152 FEVER AND AGUE IN ANCIENT BRITAIN. 



that it is never possible to be quite sure beforehand 

 where the fly will be bad and where it w r ill not. 

 We do not feel justified in attempting to kiy down 

 any general rule on this matter, for experience has 

 long since taught us the impossibility of doing so; 

 we shall therefore merely mention that in the course 

 of our wanderings in many lands we have generally 

 remarked, that wherever the waters are muddy and 

 discoloured, there as a rule flies will be bad. Whereas, 

 those favoured districts where flies are not troublesome 

 are generally regions where the waters are of crystal 

 clearness; where the lands are irrigated by swiftly flow- 

 ing streams ; and where the soil is of a somewhat dry 

 and gravelly or rocky nature it is not necessarily 

 barren land; we have seen flies very bad on exceed- 

 ingly barren land ; and we have seen flies cease from 

 troubling in sections of forest where the magnificent 

 timber showed that the soil was rich and productive. 

 The most troublesome of these insect pests, such as 

 mosquitoes, which generally seem to hover near the 

 surface of the ground, are no doubt a certain sign of 

 a more or less malarious influence. Wherever, for 

 instance, we find mosquitoes bad, in tropical regions, 

 there we may be certain malaria exists. Whereas, 

 where they are conspicuous by their absence, we may 

 regard it as a district which is probably healthy. So 

 also in forests of the temperate zone, we may, we 

 believe, even there set down a bad, fly-stricken district 

 as probably infected by some modified form of malarial 

 germs, though these may not always be strong enough 

 to affect the human system with disease of a distinctly 

 malarious type. As regards this, we know that malaria 

 does sometimes extend far up into temperate latidudes. 



