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undamental 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。 But what are genetic characters？ They are characters which must appear if any development occurs。 They are usually contrasted with 〃acquired characters；〃 using the expression 〃acquired character〃 in the Lamarckian sense。 But strictly speaking they ARE acquired characters； for the zygote at first has none of the characters which it subsequently acquires； but only the power of acquiring them in response to the action of the environment。 But the characters so acquired are not what we technically understand and what Lamarck meant by 〃acquired characters。〃 They are genetic characters； as defined above。 What then are Lamarck's 〃acquired characters〃？ They are variations in genetic characters caused in a particular way。 There are； in fact； two kinds of variation in genetic characters depending on the mode of causation。 Firstly； there are those variations consequent upon a variation in the constitution of the protoplasm of a particular zygote； and independent of the environment in which the organism develops； save in so far as this simply calls them forth： these are the so…called genetic or mutational variations。 Secondly； there are those variations which occur in zygotes of similar germinal constitution and which are caused solely by differences in the environment to which the individuals are respectively exposed： these are the 〃acquired characters〃 of Lamarck and of authors generally。 In consequence of this double sense in which the term 〃acquired characters〃 may be used； great confusion may and does occur。 If the protoplasm be compared to a machine； and the external conditions to the hand that works the machine； then it may be said that； as the machine can only work in one way； it can only produce one kind of result （genetic character）； but the particular form or quality （Lamarckian 〃acquired character〃） of the result will depend upon the hand that works the machine （environment）； just as the quality of the sound produced by a fiddle depends entirely upon the hand which plays upon it。 It would be improper to apply the term 〃mutation〃 to those genetic characters which are not new characters or new variants of old characters； but such genetic characters are of the same nature as those characters to which the term mutation has been applied。 It may be noticed in passing that it is very questionable if the modern biologist has acted in the real interests of science in applying the term mutation in the sense in which he has applied it。 The genetic characters of organisms come from one of two sources： either they are old characters and are due to the action of what we call inheritance or they are new and are due to what we call variation。 If the term mutation is applied to the actual alteration of the machinery of the protoplasm； no objection can be felt to its use； but if it be applied； as it is； to the product of the action of the altered machine； viz。 to the new genetic character； it leads to confusion。 Inheritance is the persistence of the structure of the machine； characters are the products of the wor