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 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 course of the life-history, though it is difficult to see how this has occurred. It is much more likely, if we may judge from available evidence, that every stage has had its counterpart in the ancestral form from which it has been derived by descent with modification. Just as the adult phase of the living form differs, owing to evolutionary modification, from the adult phase of the ancestor from which it has proceeded, so each larval phase will differ for the same reason from the corresponding larval phase in the life-history of the ancestor. Inasmuch as the organism is variable at every stage of its