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re we are dealing with matters of opinion。 For instance； no one would deny that the embryo of a dogfish is different from the embryo of a rabbit； but there is room for difference of opinion when it is asserted that the difference is less than the difference between an adult dogfish and an adult rabbit。 It would be perfectly true to say that the differences between the embryos concern other organs more than do the differences between the adults； but who is prepared to affirm that the presence of a cephalic coelom and of cranial segments； of external gills； of six gill slits； of the kidney tubes opening into the muscle…plate coelom； of an enormous yolk…sac； of a neurenteric canal； and the absence of any trace of an amnion； of an allantois and of a primitive streak are not morphological facts of as high an import as those implied by the differences between the adults？ The generalisation undoubtedly had its origin in the fact that there is what may be called a family resemblance between embryos and larvae； but this resemblance； which is by no means exact； is largely superficial and does not extend to anatomical detail。
It is useless to say； as Weismann has stated （〃The Evolution Theory〃； by A。 Weismann； English Translation； Vol。 II。 page 176； London； 1904。）； that 〃it cannot be disputed that the rudiments ＇vestiges his translator means＇ of gill…arches and gill…clefts； which are peculiar to one stage of human ontogeny； give us every ground for concluding that we possessed fish…like ancestors。〃 The question at issue is： did the pharyngeal arches and clefts of mammalian embryos ever discharge a branchial function in an adult ancestor of the mammalia？ We cannot therefore； without begging the question at issue in the grossest manner； apply to them the terms 〃gill… arches〃 and 〃gill…clefts〃。 That they are homologous with the 〃gill…arches〃 and 〃gill…clefts〃 of fishes is true； but there is no evidence to show that they ever discharged a branchial function。 Until such evidence is forthcoming； it is beside the point to say that it 〃cannot be disputed〃 that they are evidence of a piscine ancestry。
It must； therefore； be admitted that one outcome of the progress of embryological and palaeontological research for the last 50 years is negative。 The recapitulation theory originated as a deduction from the evolution theory and as a deduction it still remains。
Let us before leaving the subject apply another test。 If the evolution theory and the recapitulation theory are both true； how is it that living birds are not only without teeth but have no rudiments of teeth at any stage of their existence？ How is it that the missing digits in birds and mammals； the missing or reduced limb of snakes and whales； the reduced mandibulo…hyoid cleft of elasmobranch fishes are not present or relatively more highly developed in the embryo than in the adult？ How is it that when a marked variation； such as an extra digit； or a reduced limb； or an extra segment； makes its appearance； it is not confined to the adult but can be seen all through 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 liv