o that
quality of the original parent which did not appear in the second
generation was found to occur, thus showing that, though it cannot be
detected, it is present, Mendel called it the recessive quality. He
did not find transitional forms in any of his experiments, but
constantly observed that when plants were bred with regard to two
special qualities, one of those qualities became dominant in the
resultant hybrid, and the other became recessive, that is, present
though latent and ready to produce its effects upon a definite
proportion of the succeeding generation.

{210}

Remembering, then, that Mendel means by hybrid the result of the
crossing of two distinct species, his significant discovery has been
stated thus: The hybrid, whatever its own character, produces ripe
germ cells, which bear only the pure character of one parent or the
other. Thus, when one parent has the character "A," in peas, for
example, a green color, and the other the character "B," in peas once
more a yellow color, the hybrid will have in cases of simple dominance
the character "AB" or "BA," but with the second quality in either case
not noticeable. Whatever the character of the hybrid may be, that is
to say, to revert to the example of the peas, whether it be green or
yellow, its germ cells when mature will bear either the character "A"
(green), or the character "B" (yellow), but not both.

As Professor Castle says: "This perfectly simple principle is known as
the law of segregation, or the law of the purity of the germ cells. It
bids fair to prove as fundamental to a right understanding of the
facts of heredity as is the law of definite proportions in chemistry.
From it follow many important consequences."

To follow this acute observer's work still further by letting the
crossbreds fertilize themselves, Mendel raised a third generation. In
this generation were individuals which showed the dominant character
and also individuals which presented the recessive character. Such an
observation had of course been made in a good many instances before.

{211}

But Mendel noted--and this is the essence of the new discovery in his
observations--that in this third generation the numerical proportion
of dominants to recessives is in the average of a series of cases
approximately constant--being, in fact, as three to one. With almost
absolute regularity this proportion was maintained in every case of
crossing of pairs of characters, quite oppos