17(3 GNATS OR MOSQUITOES — CHAPTER VIII 



Now, except when its natural permeability is destroyed 

 by a layer of fine silt, as is the case with the cement-like 

 surface of the Punjab "put," this alluvium is considered 

 merely as a soil, by no means favourable to the development 

 of malaria, as it is for the most part so pervious that it 

 holds water badly. Owing, however, to its unbroken levels 

 the natural surface drainage is everywhere bad, and its 

 artificial improvement difficult or impracticable. 



In spite, however, of these disadvantages, it may almost 

 be said that the haunts of Anopheles larva are throughout 

 its entire area mainly the work of man, and are therefore to 

 a great extent removable ; always provided that sufficient 

 funds and intelligence be available. 



Passing south, the transition from the alluvium to the 

 broken and rocky ground of central India is often well-nigh 

 as abrupt as that from sea to land. 



In place of the monotonous plains of Northern India, the 

 surface, at very least undulates and is often mountainous ; 

 and the soil, which is seldom of very great depth, is derived 

 from the decomposition of rocks in the immediate neigh- 

 bourhood which belong mostly to primary or metamorphic 

 formations. In the northern part of this area much of the 

 cultivable land consists of what is known as the black 

 " cotton soil," which possesses certain characters which 

 have an important influence on the local seasonal incidence 

 of malaria. It absorbs water like a sponge or like so much 

 "black cotton," and once thoroughly saturated holds it 

 well enough to favour the formation of puddles, which are 

 fairly permanent as long as the air remains damp. With 

 the return, however, of the dry weather it dries rapidly, 

 splitting up into a network of deep fissures which render 

 the existence of puddles, whether of domestic or natural 

 origin, well nigh an impossibility. 



The greater part of this region is, however, to the south 

 of the line of hybernation iox Anopheles, so that adult insects 

 mav be taken at all times of the year ; but the presence of 

 larvae is practically limited to the season of the rains, or 

 from the middle of June to the end of September. 



As, however, the closure of the fissures by the swelling 



