n and complexity since organisms first
appeared on the earth? The current view is that the cycle was at first
very short and that it has increased in length by the evolutionary
creation of new adult phases, that these new phases are in addition to
those already existing and that each of them as it appears takes
over from the preceding adult phase the functional condition of the
reproductive organs. According to the same view the old adult phases are
not obliterated but persist in a more or less modified form as larval
stages. It is further supposed that as the life-history lengthens at one
end by the addition of new adult phases, it is shortened at the other by
the abbreviation of embryonic development and by the absorption of some
of the early larval stages into the embryonic period; but on the whole
the lengthening process has exceeded that of shortening, so that the
whole life-history has, with the progress of evolution, become longer
and more complicated.

Now there can be no doubt that the life-history of organisms has been
shortened in the way above suggested, for cases are known in which this
can practically be seen to occur at the present day. But the process
of lengthening by the creation of new stages at the other end of the
life-cycle is more difficult to conceive and moreover there is no
evidence for its having occurred. This, indeed, may have occurred, as
is suggested below, but the evidence we have seems to indicate
that evolutionary modification has proceeded by ALTERING and not by
SUPERSEDING: that is to say that each stage in the life-history, as we
see it to-day, has proceeded from a corresponding stage in a former era
by the modification of that stage and not by the creation of a new one.
Let me, at the risk of repetition, explain my meaning more fully by
taking a concrete illustration. The mandibulo-hyoid cleft (spiracle)
of the elasmobranch fishes, the lateral digits of the pig's foot, the
hind-limbs of whales, the enlarged digit of the ostrich's foot
are supposed to be organs which have been recently modified.
This modification is not confined to the final adult stage of the
life-history but characterises them throughout the whole of their
development. A stage with a reduced spiracle does not proceed in
development from a preceding stage in which the spiracle shows no
reduction: it is reduced at its first appearance. The same statement may
be made of organs which have entirely disappeared in the ad