crosses and accurate recording of all the facts. The result has been a
pedigree which now permits of stating the relation between all the
descendants of my original introduced plant. This pedigree at once
exhibits the laws followed by the mutating species. The main fact is,
that it does not change itself gradually, but remains unaffected during
all succeeding generations. It only throws off new forms, which are
sharply contrasted with the parent, and which are from the very
beginning as perfect and as constant, as narrowly [29] defined and as
pure of type as might be expected of any species.

These new species are not produced once or in single individuals, but
yearly and in large numbers. The whole phenomenon conveys the idea of a
close group of mutations, all belonging to one single condition of
mutability. Of course this mutable state must have had a beginning, as
it must sometime come to an end. It is to be considered as a period
within the life-time of the species and probably it is only a small part
of it.

The detailed description of this experiment, however, I must delay to a
subsequent lecture, but I may be allowed to state, that the discovery of
this period of mutability is of a definite theoretical importance. One
of the greatest objections to the Darwinian theory of descent arose from
the length of time it would require, if all evolution was to be
explained on the theory of slow and nearly invisible changes. This
difficulty is at once met and fully surmounted by the hypothesis of
periodical but sudden and quite noticeable steps. This assumption
requires only a limited number of mutative periods, which might well
occur within the time allowed by physicists and geologists for the
existence of animal and vegetable life on the earth.