Aucust 10, 1899] 
NATURE SOV 
resemblance in their constitution, or in the whole complex of 
organs and characters, and correlate their fitness for surviving. 
Now relations or members of the same family are precisely such 
individuals. If there were no selective death-rate there would 
be no correlation between the ages of death of, say, brothers. 
If there were no non-selective death-rate, we ought to find that 
the correlation between ages of death of brothers takes the value 
determined for the coefficient of heredity in brothers, e.g. 
the “4 of stature, fore-arm, cephalic index, eye colour, &c., 
Actually we find it to be something sensibly less than “4. Our 
investigation shows that, in round numbers, about 80 per cent. 
of the death-rate is selective in the case of mankind. To that 
extent natural selection is actually at work. Combined with 
the quantitative measures of heredity already published, or ob- 
tained if not yet published, we can safely conclude that Darwin’s 
theory of a progressive change due to natural selection com- 
bined with heredity applies even to mankind to an extent which 
can be quantitatively measured. The next stage must be an 
experimental one. Various types of life ought to be submitted 
to ordeals of a kind like to those which occur in nature, and 
the correlation between the powers of resistance to these ordeals 
existing in members of the same family or brood determined. 
We shall thus be able to ascertain under a variety of circum- 
stances the relative proportions of the selective and non-selective 
death-rates. A careful inspection of the characters of the 
longer-lived families may possibly enable the trained biologist 
to select some organs or characters to which a direct application 
of Prof. Weldon’s method can be made, and thus enable us to 
distribute, so to speak, the total selective death-rate previously 
discovered among its chief factors; but here it must be remem- 
bered that relationship of organs may be quite as important as 
absolute size. The present paper is merely a preliminary study 
of the selective death-rate in man; but one may venture to 
express a hope that in a comparatively few years, if enough 
workers can be found for the experimental side of the subject, 
we shall no longer hear natural selection spoken of as hypo- 
thetical, but rather its quantitative measure given for various 
organisms under divers environments. 
THE CAUSE AND PREVENTION OF 
MALARIA. 
HAVE the honour to address you, on completion of my term 
of special duty for the investigation of malaria, on the subject 
of the practical results as regards the prevention of the disease 
which may be expected to arise from my researches ; and I 
trust that this letter may be submitted to Government if the 
Director General thinks fit. 
It has been shown in my reports to you that the parasites of 
malaria pass a stage of their existence in certain species of 
mosquitoes, by the bites of which they are inoculated into the 
blood of healthy men and birds. These observations have 
solved the problem—previously thought insolvable—of the 
mode of life of these parasites in external nature. 
My results have been accepted by Dr. Laveran, the discoverer 
of the parasites of malaria; by Dr. Manson, who elaborated 
the mosquito theory,of malaria ; by Dr. Nuttall, of the Hygienic 
Institute of Berlin, who has made a special study of the 
relations between insects and disease; and, I understand, by 
M. Metchnikoff, Director of the Laboratory of the Pasteur 
Instijute in Paris. Lately, moreover, Dr. C. W. Daniels, of 
the Malaria Commission, who has been sent to study with me 
in Calcutta, has confirmed my observations in a special report 
to the Royal Society; while, lastly, Prof. Grassi and Drs. 
Bignami and Bastianelli, of Rome, have been able, after receiving 
specimens and copies of my reports from me, to repeat my 
experiments in detail, and to follow two of the parasites of 
human malaria through all their stages in a species of mosquito 
called the Anopheles claviger. 
It may, therefore, be finally accepted as a fact that malaria is 
communicated by the bites of some species of mosquito ; and, 
to judge from the general laws governing the development of 
parasitic animals, such as the parasites of malaria, this is very 
probably the only way in which infection is acquired, in which 
opinion several distinguished men of science concur with me. 
In considering this statement it is necessary to remember that 
it does not refer to the mere recurrences of fever to which 
1 Report from Major Ronald Ross to the Secretary to the Director 
General, Indian Medical Service, Simla. Dated Calcutta, February 16. 
