n of observations of the
most valuable character with a definite purpose and without any
theoretic bias.

{213}

The task of the practical breeder who seeks to establish or fix a new
variety produced by cross-breeding in a case involving two variable
characters is simply the isolation and propagation of that one in each
sixteen of the second generation offspring which will be pure as
regards the desired combination of characters. Mendel's discovery, by
putting the breeder in possession of this information enables him to
attack this problem systematically with confidence in the outcome,
whereas hitherto his work, important and fascinating as it is, has
consisted largely of groping for a treasure in the dark. The greater
the number of separately variable characters involved in a cross, the
greater will be the number of new combinations obtainable; the greater
too will be the number of individuals which it will be necessary to
raise in order to secure all the possible combinations; and the
greater again will be the difficulty of isolating the pure, that is,
the stable forms in such as are similar to them in appearance, but
still hybrid in one or more characters.

The law of Mendel reduces to an exact science the art of breeding in
the case most carefully studied by him, that of entire dominance. It
gives to the breeder a new conception of "purity." No animal or plant
is "pure," simply because it is descended from a long line of
ancestors, possessing a desired combination of characters; but any
animal or plant is pure if it produces _gametes_--that is, particles
for conjugation of only one sort--even though its grandparents may
among {214} themselves have possessed opposite characters. The
existence of purity can be established with certainty only by suitable
breeding tests, especially by crossing with recessives; but it may be
safely assumed for any animal or plant, descended from parents which
were like each other and had been shown by breeding tests to be pure.

This naturally leads us to what some biologists have considered to be
the most important part of his work--the theory which he elaborated to
explain his results, the principle which he considers to be the basis
of the laws he discovered. Mendel suggests as following logically from
the results of his experiments and observations a certain theory of
the constitution of germinal particles. He has put this important
matter so clearly himself and with such little