al 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 Cuenot 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 is to produce again pure
white mice of the composition AGAB. The second union, AY x CB is to
produce mice that appear pure _yellow_, and have the formula AYCB.
What, now, will be the result of uniting the two forms thus
produced--_i.e._ AGAB x AYCB? Cuenot's prediction was that they should
yield eight different kinds of mice, of which four should be white,
two yellow, one black and one gray. The actual aggregate result of
such unions, repeatedly performed, compared with the theoretic
expectation, is shown in the foregoing table. As will be seen, the
correspondence, though close, is not absolutely exact, yet is near
enough to prove the validity of the principle on which the prediction
was based, and we may be certain that had a much larger number of
these mice been reared the correspondence would have been still
closer. I have purposely selected a somewhat complicated example, and
time will not admit of a full explanation of the manner in which this
par