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 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 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 



