darwin and modern science-第43章
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out the less well…equipped in the matter of reproduction； and thus the relevant characters will be increased and perfected through sexual selection。 It is； of course； a necessary assumption that these secondary sexual characters may be transmitted to the female； although perhaps in rudimentary form。
As we have said； this theory of sexual selection takes up a great deal of space in Darwin's book； and it need only be considered here in so far as Darwin applied it to the descent of man。 To this latter problem the whole of Part I is devoted； while Part III contains a discussion of sexual selection in relation to man； and a general summary。 Part II treats of sexual selection in general； and may be disregarded in our present study。 Moreover； many interesting details must necessarily be passed over in what follows； for want of space。
The first part of the 〃Descent of Man〃 begins with an enumeration of the proofs of the animal descent of man taken from the structure of the human body。 Darwin chiefly emphasises the fact that the human body consists of the same organs and of the same tissues as those of the other mammals； he shows also that man is subject to the same diseases and tormented by the same parasites as the apes。 He further dwells on the general agreement exhibited by young； embryonic forms； and he illustrates this by two figures placed one above the other； one representing a human embryo； after Eaker； the other a dog embryo； after Bischoff。 （〃Descent of Man〃 （Popular Edition； 1901）； fig。 1； page 14。）
Darwin finds further proofs of the animal origin of man in the reduced structures； in themselves extremely variable； which are either absolutely useless to their possessors； or of so little use that they could never have developed under existing conditions。 Of such vestiges he enumerates： the defective development of the panniculus carnosus （muscle of the skin） so widely distributed among mammals； the ear…muscles； the occasional persistence of the animal ear…point in man； the rudimentary nictitating membrane （plica semilunaris） in the human eye； the slight development of the organ of smell； the general hairiness of the human body； the frequently defective development or entire absence of the third molar （the wisdom tooth）； the vermiform appendix； the occasional reappearance of a bony canal （foramen supracondyloideum） at the lower end of the humerus； the rudimentary tail of man （the so…called taillessness）； and so on。 Of these rudimentary structures the occasional occurrence of the animal ear…point in man is most fully discussed。 Darwin's attention was called to this interesting structure by the sculptor Woolner。 He figures such a case observed in man； and also the head of an alleged orang…foetus； the photograph of which he received from Nitsche。
Darwin's interpretation of Woolner's case as having arisen through a folding over of the free edge of a pointed ear has been fully borne out by my investigations on the external ear。 （G。 Schwalbe； 〃Das Darwin'sche Spitzohr beim menschlichen Embryo〃； 〃Anatom。 Anzeiger〃； 1889； pages 176… 189； and other papers。） In particular； it was established by these investigations that the human foetus； about the middle of its embryonic life； possesses a pointed ear somewhat similar to that of the monkey genus Macacus。 One of Darwin's statements in regard to the head of the orang… foetus must be corrected。 A LARGE ear with a point is shown in the photograph （〃Descent of Man〃； fig。3； page 24。）； but it can easily be demonstratedand Deniker has already pointed this outthat the figure is not that of an orang…foetus at all； for that form has much smaller ears with no point； nor can it be a gibbon…foetus； as Deniker supposes； for the gibbon ear is also without a point。 I myself regard it as that of a Macacus…embryo。 But this mistake； which is due to Nitsche； in no way affects the fact recognised by Darwin； that ear…forms showing the point characteristic of the animal ear occur in man with extraordinary frequency。
Finally； there is a discussion of those rudimentary structures which occur only in ONE sex； such as the rudimentary mammary glands in the male； the vesicula prostatica； which corresponds to the uterus of the female； and others。 All these facts tell in favour of the common descent of man and all other vertebrates。 The conclusion of this section is characteristic： 〃IT IS ONLY OUR NATURAL PREJUDICE； AND THAT ARROGANCE WHICH MADE OUR FOREFATHERS DECLARE THAT THEY WERE DESCENDED FROM DEMI…GODS； WHICH LEADS US TO DEMUR TO THIS CONCLUSION。 BUT THE TIME WILL BEFORE LONG COME； WHEN IT WILL BE THOUGHT WONDERFUL THAT NATURALISTS； WHO WERE WELL ACQUAINTED WITH THE COMPARATIVE STRUCTURE AND DEVELOPMENT OF MAN； AND OTHER MAMMALS； SHOULD HAVE BELIEVED THAT EACH WAS THE WORK OF A SEPARATE ACT OF CREATION。〃 （Ibid。 page 36。）
In the second chapter there is a more detailed discussion； again based upon an extraordinary wealth of facts； of the problem as to the manner in which； and the causes through which； man evolved from a lower form。 Precisely the same causes are here suggested for the origin of man； as for the origin of species in general。 Variability； which is a necessary assumption in regard to all transformations； occurs in man to a high degree。 Moreover； the rapid multiplication of the human race creates conditions which necessitate an energetic struggle for existence； and thus afford scope for the intervention of natural selection。 Of the exercise of ARTIFICIAL selection in the human race； there is nothing to be said； unless we cite such cases as the grenadiers of Frederick William I； or the population of ancient Sparta。 In the passages already referred to and in those which follow； the transmission of acquired characters； upon which Darwin does not dwell； is taken for granted。 In man； direct effects of changed conditions can be demonstrated （for instance in regard to bodily size）； and there are also proofs of the influence exerted on his physical constitution by increased use or disuse。 Reference is here made to the fact； established by Forbes； that the Quechua…Indians of the high plateaus of Peru show a striking development of lungs and thorax； as a result of living constantly at high altitudes。
Such special forms of variation as arrests of development （microcephalism） and reversion to lower forms are next discussed。 Darwin himself felt （〃Descent of Man〃； page 54。） that these subjects are so nearly related to the cases mentioned in the first chapter； that many of them might as well have been dealt with there。 It seems to me that it would have been better so； for the citation of additional instances of reversion at this place rather disturbs the logical sequence of his ideas as to the conditions which have brought about the evolution of man from lower forms。 The instances of reversion here discussed are microcephalism； which Darwin wrongly interpreted as atavistic； supernumerary mammae； supernumerary digits； bicornuate uterus； the development of abnormal muscles； and so on。 Brief mention is also made of correlative variations observed in man。
Darwin next discusses the question as to the manner in which man attained to the erect position from the state of a climbing quadruped。 Here again he puts the influence of Natural Selection in the first rank。 The immediate progenitors of man had to maintain a struggle for existence in which success was to the more intelligent； and to those with social instincts。 The hand of these climbing ancestors； which had little skill and served mainly for locomotion； could only undergo further development when some early member of the Primate series came to live more on the ground and less among trees。
A bipedal existence thus became possible； and with it the liberation of the hand from locomotion； and the one…sided development of the human foot。 The upright position brought about correlated variations in the bodily structure； with the free use of the hand it became possible to manufacture weapons and to use them； and this again resulted in a degeneration of the powerful canine teeth and the jaws； which were then no longer necessary for defence。 Above all； however； the intelligence immediately increased； and with it skull and bra
