S. T. Darling 
219 
represents infection derived from North Indian sources, for the ancylostome 
index of the Calcutta men was 27, while that of uncontaminated Fijians was nil. 
Here we are to observe two tendencies. First: a tendency for the stranger 
to part with some of his worms and to acquire a worm formula like that of 
the natives. Second a tendency for the natives to acquire certain worms 
from the strangers and thus modify the primitive formula of the region or 
race. This is actually taking place in Malaya, Fiji, Guiana and Brazil. 
Changes analogous to these that are taking place under our very eyes, may be 
assumed to have taken place centuries ago among other peoples, and if among 
a people to-day whose worm formula is 99 % Necator we encounter a group 
who are harbouring 15 % A. duodenale or any notable number of that species 
we are entitled to assume that the hookworms of the latter people are derived 
from an alien stock. 
The hookworms encountered in man are: 
Agchylostoma duodenale. Necator americanus. 
„ ceylanicum. 
„ braziliense. 
These worms probably all have identical life,cycles. A. duodenale and 
N. americanus 1 are obligate parasites for man, as they are not found in any 
of the animals. 
A. ceylanicum and the less known A. braziliense are commonly found in 
dogs in certain tropical lands particularly in the Old World. 
The adult sexually differentiated worms live in the small intestine and 
the females produce many ova which pass out in the faeces. 
Embryos from the ova-infected faeces develop in the soil under suitable 
atmospheric conditions of warmth, moisture and oxygen. 
The embryos penetrate the skin of the feet of people visiting polluted 
places and after traversing the skin, venous blood channels, right heart, lung 
and trachea, they reach the intestinal tract where they take up lodgement 
in the lumen of the small intestines, holding on to the mucous membrane by 
their strong armed mouths, and remaining in the host as long as seven or 
eight years. 
While in general it has been recognized that two species chieflv were to 
be found in man, no survey has been made of the species actually harboured 
by the different races of mankind or by the inhabitants of different zones, 
regions or localities. 
The reasons are obvious. Anti-hookworm campaigns are of very recent 
date and the doctor in charge is usually concerned merely in ridding the 
patient of worms and ameliorating his physical condition. 
The washing of faeces after employing a vermicide, and the culling and 
counting of thousands of worms under field conditions is not an easy or 
enticing vocation. 
1 Necator americanus has been found in a gorilla by Leiper and Looss. 
