and
far behind the physical sciences, both in precision and in
completeness of analysis.

What is the actual working attitude of naturalists towards the general
problem that I have endeavored to outline? It would be a piece of
presumption for me to speak for the body of working biologists, and I
will therefore speak for only one of them. It is my own conviction
that whatever be the difficulties that the mechanistic hypothesis has
to face, it has established itself as the most useful working
hypothesis that we can at present employ. I do not mean to assert that
it is adequate, or even true. I believe only that we should make use
of it as a working program, because the history of biological research
proves it to have been a more effective and fruitful means of
advancing knowledge than the vitalistic hypothesis. We should
therefore continue to employ it for this purpose until it is clearly
shown to be untenable. Whether we must in the end adopt it will
depend on whether it proves the simplest hypothesis in the large
sense, the one most in harmony with our knowledge of nature in
general. If such is the outcome, we shall be bound by a deeply lying
instinct that is almost a law of our intellectual being to accept it,
as we have accepted the Copernican system rather than the Ptolemaic. I
believe I am right in saying that the attitude I have indicated as a
more or less personal one is also that of the body of working
biologists, though there are some conspicuous exceptions.

In endeavoring to illustrate how this question actually affects
research I will offer two illustrative cases, one of which may
indicate the fruitfulness of the mechanistic conception in the
analysis of complex and apparently mysterious phenomena, the other the
nature of the difficulties that have in recent years led to attempts
to re-establish the vitalistic view. The first example is given by the
so-called law or principle of Mendel in heredity. The principle
revealed by Mendel's wonderful discovery is not shown in all the
phenomena of heredity and is probably of more or less limited
application. It possesses however a profound significance because it
gives almost a demonstration that a definite, and perhaps a relatively
simple, mechanism must lie behind the phenomena of heredity in
general. Hereditary characters that conform to this law undergo
combinations, disassociations and recombinations which in certain way
suggest those that take place in chemic