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gether and eventually assuming a definite form as special organs of the cell。 It may be also assumed that in the protoplasm and in the primitive types of nucleus； the carriers of the same hereditary unit were represented in considerable quantity； they became gradually differentiated to an extent commensurate with newly acquired characters。 It was also necessary that； in proportion as this happened； the mechanism of nuclear division must be refined。 At first processes resembling a simple constriction would suffice to provide for the distribution of all hereditary units to each of the products of division； but eventually in both organic kingdoms nuclear division； which alone insured the qualitative identity of the products of division； became a more marked feature in the course of cell…multiplication。
Where direct nuclear division occurs by constriction in the higher organisms； it does not result in the halving of hereditary units。 So far as my observations go； direct nuclear division occurs in the more highly organised plants only in cells which have lost their specific functions。 Such cells are no longer capable of specific reproduction。 An interesting case in this connection is afforded by the internodal cells of the Characeae； which possess only vegetative functions。 These cells grow vigorously and their cytoplasm increases； their growth being accompanied by a correspondingly direct multiplication of the nuclei。 They serve chiefly to nourish the plant； but； unlike the other cells； they are incapable of producing any offspring。 This is a very instructive case； because it clearly shows that the nuclei are not only carriers of hereditary characters； but that they also play a definite part in the metabolism of the protoplasts。
Attention was drawn to the fact that during the reducing division of nuclei which contain chromosomes of unequal size； gemini are constantly produced by the pairing of chromosomes of the same size。 This led to the conclusion that the pairing chromosomes are homologous； and that one comes from the father； the other from the mother。 （First stated by T。H。 Montgomery in 1901 and by W。S。 Sutton in 1902。） This evidently applies also to the pairing of chromosomes in those reduction…divisions in which differences in size do not enable us to distinguish the individual chromosomes。 In this case also each pair would be formed by two homologous chromosomes； the one of paternal； the other of maternal origin。 When the separation of these chromosomes and their distribution to both daughter…nuclei occur a chromosome of each kind is provided for each of these nuclei。 It would seem that the components of each pair might pass to either pole of the nuclear spindle； so that the paternal and maternal chromosomes would be distributed in varying proportion between the daughter…nuclei； and it is not impossible that one daughter…nucleus might occasionally contain paternal chromosomes only and its sister…nucleus exclusively maternal chromosomes。
The fact that in nuclei containing chromosomes of various sizes； the chromosomes which pair together in reduction…division are always of equal size； constitutes a further and more important proof of their qualitative difference。 This is supported also by ingenious experiments which led to an unequal distribution of chromosomes in the products of division of a sea…urchin's egg； with the result that a difference was induced in their further development。 （Demonstrated by Th。 Boveri in 1902。）
The recently discovered fact that in diploid nuclei the chromosomes are arranged in pairs affords additional evidence in favour of the unequal value of the chromosomes。 This is still more striking in the case of chromosomes of different sizes。 It has been shown that in the first division…figure in the nucleus of the fertilised egg the chromosomes of corresponding size form pairs。 They appear with this arrangement in all subsequent nuclear divisions in the diploid generation。 The longitudinal fissions of the chromosomes provide for the unaltered preservation of this condition。 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 re