232 



two tertian foriiLS are easily differentiated. The physiological dif- 

 ferences are also described and the cycle of development of the two 

 forms contrasted. At a constant temperature of 77° F. the author 

 has obtained the complete development of P. vivax in 11 days, while 

 the parasite of the mahgnant tertian form required a minimum of 14 

 days. The constant differences in these sexual cycles, which have been 

 observed by various authors, do not support the theory of the specific 

 identity of the two parasites [see this Review. Ser. B, vi, p. 205]. 

 Considered together with the morphological differences referred to 

 above, they afford, on the contrary, a weighty argument in favour 

 of the view that Plasmodium vivax and P. fraecox are distinct species. 



The following explanation is offered of the seasonal and geographic 

 differentiation of the two tertian parasites, based upon the duration 

 of the sexual cycle. It is known that in temperate climates where 

 the two organisms co-exist, it is P. vivax that appears first in the spring 

 attacks of fever, while P. praecox is dominant in the summer and 

 autumn. Recent observations made in regard to the Balkan army 

 have confirmed this periodicity [see this Review, Ser. B, v, p. 98]. 

 The earlier appearance of P. vivax is recognised as a direct consequence 

 of its more rapid development in the Anopheline. At that season 

 the sporozoites of P. vivax are largely infecting the salivary glands of 

 the Anophelines and it is therefore the benign tertian form that appears. 

 The early activity of P. vivax is followed by a premature invasion of 

 human habitations ; it is therefore easily understood how P. vivax 

 is earUer discovered in the blood. When P. praecox, having completed 

 its sporogonic development, begins to be disseminated by the 

 Anophehnes, it finds its human victims already infected to a great 

 extent by the parasite of benign tertian malaria. In the majority 

 of cases it is therefore a mixed infection that is produced, but owing 

 to the previous invasion of the human blood by asexual generations 

 of P. vivax, it may be assumed that the presence of P. praecox passes 

 for a time unnoticed. It is only later in the summer and autumn 

 that, owing to their increased virulence and perhaps to many re- 

 infections, the asexual generations of P. praecox take up the dominant 

 role and mask the primary infection. The final re-appearance of 

 P. vivax in the winter, in the case of secondary malaria, is doubtless 

 a result of the reverse phenomenon, P. vivax reappearing in patients 

 attacked by a mixed infection when P. praecox has spent its first 

 violence. This preponderance of P. vivax over P. praecox, owing to 

 its more rapid development, explains the almost exclusive occurrence 

 of the benign tertian form in temperate countries where malaria is 

 not intense, such as France. The locally-acquired malignant tertian 

 form is so rare in France that the question of its possible transmission 

 by the Anophelines found in the country has been disputed by various 

 authors. 



While it may be assumed that the temperature conditions in France 

 in summer and early autumn are perfectly compatible with the develop- 

 ment of P. praecox, and while this theory is confirmed by observations 

 on cases of malignant tertian malaria, it is nevertheless obvious 

 that some factor must exist that limits the extension of that parasite in 

 France. The explanation of this seems to be the competition of 

 P. vivax, with its more rapid development, which masks and counteracts 

 the asexual generations of P. praecox in a manner disadvantageous 