NO. 1554, VOL. 60] 
people previously infected are often subject as the result of 
chill, fatigue, and so on. When I say that malaria is com- 
municated by the bites of mosquitoes, I allude only to the 
original infection. 
It is also necessary to guard against assertions to the effect 
that malaria is prevalent where mosquitoes and gnats do not 
exist. In my experience, when the facts come to be inquired 
into, such assertions are found to be untrue. Scientific research 
has now yielded so absolute a proof of the mosquito theory of 
malaria that hearsay evidence opposed to it can no longer carry 
any weight. 
Hence it follows that, in order to eliminate malaria wholly or 
partly from a given locality, it is necessary only to exterminate 
the various species of insect which carry the infection. This 
will certainly remove the malaria to a large extent, and will 
almost certainly remove it altogether. It remains only to 
consider whether such a measure is practicable. 
Theoretically the extermination of mosquitoes is a very 
simple matter. These insects are always hatched from aquatic 
larvee or grubs which can live only in small stagnant collections 
of water, such as pots and tubs of water, garden cisterns, wells, 
ditches and drains, small ponds, half-dried watercourses, and 
temporary pools of rain-water. So far as I have yet observed 
the larvee are seldom to be found in larger bodies of water, such 
as tanks, rice-fields, streams and rivers and lakes, because in 
such places they are devoured by minnows and other small fish. 
Nor have I ever seen any evidence in favour of the popular 
view that they breed in damp grass, dead leaves, and so on. 
Hence, in order to get rid of these insects from a locality, it 
will suffice to empty out or drain away, or treat with certain 
chemicals, the small collections of water in which their larvae 
must pass their existence. 
But the practicability of this will depend on circumstances— 
especially, I think, on the species of mosquito with which we 
wish to deal. In my experience, different species select 
different habitations for tleir larve. Thus the common 
“*brindled mosquitoes” breed almost entirely in pots and tubs 
of water ; the common “‘ grey mosquitoes” only in cisterns, 
ditches and drains; while the rarer ‘‘spotted-winged 
mosquitoes” seem to choose only shallow rain-water puddles and 
ponds too large to dry up under a week or more, and too small 
or too foul and stagnant for minnows. 
Hence the larvz of the first two varieties are found in large 
numbers round almost all human dwellings in India ; and, 
because their breeding grounds—namely, vessels of water, drains 
and wells—are so numerous and are so frequently contained in 
private tenements, it will be almost impossible to exterminate 
them on a large scale. 
On the other hand, spotted-winged mosquitoes are generally 
much more rare than the other two varieties. They do not 
appear to breed in wells, cisterns and vessels of water, and 
therefore have no special connection with human habitations. 
In fact it is usually a matter of some difficulty to obtain their 
larvee. Small pools of any permanence—such as they require 
—are not common in most parts of India, except during the 
rains, and then pools of this kind are generally full of minnows 
which make short work of any mosquito larve they may find. 
In other words, the breeding grounds of the spotted-winged: 
varieties seem to be so isolated and small that I think it may be 
possible to exterminate this species under certain circumstances. 
The importance of these observations will be apparent when 
I add that hitherto the parasites of human malaria have been 
found only in spotted-winged mosquitoes—namely, in two 
species of them in India and in one species in Italy. As a 
result of very numerous experiments I think that the common 
brindled and grey mosquitoes are quite innocuous as regards 
human malaria—a fortunate circumstance for the human race in 
the tropics. And Prof. Grassi seems to have come to the 
same conclusion as the result of his inquiries in Italy. 
But I wish to be understood as writing with all due caution 
on these points. Up to the present our knowledge, both as 
regards the habits of the various species of mosquito and as 
regards the capacity of each for carrying malaria, is not 
complete. All I can now say is that if my anticipations be 
realised—if it be found that the malaria-bearing species of 
mosquito multiply only in small isolated collections of water 
which can easily be dissipated—we shall possess a simple mode 
of eliminating malaria from certain localities. 
I limit this statement to certain localities only, because it is 
obyious that where the breeding pools are very numerous, 
