s all within each class have； according to our theory； been connected together by fine gradations； the best； and； if our collections were nearly perfect； the only possible arrangement； would be genealogical； descent being the hidden bond of connexion which naturalists have been seeking under the term of the Natural System。 On this view we can understand how it is that； in the eyes of most naturalists； the structure of the embryo is even more important for classification than that of the adult。 In two or more groups of animals； however much they may differ from each other in structure and habits in their adult condition； if they pass through closely similar embryonic stages； we may feel assured that they all are descended from one parent… form； and are therefore closely related。 Thus； community in embryonic structure reveals community of descent； but dissimilarity in embryonic development does not prove discommunity of descent； for in one of two groups the developmental stages may have been suppressed； or may have been so greatly modified through adaptation to new habits of life； as to be no longer recognisable。 Even in groups； in which the adults have been modified to an extreme degree； community of origin is often revealed by the structure of the larvae； we have seen； for instance； that cirripedes； though externally so like shell…fish； are at once known by their larvae to belong to the great class of crustaceans。 As the embryo often shows us more or less plainly the structure of the less modified and ancient progenitor of the group； we can see why ancient and extinct forms so often resemble in their adult state the embryos of existing species of the same class。 Agassiz believes this to be a universal law of nature； and we may hope hereafter to see the law proved true。 It can； however； be proved true only in those cases in which the ancient state of the progenitor of the group has not been wholly obliterated； either by successive variations having supervened at a very early period of growth； or by such variations having been inherited at an earlier stage than that at which they first appeared。 It should also be borne in mind； that the law may be true； but yet； owing to the geological record not extending far enough back in time； may remain for a long period； or for ever； incapable of demonstration。 The law will not strictly hold good in those cases in which an ancient form became adapted in its larval state to some special line of life； and transmitted the same larval state to a whole group of descendants； for such larvae will not resemble any still more ancient form in its adult state。〃
As this passage shows； Darwin held that embryology was of interest because of the light it seems to throw upon ancestral history （phylogeny） and because of the help it would give in enabling us to arrive at a natural system of classification。 With regard to the latter point； he quotes with approval the opinion that 〃the structure of the embryo is even more important for classification than that of the adult。〃 What justification is there for this view？ The phase of life chosen for the ordinary anatomical and physiological studies； namely； the adult phase； is merely one of the large number of stages of structure through which the organism passes。 By far the greater number of these are included in what is specially called the developmental or （if we include larvae with embryos） embryonic period； for the developmental changes are more numerous and take place with greater rapidity at the beginning of life than in its later periods。 As each of these stages is equal in value； for our present purpose； to the adult phase； it clearly follows that if there is anything in the view that the anatomical study of organisms is of importance in determining their mutual relations； the study of the organism in its various embryonic （and larval） stages must have a greater importance than the study of the single and arbitrarily selected stage of life called the adult。
But a deeper reason than this has been assigned for the importance of embryology in classification。 It has been asserted； and is implied by Darwin in the passage quoted； that the ancestral history is repeated in a condensed form in the embryonic； and that a study of the latter enables us to form a picture of the stages of structure through which the organism has passed in its evolution。 It enables us on this view to reconstruct the pedigrees of animals and so to form a genealogical tree which shall be the true expression of their natural relations。
The real question which we have to consider is to what extent the embryological studies of the last 50 years have confirmed or rendered probable this 〃theory of recapitulation。〃 In the first place it must be noted that the recapitulation theory is itself a deduction from the theory of evolution。 The facts of embryology； particularly of vertebrate embryology； and of larval history receive； it is argued； an explanation on the view that the successive stages of development are； on the whole； records of adult stages of structure which the species has passed through in its evolution。 Whether this statement will bear a critical verbal examination I will not now pause to inquire； for it is more important to determine whether any independent facts can be alleged in favour of the theory。 If it could be shown； as was stated to be the case by L。 Agassiz； that ancient and extinct forms of life present features of structure now only found in embryos； we should have a body of facts of the greatest importance in the present discussion。 But as Huxley （See Huxley's 〃Scientific Memoirs〃； London； 1898； Vol。 I。 page 303： 〃There is no real parallel between the successive forms assumed in the development of the life of the individual at present； and those which have appeared at different epochs in the past。〃 See also his Address to the Geological Society of London （1862） 'On the Palaeontological Evidence of Evolution'； ibid。 Vol。 II。 page 512。） has shown and as the whole course of palaeontological and embryological investigation has demonstrated； no such statement can be made。 The extinct forms of life are very similar to those now existing and there is nothing specially embryonic about them。 So that the facts； as we know them； lend no support to theory。
But there is another class of facts which have been alleged in favour of the theory； viz。 the facts which have been included in the generalisation known as the Law of v。 Baer。 The law asserts that embryos of different species of animals of the same group are more alike than the adults and that； the younger the embryo； the greater are the resemblances。 If this law could be established it would undoubtedly be a strong argument in favour of the 〃recapitulation〃 explanation of the facts of embryology。 But its truth has been seriously disputed。 If it were true we should expect to find that the embryos of closely similar species would be indistinguishable from one another； but this is notoriously not the case。 It is more difficult to meet the assertion when it is made in the form given above； for here 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； whet