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7 — 
THE PRESIDENTIAL ADDRESS. 25 
those caused by intestinal parasites, prominent among which are the 
hookworms and bilharzia. 
This advance has not been limited to the infectious group : it has been 
shared by other groups, notably those due to dietetic deficiencies, the so- 
called deficiency diseases. These deficiency diseases are just as important, 
or even more important, than the infectious, since they are always with 
us and exact an enormous toll in lowered health, lowered vitality, mal- 
formation, and inefficiency. 
Until a few years ago it was taught in the schools that a complete diet 
consisted of certain proportions of proteins, carbohydrates, fats, and salts. 
But our knowledge is constantly increasing, our ideas about things con- 
stantly changing, and what is looked on to-day as absolute immutable 
truth to-morrow is seen in the light of some newer knowledge to be but a 
crude beginning. So the teaching concerning what constitutes a complete 
and healthy diet has changed, inasmuch as certain substances have been 
discovered in food-stuffs in the absence of which an adequate number of 
calories supplied in the form of proteins, carbohydrates, fats, and salts can 
alone neither promote growth nor support life indefinitely. These acces- 
sory food factors, or vitamins as they have been named, are present in such 
minute quantities in foods that they have never been isolated, and their 
chemical composition is therefore unknown. It is still a matter of opinion 
as to whether they really constitute parts of the structure of living tissues, 
or whether they merely act as catalysts or stimulators in the processes 
of growth and metabolism. That they are definite chemical substances 
which can be added to or removed from a food-stuff, with good or evil 
results, has, however, been abundantly proved. 
The untutored savage living on the natural fruits of the earth and the 
chase knows no deficiency diseases. It is only when man begins by arti- 
ficial means to polish his rice, whiten his flour, and tin his beef and vege- 
tables that the trouble begins. Civilised man living in comfort, drawing 
his food supply from fhe whole earth and able to vary his dietary at will, is 
in little danger ;_ but it is otherwise with children and adults living under 
institutional conditions, with armies on active service, encountering 
extremes of climate, and with young infants on their naturally restricted 
diet. While it is true that deficiency diseases will only develop to their 
well-marked dangerous stage if the deficiency of accessory factor is severe 
and protracted, a slighter deficiency, if prolonged, may cause a condition 
of general ill-health and inefficiency not less important although ill defined 
and difficult to diagnose. This fact is of special importance in the case of 
infants and young children. 
