darwin and modern science-第48章
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ses further bring home to us the fact； to which I have already referred； that the full development of the functional reproductive organs is nearly always associated with the final stages of the life…history。
On this view of the succession of characters in the life…history of organisms； how shall we explain the undoubted fact that the development of buds hardly ever presents any phenomena corresponding to the embryonic and larval changes？ The reason is clearly this； that budding usually occurs after the embryonic stage is past； when the characters of embryonic life have been worked out by the machine。 When it takes place at an early stage in embryonic life； as it does in cases of so…called embryonic fission； the product shows； either partly or entirely； phenomena similar to those of embryonic development。 The only case known to me in which budding by the adult is accompanied by morphological features similar to those displayed by embryos is furnished by the budding of the medusiform spore…sacs of hydrozoon polyps。 But this case is exceptional； for here we have to do with an attempt； which fails； to form a free…swimming organism； the medusa； and the vestiges which appear in the buds are the umbrella…cavity； marginal tentacles； circular canal； etc。； of the medusa arrested in development。
But the question still remains； are there no cases in which； as implied by the recapitulation theory； variations in any organ are confined to the period in which the organ is functional and do not affect it in the embryonic stages？ The teeth of the whalebone whales may be cited as a case in which this is said to occur； but here the teeth are only imperfectly developed in the embryo and are soon absorbed。 They have been affected by the change which has produced their disappearance in the adult； but not to complete extinction。 Nor are they now likely to be extinguished； for having become exclusively embryonic they are largely protected from the action of natural selection。 This consideration brings up a most important aspect of the question； so far as disappearing organs are concerned。 Every organ is laid down at a certain period in the embryo and undergoes a certain course of growth until it obtains full functional development。 When for any cause reduction begins； it is affected at all stages of its growth； unless it has functional importance in the larva； and in some cases its life is shortened at one or both ends。 In cases； as in that of the whale's teeth； in which it entirely disappears in the adult； the latter part of its life is cut off； in others； the beginning of its life may be deferred。 This happens； for instance； with the spiracle of many Elasmobranchs； which makes its appearance after the hyobranchial cleft； not before it as it should do； being anterior to it in position； and as it does in the Amniota in which it shows no reduction in size as compared with the other pharyngeal clefts。 In those Elasmobranchs in which it is absent in the adult but present in the embryo （e。g。 Carcharias） its life is shortened at both ends。 Many more instances of organs； of which the beginning and end have been cut off； might be mentioned； e。g。 the muscle…plate coelom of Aves； the primitive streak and the neurenteric canal of amniote blastoderms。 In yet other cases in which the reduced organ is almost on the verge of disappearance； it may appear for a moment and disappear more than once in the course of development。 As an instance of this striking phenomenon I may mention the neurenteric canal of avine embryos； and the anterior neuropore of Ascidians。 Lastly the reduced organ may disappear in the developing stages before it does so in the adult。 As an instance of this may be mentioned the mandibular palp of those Crustacea with zoaea larvae。 This structure disappears in the larva only to reappear in a reduced form in later stages。 In all these cases we are dealing with an organ which； we imagine； attained a fuller functional development at some previous stage in race…history； but in most of them we have no proof that it did so。 It may be； and the possibility must not be lost sight of； that these organs never were anything else than functionless and that though they have been got rid of in the adult by elimination in the course of time； they have been able to persist in embryonic stages which are protected from the full action of natural selection。 There is no reason to suppose that living matter at its first appearance differed from non…living matter in possessing only properties conducive to its well…being and prolonged existence。 No one thinks that the properties of the various forms of inorganic matter are all strictly related to external conditions。 Of what use to the diamond is its high specific gravity and high refrangibility； and to gold of its yellow colour and great weight？ These substances continue to exist in virtue of other properties than these。 It is impossible to suppose that the properties of living matter at its first appearance were all useful to it； for even now after aeons of elimination we find that it possesses many useless organs and that many of its relations to the external world are capable of considerable improvement。
In writing this essay I have purposely refrained from taking a definite position with regard to the problems touched。 My desire has been to write a chapter showing the influence of Darwin's work so far as Embryology is concerned； and the various points which come up for consideration in discussing his views。 Darwin was the last man who would have claimed finality for any of his doctrines； but he might fairly have claimed to have set going a process of intellectual fermentation which is still very far from completion。
