The history of blood flagellates goes back to the year 1841, in which Valentin discovered in the blood of a brook-trout (_Salmo fario_ L.) minute bodies, from 7 µ to 13 µ in length, with active movements and presenting marked changes in form. Valentin considered the parasite a new species of the old genus _Proteus_ or _Amœba_, Ehrbg. This announcement led Gluge (1842) to publish a similar discovery he had made in frog’s blood. The latter forms were called by Mayer (1843) _Amœba rotatoria_, _Paramœcium loricatum_ and _P. costatum_, while Gruby (1843) called them _Trypanosoma sanguinis_.[53] Later it was discovered that similar organisms occurred also in the blood of birds (Wedl (1850), Danilewsky) and of mammals. Gros (1845) found them in the mouse and mole, Chaussat (1850) in the house rat, Lewis (1879) in the Indian rat, Wittich (1881) in the hamster. Danilewsky (1886–89) and Chalachnikow (1888) investigated the structure and division of trypanosomes. [53] Gruby’s generic name is generally accepted. Still others have been used, _e.g._, _Undulina_, Ray _Lankester_, _Globularia_ Wedl, _Paramecioides_ Grassi, _Trypanomonas_ Danilewsky, _Hæmatomonas_ Mitrophanow. In the case of all these forms, there was no discussion as to a pathogenic influence on the host. Opinion, however, as to the action of trypanosomes changed when, in 1880, Evans found flagellates in the blood of horses in India that suffered from a disease endemic there called “surra,” and associated the parasites with the disease. Steel and Evans were successful in transmitting the parasites--first known as _Spirochæta evansi_, Steel, then as _Trichomonas evansi_, Crookshank, and finally as _Trypanosoma evansi_--to dogs, mules and horses. They recognized that the above mentioned flagellates in the blood of the experimental animals were the causal agents of the disease. From that time there was a considerable increase in the literature, the contents of which have been summarized by Laveran and Blanchard. In 1894 Rouget discovered trypanosomes in the blood of African horses that suffer from “stallion’s disease” (dourine). In 1894 Bruce found similar forms (_T. brucei_) in the blood of South African mammals suffering from “nagana,” and in consequence attention was drawn to the part which the much dreaded tsetse-fly played in the transmission of “nagana.” In 1901 Elmassian discovered trypanosomes in the blood of horses that were stricken with “mal de caderas,” which is very common in the Argentine. The disease in cattle named “galziekte” (gall-sickness), occurring in the Transvaal, was also at one time attributed to a trypanosome remarkable for its great size, and like some other species, bearing the name of its discoverer (_T. theileri_).