40 
MALARIA 
with the effect of the host on the parasite. 
The immunology of the malarial organisms 
will be discussed elsewhere, but it may be 
pointed out here that so far only man has 
been found susceptible to P. vivax, although 
there are very similar species occurring in 
monkeys and some other mammals. What 
the factors are in the physiology of host 
and parasite which make for so strict a 
host-specificity would make a very interest¬ 
ing study, but are so far quite unknown. 
Some of the most significant of the re¬ 
cent work on malaria has been concerned 
with the factors responsible for the remark¬ 
able periodicity exhibited in the asexual 
cycle by many of the species of malarial 
plasmodia. It was shown first from work 
on birds and later by similar experiments 
on monkeys (L. G. Taliaferro 1928; G. H. 
Boyd 1929; L. G. and W. H. Taliaferro 
1934) that diurnal variations in the physi¬ 
ology of the host somehow cause the para¬ 
site to sporulate at certain definite times. 
When the length of the day is artificially 
changed the length of the cycle of the para¬ 
site. soon exhibits a corresponding change. 
It is of course obvious that there is some¬ 
thing in the genetic constitution of a given 
species of plasmodium which determines 
the way in which it will be affected by 
variations in its environment caused by the 
metabolic activities of the host, and thus 
we have illustrated the very delicate and 
intricate nature of the relationship which 
exists between host and parasite. We may 
feel fairly confident, though of course we 
cannot be sure, that the human species of 
parasites, including P. vivax, would be 
found to exhibit similar changes in perio¬ 
dicity with a similar alteration in condi¬ 
tions. It is stated by Hewitt (1940) in his 
recent monograph on bird malaria that 
Young, , Coatney and Stubbs (unpub¬ 
lished) have found that periodicity may be 
reversed in P. malariae infections by re¬ 
versing the host’s period of rest and 
activity. On this point, as on many others, 
we see that more research is needed. It is 
indeed a rather remarkable fact that so 
many problems of fundamental importance 
relating to the malarial parasites of man 
remain unsolved, especially when we re¬ 
call the place of leadership among human 
diseases which malaria holds. 
