darwin and modern science-第30章
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dition。 In the reduction nucleus of the gonotokonts the homologous chromosomes being near together need not seek out one another； they are ready to form gemini。 The next stage is their separation to the haploid daughter…nuclei； which have resulted from the reduction process。
Peculiar phenomena in the reduction nucleus accompany the formation of gemini in both organic kingdoms。 （This has been shown more particularly by the work of L。 Guignard； M。 Mottier； J。B。 Farmer； C。B。 Wilson； V。 Hacker and more recently by V。 Gregoire and his pupil C。A。 Allen； by the researches conducted in the Bonn Botanical Institute； and by A。 and K。E。 Schreiner。） Probably for the purpose of entering into most intimate relation； the pairs are stretched to long threads in which the chromomeres come to lie opposite one another。 （C。A。 Allen； A。 and K。E。 Schreiner； and Strasburger。） It seems probable that these are homologous chromomeres； and that the pairs afterwards unite for a short time； so that an exchange of hereditary units is rendered possible。 （H。 de Vries and Strasburger。） This cannot be actually seen； but certain facts of heredity point to the conclusion that this occurs。 It follows from these phenomena that any exchange which may be effected must be one of homologous carriers of hereditary units only。 These units continue to form exchangeable segments after they have undergone unequal changes； they then constitute allelotropic pairs。 We may thus calculate what sum of possible combinations the exchange of homologous hereditary units between the pairing chromosomes provides for before the reduction division and the subsequent distribution of paternal and maternal chromosomes in the haploid daughter…nuclei。 These nuclei then transmit their characters to the sexual cells； the conjugation of which in fertilization again produces the most varied combinations。 （A。 Weismann gave the impulse to these ideas in his theory on 〃Amphimixis〃。） In this way all the cooperations which the carriers of hereditary characters are capable of in a species are produced； this must give it an appreciable advantage in the struggle for life。
The admirers of Charles Darwin must deeply regret that he did not live to see the results achieved by the new Cytology。 What service would they have been to him in the presentation of his hypothesis of Pangenesis； what an outlook into the future would they have given to his active mind！
The Darwinian hypothesis of Pangenesis rests on the conception that all inheritable properties are represented in the cells by small invisible particles or gemmules and that these gemmules increase by division。 Cytology began to develop on new lines some years after the publication in 1868 of Charles Darwin's 〃Provisional hypothesis of Pangenesis〃 （〃Animals and Plants under Domestication〃； London； 1868； Chapter XXVII。）； and when he died in 1882 it was still in its infancy。 Darwin would have soon suggested the substitution of the nuclei for his gemmules。 At least the great majority of present…day investigators in the domain of cytology have been led to the conclusion that the nucleus is the carrier of hereditary characters； and they also believe that hereditary characters are represented in the nucleus as distinct units。 Such would be Darwin's gemmules； which in conformity with the name of his hypothesis may be called pangens （So called by H。 de Vries in 1889。）： these pangens multiply by division。 All recently adopted views may be thus linked on to this part of Darwin's hypothesis。 It is otherwise with Darwin's conception to which Pangenesis owes its name； namely the view that all cells continually give off gemmules； which migrate to other places in the organism； where they unite to form reproductive cells。 When Darwin foresaw this possibility； the continuity of the germinal substance was still unknown （Demonstrated by Nussbaum in 1880； by Sachs in 1882； and by Weismann in 1885。）； a fact which excludes a transference of gemmules。
But even Charles Darwin's genius was confined within finite boundaries by the state of science in his day。
It is not my province to deal with other theories of development which followed from Darwin's Pangenesis； or to discuss their histological probabilities。 We can； however； affirm that Charles Darwin's idea that invisible gemmules are the carriers of hereditary characters and that they multiply by division has been removed from the position of a provisional hypothesis to that of a well…founded theory。 It is supported by histology； and the results of experimental work in heredity； which are now assuming extraordinary prominence； are in close agreement with it。
VII。 〃THE DESCENT OF MAN〃
By G。 SCHWALBE。 Professor of Anatomy in the University of Strassburg。
The problem of the origin of the human race； of the descent of man； is ranked by Huxley in his epoch…making book 〃Man's Place in Nature〃； as the deepest with which biology has to concern itself； 〃the question of questions；〃the problem which underlies all others。 In the same brilliant and lucid exposition； which appeared in 1863； soon after the publication of Darwin's 〃Origin of Species〃； Huxley stated his own views in regard to this great problem。 He tells us how the idea of a natural descent of man gradually grew up in his mind； it was especially the assertions of Owen in regard to the total difference between the human and the simian brain that called forth strong dissent from the great anatomist Huxley； and he easily succeeded in showing that Owen's supposed differences had no real existence； he even established； on the basis of his own anatomical investigations； the proposition that the anatomical differences between the Marmoset and the Chimpanzee are much greater than those between the Chimpanzee and Man。
