rapidly taking a foremost
place. Of its practical achievements in connexion with disease, and with
the part played by bacteria and other minute organisms in the life and
affairs of man, it is not necessary to speak. Every one knows the great
advances that have been made in recent years in identifying (and to a
less extent in controlling) disease-producing organisms, whether
bacteria, protozoa (such as the organisms causing malaria, dysentery,
etc.), or more highly organized parasites. The attempt, however, to
combat these pathogenic bacteria has led to discoveries of the highest
importance with regard to the production of immunity, not only against
specific germs, but against many organic poisons such as snake venom and
various vegetable toxins. That an attack of certain diseases leaves the
patient immune to that disease for a longer or shorter time has of
course been known for centuries, but it is a modern discovery that a
specific poison induces the body to produce a specific antidote which
neutralizes it, and the detailed working out of this principle and the
study of the means by which the immunity is brought about promise to
lead us a long way towards the central problem of the nature and
activities of life itself.

We have seen how zoology has been led back into physiological channels
of research, and how the study of bacteria is opening up some of the
deepest problems of the reaction of living things to environmental
stimuli, and just as the various branches of these sciences interlace
and influence one another, so all of them, in recent years, have been
coming into contact with the inorganic sciences of chemistry and
physics. One of the noteworthy features of science in all its branches
in recent years has been the tendency of subjects which were at one time
regarded as distinct to come together again and to find that the
problems of each can only be successfully attacked by the co-operation
of the others. In their earlier days the biological sciences were in
most respects far removed from chemistry and physics; it was recognized,
of course, that organisms were in one sense at least physico-chemical
mechanisms, consisting of chemical elements and subject to the
fundamental laws of matter and energy. With the advent of the theory of
evolution this conception of the organism as a mechanism took more
definite shape, and among many biologists the belief was held that in no
very long time all the phenomena of life w