rough the development？ All the clear evidence we can get tends to show that marked variations； whether of reduction or increase； of organs are manifest during the whole of the development of the organ and do not merely affect the adult。 And on reflection we see that it could hardly be otherwise。 All such evidence is distinctly at variance with the theory of recapitulation； at least as applied to embryos。 In the case of larvae of course the case will be different； for in them the organs are functional； and reduction in the adult will not be accompanied by reduction in the larva unless a change in the conditions of life of the larva enables it to occur。
If after 50 years of research and close examination of the facts of embryology the recapitulation theory is still without satisfactory proof； it seems desirable to take a wider sweep and to inquire whether the facts of embryology cannot be included in a larger category。
As has been pointed out by Huxley； development and life are co…extensive； and it is impossible to point to any period in the life of an organism when the developmental changes cease。 It is true that these changes take place more rapidly at the commencement of life； but they are never wholly absent； and those which occur in the later or so…called adult stages of life do not differ in their essence； however much they may differ in their degree； from those which occur during the embryonic and larval periods。 This consideration at once brings the changes of the embryonic period into the same category as those of the adult and suggests that an explanation which will account for the one will account for the other。 What then is the problem we are dealing with？ Surely it is this： Why does an organism as soon as it is established at the fertilisation of the ovum enter upon a cycle of transformations which never cease until death puts an end to them？ In other words what is the meaning of that cycle of changes which all organisms present in a greater or less degree and which constitute the very essence of life？ It is impossible to give an answer to this question so long as we remain within the precincts of Biologyand it is not my present purpose to penetrate beyond those precincts into the realms of philosophy。 We have to do with an ultimate biological fact； with a fundamental property of living matter； which governs and includes all its other properties。 How may this property be stated？ Thus： it is a property of living matter to react in a remarkable way to external forces without undergoing destruction。 The life…cycle； of which the embryonic and larval periods are a part； consists of the orderly interaction between the organism and its environment。 The action of the environment produces certain morphological changes in the organism。 These changes enable the organism to come into relation with new external forces； to move into what is practically a new environment； which in its turn produces further structural changes in the organism。 These in their turn enable； indeed necessitate； the organism to move again into a new environment； and so the process continues until the structural changes are of such a nature that the organism is unable to adapt itself to the environment in which it finds itself。 The essential condition of success in this process is that the organism should always shift into the environment to which its new structure is suitedany failure in this leading to the impairment of the organism。 In most cases the shifting of the environment is a very gradual process （whether consisting in the very slight and gradual alteration in the relation of the embryo as a whole to the egg…shell or uterine wall； or in the relations of its parts to each other； or in the successive phases of adult life）； and the morphological changes in connection with each step of it are but slight。 But in some cases jumps are made such as we find in the phenomena known as hatching； birth； and metamorphosis。
This property of reacting to the environment without undergoing destruction is； as has been stated； a fundamental property of organisms。 It is impossible to conceive of any matter； to which the term living could be applied； being without it。 And with this property of reacting to the environment goes the further property of undergoing a change which alters the relation of the organism to the old environment and places it in a new environment。 If this reasoning is correct； it necessarily follows that this property must have been possessed by living matter at its first appearance on the earth。 In other words living matter must always have presented a life…cycle； and the question arises what kind of modification has that cycle undergone？ Has it increased or diminished in duration 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 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 independent existence and is exposed to the action of natural selection there is no reason why it should escape modification at any stage。
If there is any truth in these considerations it would seem to follow that at the dawn of life the life…cycle must have been； either in posse or in esse； at least as long as it is at the present time； and that the peculiarity of passing through a series of stages in which new characters are successively evolved is a primordial quality of living matter。
Before leaving this part of the subject； it is necessary to touch upon another aspect of it。 What are these variations in structure which succeed one another in the life…history of an organism？ I am conscious that I am here on the threshold of a chamber which contains the clue to some of our difficulties； and that I cannot enter it。 Looked at from one point of view they belong to the class of genetic variations； which depend upon the structure or constitution of the protoplasm； but instead of appearing in different zygotes （A zygote is a fertilised ovum； i。e。 a new organism resulting from the fusion of an ovum and a spermatozoon。）； they are present in the same zygote though at different times in its life…history。 They are of the same order as the mutational variations of the modern biologist upon which the appearance of a new character depends。 What is a genetic or mutational variation？ It is a genetic character which was not present in either of the parents。 But these 〃growth variations〃 were present in the parents； and in this they differ from mutational variations