[18] This conception was clearly formed by Naudin simultaneously
with Mendel, but it was not worked out by him and remained a mere
suggestion. In one place also Focke came very near to the same idea
(see Bibliography).
We have seen that Mendel makes no prediction as to the outward and
visible characters of _AB_, but only as to the essential constitution
and statistical condition of its gametes in regard to the characters
_A_ and _B_. Nevertheless in a large number of cases the character of
_AB_ is known to fall into one of three categories (omitting mosaics).
(1) The cross-bred may almost always resemble one of its pure
parents so closely as to be practically indistinguishable from that
pure form, as in the case of the yellow cotyledon-colour of certain
varieties of peas when crossed with green-cotyledoned varieties; in
which case the parental character, yellow, thus manifested by the
cross-bred is called “dominant” and the parental character, green,
not manifested, is called recessive.
(2) The cross-bred may present some condition intermediate between
the two parental forms, in which case we may still retain the term
“blend” as applied to the zygote.
Such an “intermediate” may be the apparent mean between the two
parental forms or be nearer to one or other in any degree. Such
a case is that of a cross between a rich crimson Magenta Chinese
Primrose and a clear White, giving a flower of a colour appropriately
described as a “washy” magenta.
(3) The cross-bred may present some form quite different from that
of either pure parent. Though, as has been stated, nothing can be
predicted of an unknown case, we already know a considerable number
of examples of this nature in which the mule-form _approaches
sometimes with great accuracy to that of a putative ancestor, near
or remote_. It is scarcely possible to doubt that several--though
perhaps not all--of Darwin’s “reversions on crossing” were of this
nature.
Such a case is that of the “wild grey mouse” produced by the union
of an albino tame mouse and a piebald Japanese mouse[19]. These
“reversionary” mice bred together produce the parental tame types,
some other types, and “reversionary” mice again.
[19] See von Guaita, _Ber. naturf. Ges. Freiburg_ X. 1898 and XI.
1899, quoted by Professor Weldon (see later).
From what has been said it will now be clear that the applicability of
the Mendelian hypothesis has, intrinsically, nothing whatever to do
with the question of the inheritance being _blended_ or _alternative_.
In fact, as soon as the relation of zygote characters to gamete
characters is appreciated, it is difficult to see any reason for
supposing that the manifestation of characters seen in the zygotes
should give any indication as to their mode of allotment among the
gametes.
On a previous occasion I pointed out that the terms “Heredity” and
“Inheritance” are founded on a misapplication of metaphor, and in the
light of our present knowledge it is becoming clearer that the ideas of
“transmission” of a character by parent to offspring, or of there being
any “contribution” made by an ancestor to its posterity, must only be
admitted under the strictest reserve, and merely as descriptive terms.
* * * * *