REPORT OF THE MALARIA EXPEDITION. 
33 
parasites may infect other animals, and tlie converse, through Anophela ; and it will be noted tliat 
monkeys and bats, in which parasites like the human ones have been found, are closely allied to 
man. But while no strong argument can be adduced against this view, not much can be said for 
it. In fact the only thing which we can find in its favour is the popular notion that malarial fever 
can be acquired in uniniiabited localities —a notion wiiich we will presently discuss. 
24. Is the Life-History of the Haemamoebidae fully known ?— According to the 
known facts, malarial fever is communicated from sick to healthy persons by the bite of Anophrlrs. 
But a very important question remains. Is this the only way in which the disease can be 
propagated ; or do the parasites possess some other cycle of existence, and, perhaps, some other 
mode of entry into man ? 
Nature is so full of potentialities that we can scarcely ever venture to say absolutely, at 
least with regard to any of the lowest forms of life, that we know all their life-history ; but when 
one life-cycle is known, we are not justified in believing that there may or must be another, unless 
there are grounds for this belief. The lower forms of life, like the higher ones, cannot have an 
unlimited number of life-cycles. The life-cycle of the Haemamoebidae as already known is 
complex enough — are we to suppose that they possess others r If they can live freely in air and 
water ; if they can enter man by the respiratory or digestive tracts ; even if they can pass, not 
only from the gnat to man, but also indefinitely from gnat to gnat, they must possess different 
phases adapted to these modes of existence and transference, and their whole life-history must be 
complex indeed. They must possess not only the known phase which adapts them for life in the 
gnat, but other phases which adapt them for life in the soil or water — which enable them to rise 
in the air and pierce the epithelium of our respiratory passages, and so on. When we consider 
the amount of variation in structure and function which this would entail, we can scarcely be 
blamed for refusing to believe in other life-cycles without good evidence. 
Is there any evidence which will lead us to suppose that there is or may be another 
cycle ? First, is there any evidence to be drawn from our knowledge of the disease, malarial 
fever r Secotuily, is there any evidence to be drawn from our knowledge regarding the parasites 
themselves ? 
It is often stated tliat malarial fever exists where there are no gnats ; and that persons 
become infected without being bitten by gnats. Were cases of this kind to be substantiated, 
they would certainly prove the existence of other modes of infection, either by other suctorial 
insects, or by some quite different route. But what value can we attach to popular statements 
of this kind ? A perusal of paragraph 16 will suggest how easy it may be for Anopheles to 
be overlooked — especially by persons who do not make an express search for them. Moreover, 
when we remember that a large number of cases of fever occurring anywhere are rr/apses, and that 
Anopheles may be present only at certain seasons, we must hesitate still more in accepting such 
statements ; while the known fact that persons can be bitten without observing it discredits 
the other notion. We do not sav that all such statements are absolutely untrue ; but we are 
scarcely justified in accepting them until cases have been established by competent observers. 
E 
