178 Darwin and Embryology 
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 lhfe-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 pro- 
ceeded 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 repeti- 
tion, 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 adult, such as bird’s teeth and snake’s fore-limbs: the adult 
stage in which they have disappeared is not preceded by embryonic 
stages in which the teeth and limbs or rudiments of them are present. 
In fact the evidence indicates that adult variations of any part are 
accompanied by precedent variations in the same direction in the 
embryo. The evidence seems to show, not that a stage is added on 
at the end of the life-history, but only that some of the stages in the 
life-history are modified. Indeed, on the wider view of development 
taken in this essay, a view which makes it coincident with life, one 
would not expect often to find, even if new stages are added in the 
course of evolution, that they are added at the end of the series when 
the organism has passed through its reproductive period. It is 
possible of course that new stages have been intercalated in the 
