possible to determine exactly
what constitutes the active principle in use in the milk consumed in
these countries, and, as we shall see, this principle has been applied
so that, at the present day, a pure fermented milk may be obtained in
any country, and there is every reason to believe that should such be
adopted as a general article of food, it would contribute to the
prolongation of human existence.

It is due to Metchnikoff, of the Pasteur Institute, that so much
prominence has been given to the use of fermented milks. He gave it as
his opinion[40] that senility was caused partly by auto-intoxication or
by the poison derived from putrefactive micro-organisms which inhabit
the digestive track. These organisms increase with age, and under
certain unhealthy conditions multiply enormously, particularly in the
large intestine. Having arrived at this knowledge, Metchnikoff set to
work to devise some means of combating the influence of these harmful
microbes, and set up the hypothesis that the tendency to longevity which
is exhibited in Eastern countries is due to the consumption of lactic
acid organisms in the shape of soured milk. These organisms are more
powerful than those of a putrefactive character and inhibit their
growth.

"In the presence of such facts," says Metchnikoff, "it becomes
exceedingly important to find some means of combating the intestinal
putrefaction which constitutes so incontestable a source of danger. Such
putrefaction is not only capable of producing diseases of the digestive
tube--_enteritis_ and _colitis_--but even of becoming a source of
intoxication of the organism in its most varied manifestations.

"It is some years since I proposed to combat intestinal putrefaction and
its injurious consequences by means of lactic ferments. I thought the
acidity produced by such microbes would be much more effective in
preventing the germination of putrefying microbes than the small
quantity of acids produced by _Bacillus coli_. On the other hand, I had
no illusion as to the difficulty sure to be encountered in any effort to
introduce lactic microbes into the intestinal flora which has been
preoccupied by a multitude of other microbes. To make surer of the
result, I chose the lactic microbe, which is the strongest as an acid
producer. It is found in the _yahourt_ (yoghourt), which originates in
Bulgaria. The same bacillus has also been isolated from the _leben_ of
Egypt; and it is now proved that it i