To reach such a point of view it is necessary to compare the evidence
given by hybrids, with the conclusions already attained by the
comparison of the differentiating characteristics of allied forms.
On this ground we first have to inquire what may be expected respecting
the internal nature and the outcome of the process of crossing in the
various cases cited in our former discussion.
We must always distinguish the qualities, which are the same in both
parents, from those that constitute the differentiating marks in every
single cross. In respect to the first [252] group the cross is not at
all distinguished from a normal fertilization, and ordinarily these
characters are simply left out of consideration. But it should never be
forgotten that they constitute the enormous majority, amounting to
hundreds and thousands, whereas the differentiating marks in each case
are only one or two or a few at most. The whole discussion is to be
limited to these last-named exceptions. We must consider first what
would be the nature of a cross when species are symmetrically combined,
and what must be the case when varieties are subjected to the same
treatment. In so doing, I intend to limit the discussion to the most
typical cases. We may take the crosses between elementary species of the
same or of very narrowly allied systematic species on the one side, and
on the other, limit treatment to the crossing of varieties with the
species, from which they are supposed to have sprung by a retrograde
modification.
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