ther by the theory of types； which owed its origin to the great anatomists of the preceding generation； to Cuvier； L。 Agassiz； J。 Muller； and R。 Owen； they were now combined together into one organic whole by the theory of descent and by the hypothesis of recapitulation which was deduced from that theory。 The view （First clearly enunciated by Fritz Muller in his well…known work； 〃Fur Darwin〃； Leipzig； 1864； （English Edition； 〃Facts for Darwin〃； 1869）。） that a knowledge of embryonic and larval histories would lay bare the secrets of race…history and enable the course of evolution to be traced； and so lead to the discovery of the natural system of classification； gave a powerful stimulus to morphological study in general and to embryological investigation in particular。 In Darwin's words： 〃Embryology rises greatly in interest； when we look at the embryo as a picture； more or less obscured； of the progenitor； either in its adult or larval state； of all the members of the same great class。〃 （〃Origin〃 （6th edition）； page 396。） In the period under consideration the output of embryological work has been enormous。 No group of the animal kingdom has escaped exhaustive examination and no effort has been spared to obtain the embryos of isolated and out of the way forms； the development of which might have an important bearing upon questions of phylogeny and classification。 Marine zoological stations have been established； expeditions have been sent to distant countries； and the methods of investigation have been greatly improved。 The result of this activity has been that the main features of the developmental history of all the most important animals are now known and the curiosity as to developmental processes； so greatly excited by the promulgation of the Darwinian theory； has to a considerable extent been satisfied。
To what extent have the results of this vast activity fulfilled the expectations of the workers who have achieved them？ The Darwin centenary is a fitting moment at which to take stock of our position。 In this inquiry we shall leave out of consideration the immense and intensely interesting additions to our knowledge of Natural History。 These may be said to constitute a capital fund upon which philosophers； poets and men of science will draw for many generations。 The interest of Natural History existed long before Darwinian evolution was thought of and will endure without any reference to philosophic speculations。 She is a mistress in whose face are beauties and in whose arms are delights elsewhere unattainable。 She is and always has been pursued for her own sake without any reference to philosophy； science； or utility。
Darwin's own views of the bearing of the facts of embryology upon questions of wide scientific interest are perfectly clear。 He writes （〃Origin〃 （6th edition）； page 395。）：
〃On the other hand it is highly probable that with many animals the embryonic or larval stages show us； more or less completely； the condition of the progenitor of the whole group in its adult state。 In the great class of the Crustacea； forms wonderfully distinct from each other； namely； suctorial parasites； cirripedes； entomostraca； and even the malacostraca； appear at first as larvae under the nauplius…form； and as these larvae live and feed in the open sea； and are not adapted for any peculiar habits of life； and from other reasons assigned by Fritz Muller； it is probable that at some very remote period an independent adult animal； resembling the Nauplius； existed； and subsequently produced； along several divergent lines of descent； the above…named great Crustacean groups。 So again it is probable； from what we know of the embryos of mammals； birds； fishes； and reptiles； that these animals are the modified descendants of some ancient progenitor； which was furnished in its adult state with branchiae； a swim… bladder； four fin…like limbs； and a long tail； all fitted for an aquatic life。
〃As all the organic beings； extinct and recent； which have ever lived； can be arranged within a few great classes； and as 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 ver