These observations then showed us that an almost exact parallelism existed between the presence of Anophelinse and distance of breeding grounds, and the value of the endemic index, and we would mention here, incidentally, a conclusion that resulted from these observations, viz., that a distance of half a mile to a mile from breeding grounds was sufficient to place a village in a zone free from malaria ; in other words, that Anophelince did not, under these circumstances fly more than a half a mile or one mile. While, then, our observations show an almost exact parallelism between the existence of
Anophelince (and their breeding places) and the existence of malaria , yet on examining the figures closely and comparing them with others obtained elsewhere facts difficult of explanation presented themselves. Thus in the district where 10 villages were examined, nowhere did the endemic index exceed 45, while in some it was as low as 5. How, then, is one to explain this comparatively low intensity when we compare it with that of another 10 villages examined in Madras (Jeypore), where the endemic index was almost uniformly high, not sinking below 20 and reaching as high as 86; and, indeed, the difficulty of explaining these variations became even greater, when, as in this latter case, the comparison was made with another series of eight villages distant only some 30 miles from the former, where the climatic conditions were identical. In these latter villages four were found where the endemic index was zero and the highest figure reached was 27. Of these facts no explanation is at present forthcoming. To climatic differences are generally assigned the differences between the mild malaria of northern Italy and the severe malaria of southern Italy, but these instances we have just adduced occur where the explanation, so to call it, of climate would hardly suffice. What factor is it, for instance, that makes the malaria of West Africa the most severe form we know? That malaria is a virulent disease in some countries and a mild one in others has often been overlooked. Why is it that blackwater fever dependent on malaria is practically coterminous with severe malaria, but even then not always? Why, again, is the west coast of Africa the home par excellence of the malignant tertian parasite, so that in a European returning from there, in 99 out of 100 cases, it is this parasite that is present, while in Europeans returning from the West Indies and South America it is the simple tertian that is most commonly found? Why, again, in certain parts of India, in the tea gardens of the Duars, is the parasite in the native children exclusively quartan, while in Lahore (Panjab) it is almost entirely simple tertian? These and many other similar questions we cannot at present explain; we have, however, said enough to make it clear that before these problems can be solved far more precise knowledge of endemic malaria and the accompanying Anophelince will have to be acquired. The "regional factor," an expression used by Christophers and myself to embrace these unknown causes, must be elucidated, and we shall then be enabled to construct maps of malarial distribution having some more definite value than those dubiously shaded examples which we at present possess.