620 
Insects and Disease 
long quotation from Ross, taken from a lecture delivered by him on March 2, 
1900, before the Royal Institution of Great Britain, gives a detailed account 
of this work, answers both the questions asked above, and at the same time 
serves to reveal a typical instance of the faith and persistence of the men 
to whom we owe scientific progress. 
“It was reserved for Manson,” says Ross, “to detect the ultimate (though 
not the immediate) functions of these bodies [the motile filaments]. He 
asked why the escape of the motile filaments occurs only after the blood 
is abstracted from the host (a fact agreed upon by many observers). From 
his study of these filaments, of their form and their characteristic movements, 
he rejected the Italian view that they are regressive forms; he was convinced 
that they are living elements. Hence he felt that the fact of their appearance 
■only ajter abstraction from the blood (about fifteen minutes afterwards) 
must have some definite purpose in the life-scheme of the parasites. What 
is that purpose ? It is evident that these parasites, like all others, must pass 
from host to host; all known parasites are capable of not only entering the 
host, but, either in themselves or their progeny, of leaving him. Manson 
himself had already pushed such methods of inductive reasoning to a bril¬ 
liantly successful issue in discovering by their means the development of 
Filaria nocturna in the gnat. He now applied the same methods to the 
study of the parasites of malaria. Why should the motile filaments appear 
only after abstraction of the blood? There could be only one explanation. 
The phenomenon, though it is usually observed in a preparation for the 
microscope, is really meant to occur within-the stomach-cavity oj some suctorial 
insect , and constitutes the first step in the lije-history oj the parasite outside 
the vertebrate host. 
“It is perhaps impossible for any one, except one who has spent years in 
revolving the subject, to understand the full value and force of this remarkable 
induction. To my mind the reasoning is complete and exigent. It was 
from the first impossible to consider the subject in the light which Manson 
placed it without feeling convinced that the parasite requires a suctorial 
insect for its further development. And subsequent events have proved 
Manson to have been right. 
“The most evident reasoning—the connection between malarial fever 
and low-lying water-logged areas in warm countries—suggested at once 
that the suctorial insect must be the gnat (called mosquito in the tropics); 
and this view was fortified by numerous analogies which must occur at once 
to any one who considers the subject at all, and which it is not necessary to 
discuss in this place. 
“Needless to say, since Manson’s theory was proved to be the right 
one it has been shown to be not entirely original. Nuttall, in his admirable 
history of the mosquito theory, demonstrates its antiquity. Eleven years 
