of organic structure indicated an order of evolution, because it seemed impossible that an animal so simple as a protozoan could produce offspring with the complex organization of a frog or a cat, even in long ages. But development delivers its evidence relating to this matter with telling and impressive force.
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