XI。 THE PALAEONTOLOGICAL RECORD。
I。 ANIMALS。
By W。B。 SCOTT。 Professor of Geology in the University of Princeton； U。S。A。
To no branch of science did the publication of 〃The Origin of Species〃 prove to be a more vivifying and transforming influence than to Palaeontology。 This science had suffered； and to some extent； still suffers from its rather anomalous position between geology and biology； each of which makes claim to its territory； and it was held in strict bondage to the Linnean and Cuvierian dogma that species were immutable entities。 There is； however； reason to maintain that this strict bondage to a dogma now abandoned； was not without its good side； and served the purpose of keeping the infant science in leading…strings until it was able to walk alone； and preventing a flood of premature generalisations and speculations。
As Zittel has said： 〃Two directions were from the first apparent in palaeontological researcha stratigraphical and a biological。 Stratigraphers wished from palaeontology mainly confirmation regarding the true order or relative age of zones of rock…deposits in the field。 Biologists had； theoretically at least； the more genuine interest in fossil organisms as individual forms of life。〃 （Zittel； 〃History of Geology and Palaeontology〃； page 363； London； 1901。） The geological or stratigraphical direction of the science was given by the work of William Smith； 〃the father of historical geology；〃 in the closing decade of the eighteenth century。 Smith was the first to make a systematic use of fossils in determining the order of succession of the rocks which make up the accessible crust of the earth； and this use has continued； without essential change； to the present day。 It is true that the theory of evolution has greatly modified our conceptions concerning the introduction of new species and the manner in which palaeontological data are to be interpreted in terms of stratigraphy； but； broadly speaking； the method remains fundamentally the same as that introduced by Smith。
The biological direction of palaeontology was due to Cuvier and his associates； who first showed that fossils were not merely varieties of existing organisms； but belonged to extinct species and genera； an altogether revolutionary conception； which startled the scientific world。 Cuvier made careful studies； especially of fossil vertebrates； from the standpoint of zoology and was thus the founder of palaeontology as a biological science。 His great work on 〃Ossements Fossiles〃 （Paris； 1821） has never been surpassed as a masterpiece of the comparative method of anatomical investigation； and has furnished to the palaeontologist the indispensable implements of research。
On the other hand； Cuvier's theoretical views regarding the history of the earth and its successive faunas and floras are such as no one believes to… day。 He held that the earth had been repeatedly devastated by great cataclysms； which destroyed every living thing； necessitating an entirely new creation； thus regarding the geological periods as sharply demarcated and strictly contemporaneous for the whole earth； and each species of animal and plant as confined to a single period。 Cuvier's immense authority and his commanding personality dominated scientific thought for more than a generation and marked out the line which the development of palaeontology was to follow。 The work was enthusiastically taken up by many very able men in the various European countries and in the United States； but； controlled as it was by the belief in the fixity of species； it remained almost entirely descriptive and consisted in the description and classification of the different groups of fossil organisms。 As already intimated； this narrowness of view had its compensations； for it deferred generalisations until some adequate foundations for these had been laid。
Dominant as it was； Cuvier's authority was slowly undermined by the progress of knowledge and the way was prepared for the introduction of more rational conceptions。 The theory of 〃Catastrophism〃 was attacked by several geologists； most effectively by Sir Charles Lyell； who greatly amplified the principles enunciated by Hutton and Playfair in the preceding century； and inaugurated a new era in geology。 Lyell's uniformitarian views of the earth's history and of the agencies which had wrought its changes； had undoubted effect in educating men's minds for the acceptance of essentially similar views regarding the organic world。 In palaeontology too the doctrine of the immutability of species； though vehemently maintained and reasserted； was gradually weakening。 In reviewing long series of fossils； relations were observed which pointed to genetic connections and yet were interpreted as purely ideal。 Agassiz； for example； who never accepted the evolutionary theory； drew attention to facts which could be satisfactorily interpreted only in terms of that theory。 Among the fossils he indicated 〃progressive；〃 〃synthetic；〃 〃prophetic；〃 and 〃embryonic〃 types； and pointed out the parallelism which obtains between the geological succession of ancient animals and the ontogenetic development of recent forms。 In Darwin's words： 〃This view accords admirably well with our theory。〃 （〃Origin of Species〃 （6th edition）； page 310。） Of similar import were Owen's views on 〃generalised types〃 and 〃archetypes。〃
The appearance of 〃The Origin of Species〃 in 1859 revolutionised all the biological sciences。 From the very nature of the case； Darwin was compelled to give careful consi
