MALARIA 59' 



the east over which the prevailing winds blow before 

 reaching the towns. 



If swamps or expanses of stagnant water were inju- 

 rious to health in themselves, we should find it impossible 

 to account for such an anomaly as that presented by the 

 towns I have just described. If, on the other hand, we 

 accept the theory that many diseases are propagated by 

 insect agency, as has been proved beyond the shadow of 

 a doubt in the case of malarial and yellow fevers, we can 

 understand how it may happen that diseases dependent 

 on insect agency for their transmission will be prevalent 

 in the neighbourhood of one marsh and unknov^m in the 

 vicinity of another. Different causes may contribute to 

 this. In the first place, it is quite possible that the par- 

 ticular insect capable of harbouring some special parasite 

 causing disease in man during certain phases of its exist- 

 ence may be abundant in a locality without affecting the 

 health of the inhabitants ; and this would be the case 

 where the insects themselves happen to be free from 

 infection through the non-existence in the country fre- 

 quented by them of the organisms constituting at some 

 period in their lives the parasites to which these insects 

 are liable. Again, the organism may be present and the 

 insect absent. To account for a regular malarial district 

 both the insect and its parasite must occur together, and the 

 conditions must be favourable to both. Nothing is more 

 puzzling to the naturalist at times than to try to deter- 

 mine why some particular form of life should be abundant 

 in one locality and absent in another, while nearly allied 

 forms may be common to both. Causes which we are 

 frequently unable to appreciate, but which must be very 

 potent in themselves, are responsible for this, to us, un- 

 accountable distribution of life ; and when we speak of 



