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only infective to a much less degree than the others. Although about 
2 per cent, of sinensis have been found to be infected in Malaya, the 
presence of this mosquito in great numbers in certain districts without 
causing malaria is in marked contrast to what we read of the amount 
of sickness caused by umbrosus and ludlowi : it is therefore probably 
less infective than these two species. We have only a little indirect 
evidence that umbrosus is probably less infective than ludlowi, viz.: 
at Jeram where there was no malaria, we caught large numbers of 
umbrosus in a few minutes, yet at Morib only thirty-eight were captured 
in nine days. The conditions, as said before, were almost identical. 
It is probable that at Morib very little of the fever was being caused 
by umbrosus and most of it by ludlowi. 
At Morib it would seem that rossi and sinensis are both negligible 
factors in the situation; and this view is borne out by observations in 
other localities mentioned above, where, in spite of great numbers of 
these species, there was no fever. The conclusion we came to was that 
ludlowi was the greater cause of trouble, not only because of its 
probably greater infectivity, but also because of its greater numbers 
than umbrosus. 
These conclusions by their necessary lack of precision show us how 
useful it would be if we had an accurate table of the relative infectivity 
of the different species, for we might then by merely counting the relative 
numbers of the prevalent species in any place find out exactly their 
relative culpability, and act accordingly. 
With regard to preventive measures it would seem that greater 
success might be anticipated from a campaign against a mosquito, which 
was causing trouble more by reason of its large numbers than its high 
infectivity, than against a mosquito which was of high infectivity and 
low prevalence, for it is obvious that a larger proportion of the former 
type would be found and destroyed. This hypothesis might account for 
the great success which has attended Watson’s antimosquito measures 
in the Federated Malay States in places like Port Swettenham, and for 
the comparative lack of results in the hill-country, for in a place like 
the former, ludlowi, the chief cause, as we think, of the fever, is a 
mosquito of relatively low infectivity, but of usually great prevalence. 
It breeds in enormous quantities, this being probably due to the larval 
characteristic, as in rossi, of disappearing on the slightest approach of 
danger. On the hill-land, on the other hand, maculatus and albirostris 
are never to be found in such great numbers, but they possess great 
infective powers. It follows that if malaria is easily allayed by anti- 