But why do we thus introduce the study of Darwin's 〃Descent of Man〃； which is to occupy us here； by insisting on the fact that Huxley had taken the field in defence of the descent of man in 1863； while Darwin's book on the subject did not appear till 1871？ It is in order that we may clearly understand how it happened that from this time onwards Darwin and Huxley followed the same great aim in the most intimate association。
Huxley and Darwin working at the same Problema maximum！ Huxley fiery； impetuous； eager for battle； contemptuous of the resistance of a dull world； or energetically triumphing over it。 Darwin calm； weighing every problem slowly； letting it mature thoroughly；not a fighter； yet having the greater and more lasting influence by virtue of his immense mass of critically sifted proofs。 Darwin's friend； Huxley； was the first to do him justice； to understand his nature； and to find in it the reason why the detailed and carefully considered book on the descent of man made its appearance so late。 Huxley； always generous； never thought of claiming priority for himself。 In enthusiastic language he tells how Darwin's immortal work； 〃The Origin of Species〃； first shed light for him on the problem of the descent of man； the recognition of a vera causa in the transformation of species illuminated his thoughts as with a flash。 He was now content to leave what perplexed him； what he could not yet solve； as he says himself； 〃in the mighty hands of Darwin。〃 Happy in the bustle of strife against old and deep…rooted prejudices； against intolerance and superstition； he wielded his sharp weapons on Darwin's behalf； wearing Darwin's armour he joyously overthrew adversary after adversary。 Darwin spoke of Huxley as his 〃general agent。〃 （〃Life and Letters of Thomas Henry Huxley〃； Vol。 I。 page 171； London； 1900。） Huxley says of himself 〃I am Darwin's bulldog。〃 （Ibid。 page 363。）
Thus Huxley openly acknowledged that it was Darwin's 〃Origin of Species〃 that first set the problem of the descent of man in its true light； that made the question of the origin of the human race a pressing one。 That this was the logical consequence of his book Darwin himself had long felt。 He had been reproached with intentionally shirking the application of his theory to Man。 Let us hear what he says on this point in his autobiography： 〃As soon as I had become； in the year 1837 or 1838； convinced that species were mutable productions； I could not avoid the belief that man must come under the same law。 Accordingly I collected notes on the subject for my own satisfaction； and not for a long time with any intention of publishing。 Although in the 'Origin of Species' the derivation of any particular species is never discussed； yet I thought it best； in order THAT NO HONOURABLE MAN SHOULD ACCUSE ME OF CONCEALING MY VIEWS （No italics in original。）； to add that by the work 'light would be thrown on the origin of man and his history。' It would have been useless and injurious to the success of the book to have paraded； without giving any evidence； my conviction with respect to his origin。〃 （〃Life and Letters of Charles Darwin〃； Vol。 1。 page 93。）
In a letter written in January； 1860； to the Rev。 L。 Blomefield； Darwin expresses himself in similar terms。 〃With respect to man； I am very far from wishing to obtrude my belief； but I thought it dishonest to quite conceal my opinion。〃 （Ibid。 Vol。 II。 page 263。）
The brief allusion in the 〃Origin of Species〃 is so far from prominent and so incidental that it was excusable to assume that Darwin had not touched upon the descent of man in this work。 It was solely the desire to have his mass of evidence sufficiently complete； solely Darwin's great characteristic of never publishing till he had carefully weighed all aspects of his subject for years； solely； in short； his most fastidious scientific conscience that restrained him from challenging the world in 1859 with a book in which the theory of the descent of man was fully set forth。 Three years； frequently interrupted by ill…health； were needed for the actual writing of the book （〃Life and Letters〃； Vol。 I。 page 94。）： the first edition； which appeared in 1871； was followed in 1874 by a much improved second edition； the preparation of which he very reluctantly undertook。 （Ibid。 Vol。 III。 page 175。）
This； briefly； is the history of the work； which； with the 〃Origin of Species〃； marks an epoch in the history of biological sciencesthe work with which the cautious； peace…loving investigator ventured forth from his contemplative life into the arena of strife and unrest； and laid himself open to all the annoyances that deep…rooted belief and prejudice； and the prevailing tendency of scientific thought at the time could devise。
Darwin did not take this step lightly。 Of great interest in this connection is a letter written to Wallace on Dec。 22； 1857 （Ibid。 Vol。 II。 page 109。）； in which he says 〃You ask whether I shall discuss 'man。' I think I shall avoid the whole subject； as so surrounded with prejudices； though I fully admit that it is the highest and most interesting problem for the naturalist。〃 But his conscientiousness compelled him to state briefly his opinion on the subject in the 〃Origin of Species〃 in 1859。 Nevertheless he did not escape reproaches for having been so reticent。 This is unmistakably apparent from a letter to Fritz Muller dated February 22 （1869？）； in which he says： 〃I am thinking of writing a little essay on the Origin of Manki
