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Show 438 DARWINISM OHAP. although it is very difficult to make them intelligible to persons unfamiliar with the main facts of modern ombryology.1 The problem is thus stated by Weismann : "How is it that in the case of all higher animals and plants a single cell is able to separate itself from amongst the millions of mo t various kinds of which an organism is composed, and by division and complicated differentiation to reconstruct a new individual with marvellous likeness, unchanged in many ca. cs even throughout whole geological periods 1" Darwin attempted to solve the problem by his theory of "Pan gene. ·i , " which supposed that every individual cell in the body gn;ve off gcmmules or germs capable of reproducing themselves, and that portions of these germs of each of the almost infinite num bcr of cells permeate the whole body and become collected in the generative cells, and are thus able to roprodnce the whole organism. This theory is felt to be so ponderously complc.r and difficult that it has met with no general acceptance among physiologists. The fact that the germ-cells do reproduce with wonderful accuracy not only the general characters of the specie:;, but many of the individual characteristics of the pa.rcnt or more remote ancestors, and that this process is continued from generation to generation, can be accounted for, \Yei. mann think., only on two suppositions which are physiologically possible. Either the sub tance of the parent germ-cell, after passing through a cycle of changes required for the construction of a new individual, possesses the capability of producing a.n •w germ-cells identical with those from which that indiviflual was developed, or the new germ-cell::; arise, as far as their essenfil/1 and characteristic S?.tbstance is concerned, not at all o1.tt of the /)()d.'f of the individnal, bnt di1·ect from the pa1·ent germ-cell. Thi · hll t•r view vVei mann holds to be the correct one, and, on thi theory, heredity depends on the fact that a substance of pecial molccular composition passes over from one generation to another. This is the "germ-plasm," the power of which to develop itf-clf into a perfect organism depends on the extraordinary com pl ication of its minutest structure. At every new birth a portion 1 The outline here given is derived from two arti cles in ..Y. atu rt•, \Ol. xxxiii. p.l54, and vol. xxxiv. p. 629, in which \¥ei mann's paper. nr • summarised aml partly translated. XIV FUNDAMENTAL 1)ROBLEMS 439 of the specific germ-plasm, which the parent egg-cell contains, is not usctl up in producing the offspring, but is rcRcrvcd uncha. ngcd to produce the g rm-cclls of the following generation. Tlm. the germ-cells- so far as reganl their essential part the germ-plasm-are not a product of the body it elf, but arc rela.tcd to one :mother in tho same way as arc a series of generations of unicellular organisms derived from one another hy a. continuous course of simple eli vision. Thus the question of heredity is reduced to one of growth. A 1~inutc portion of the very same germ-plasm from which, first the germ-cell, and then the whole organi. m of the parent, were developed, becomes the sta.rting-point of the growth of the child. 1'/w Cause of ri.L1'i££tion. But if this were all, the offspring would ·reproduce the parent exactly, in every detail of form and structmc; and here we see the importance of sex, for each new germ oTows out of the united germ-plasms of two parents, whence ari:cs a mingling of their characters in the offspring. This occurs in each generation; hence every individual is a complex rc ult reproducing in ever-varying dco-recs the diverse ·haracteristics f his two parents, four grandparents, eight grc:1t-grandparcnts, and other more remote ance tors· and that ever-pre ent individual variation ari. cs which furnishes the material for natura.! selection to act upon. Diversity of sex becomes, therefore, of primary importa.nce as the cause of Vl~?·iation. ·where a exual generation prevails, the characteristics of the individual alone are reproclnccd, n.nd there arc thus no means of effecting the chn.ngc of form or stmctnrc required by chrmged condition. of existence. U ndcr :nch chan crcd condition: a compl 'X organism, if nly asexually propagated, wonl<l become extinct. But when a complex orgn.nism is sexually propngatccl, there is an ever-present cause of chn,ngc which, th01wh light in any one generation, is cumnhti ve, :uHl under the in Huence of election is sufficient to keep up the harmony between the organism and its slowly ch:mging cnvironment.l 1 cr'here are many inclications tl1at this explanation of the rause of variation is the true one. M.r. E. B. Poulton suggests one, iu the fa ·t thnt parthenogenetic reproduction 011ly o cur:-; in isolated !ipecies, 11ot in groups of related species ; as this shows that partlleuogcnesi::; cannot lead lo tl1 e evolution of |