DR. 8. It. LONG ON THE MOSQUlTO-MALARlA THEORY. 199 
of there being another intermediary host besides man capable of 
harbouring the parasite, and, assuming that this were so, this host 
may have become extinct in the lowlands, where it is known that 
the fauna and flora have altered.” 
Preventive Measures. 
So prevalent is Malaria in many parts of the world, rendering 
many thousands of square miles of fertile country practically 
uninhabitable, at any rate by white races, and causing a yearly 
mortality of millions of human lives (in India alone it is estimated 
that it kills every year some five million people), that any 
suggestions based upon our present knowledge which are likely 
to diminish in any way the spread of the disease cannot fail to 
bo of inestimable value to mankind. Such methods can be 
directed towards (1) diminishing the number of the Mosquitoes; 
(2) protection against the bite of the Mosquito ; (3) extermination 
of the malarial parasite ; (4) isolation of the infected. The best 
way to diminish the number of Mosquitoes is obviously to diminish 
the number of their breeding-places. This can be effected in some 
districts by a system of drainage as has been carried out in the 
Fens and other parts of England. At other times the tilling up of 
pools and small lakes will effect the same object. In many parts 
of the world, however, so vast are the areas to be drained, and so 
stupendous would be the work necessary to carry this out, that 
other means of attaining the same end must be thought of. That 
which naturally first suggests itself, is a destruction of the larvie 
of the insects in the water in which they are hatched. Various 
methods have already been employed to secure this end, the chief 
of which are : — 
(1) The breeding of fish in ponds, with the idea that the larvae 
would fall a prey to the fish as food. This method has been 
unsuccessful. 
(2) The rearing of water Dragon Flies, knowing that these insects 
are carnivorous and destroy the Mosquitoes and their larvae. So 
prolific, however, are the Mosquitoes that this means has failed. 
(3) The addition to the water of various soluble mineral salts 
that are known to be lethal to the larvae, e.g., corrosive sublimate, 
potassium bichromate, potassium permanganate, &c. The objection 
to the use of such substances as these is obvious. 
