Biology | Page 5

half the usual size, and a pair of twins results. In each of these cases
the astonishing fact is that a mechanical injury sets up in the organism a
complicated adaptive response in the form of operations which in the
end counteract the initial mechanical effect. It is no doubt true that

somewhat similar self-adjustments or responses may be said to take
place in certain non-living mechanical systems, such as the spinning
top or the gyroscope; but those that occur in the living body are of such
general occurrence, of such complexity and variety, and of so
design-like a quality, that they may fairly be regarded as among the
most characteristic of the vital activities. It is precisely this
characteristic of many vital phenomena that renders their accurate
analysis so difficult and complex a task; and it is largely for this reason
that the biological sciences, as a whole, still stand 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
chemical reactions; and like the latter they conform to definite
quantitative rules that are capable of arithmetical formulation. This
analogy must not be pressed too far; for chemical reactions are
individually definite and fixed, while those of the hereditary characters
involve a fortuitous element of such a nature that the numerical result is
not fixed or constant in the individual case but follows the law of
probability in the aggregate of individuals. Nevertheless, it is possible,
and has already become the custom, to designate the hereditary
organization by symbols or formulas that resemble those of the chemist
in that they imply the quantitative results of heredity that follow the
union of compounds of known composition. Quantitative
prediction--not precisely accurate, but in accordance with the law of
probability--has thus become possible to the biological experimenter on
heredity. I will give one example of such a prediction made by
Professor Cuénot in experimenting on the heredity of color in mice (see
the following table). The experiment extended through three
generations. Of the four grandparents three were pure white albinos,
identical in outward appearance, but of different hereditary capacity,
while the fourth was a pure black mouse. The first pair of grandparents
consisted of an albino of gray ancestry, AG, and one of black ancestry,
AB. The second pair consisted of an albino of yellow ancestry, AY, and
a black mouse, CB. The result of the first union, AG x AB