HEREDITY 



677 



from those which ar> forming the 'body.' The latter 

 i(.-vfl..|. in iiuuiifitld variety, and soon lose almost 

 all likeness to tlie fertilised ovum. The former 

 tin- reproductive rudiments are not implicated in 

 tin- up-building of the ' body,' remain virtually 

 niicli.-iM-.'il, and continue the protoplasmic tradi- 

 tion unaltered so as to start a new organism on 

 tin* -.UN.- lini- l.i'. \\ith the same protoplasmic 

 material. It is evident that a fertilised egg-cell 

 with certain characters, a, b, c, will develop into 

 an organism in which these characters a, 6, c 

 are variously expressed ; but if at an early stage 



ABO 



Fig. 1. 



I, original ovum ; ABC, body of organism to which it gives rise ; 

 o, chain of ovarian ova witU properties a, b, c retained ; I', 

 liberated ovum of next generation, virtually equivalent to I. 



certain cells are set apart, retaining the characters 

 a, b, c in all their entirety, then these will be 

 on the same footing as the original fertilised 

 egg-cell, able like it to give rise to an organism, 

 and necessarily to a similar organism. This ex- 

 planation of heredity is at once so simple and so 

 satisfactory that it becomes a most important 

 question to determine how far the above facts are 

 actually true among plants and animals. The 

 answer is that they are as yet demonstrable only 

 in a minority of cases. Thus, it is true that an 

 early appearance or insulation of reproductive 

 cells, materially continuous and presumably iden- 

 tical with the ovum itself, has oeen observed in 

 some worm-types (leeches, Sagitta, thread-worms, 

 Polyzoa), in some Arthropods (e.g. Moina among 

 Crustaceans, Chironomus among Insects, Phalan- 

 gida- among Arachnids), and with less distinctness 

 in a number of other organisms. But it must be 

 distinctly allowed that in most cases it is only after 

 development has progressed for some time that the 

 future reproductive cells make their distinct appear- 

 ance. Therefore, if distinct cellular continuity be 

 only demonstrable in a minority, it becomes neces- 

 sary to modify the generalisation. The required 

 modification is due to Weismann, whose theory 

 must IKJ briefly stated. ( 1 ) A small portion of the 

 effective substance of the fertilised egg-cell remains 

 unchanged during the development, and serves as 

 a foundation from which the germ-cells of the new 

 organism are produced. (2) This important sub- 

 stance the 'germ-plasma' which keeps up con- 

 tinuity from one generation to another, is part of 

 the nucleus, possesses an exceedingly complex 

 minute structure, but has great stability, for it 

 absorbs nourishment and grows enormously with- 

 out the least change in its molecular constitution. 

 (3) Hut while part of this special nuclear substance 

 or germ-plasma of the egg-cell is reserved unchanged 

 for the formation of the germ-cells of the resulting 

 organism, part of it is changed into the nuclei of 

 the ordinary Ixxly-cells, where, however, it some- 

 times retains enough of its original efficiency to be 

 able to repair serious injuries or start the develop- 

 ment of a new organism in asexual reproduction. 

 Weismann has given a more complete expression to 

 the fact of the continuity of generations than has 

 hitherto been proposed, hut it cannot be denied 

 that there is much that is entirely hypothetical 

 about the 'germ-plasma' and its history. For 

 thorough exposition, reference must be made to his 



translated papers, and for detailed criticinin to 

 works cited in the bibliography. 



We may further notice an important work by II. 

 de VrifM ( 1HS!), which seeks to combine the fact of 

 continuity with part of the theory of pangenexui. 

 He maintains that every characteristic of the 

 organism is represented by a i-peci.-d ' pangene, 1 

 and that the germ-cells contain samples of all 

 This pangenetic accumulation in the germ-cell* 

 is not, however, the result of contributions travel- 

 ling from the various parts of the body, but in 

 the result of a definite, more or less direct 

 continuity between the germ-cells and the fertilised 

 ovum which started the organism to which they 

 belong. 



Theories of Continuity. It can hardly be doubted 

 that in the more or less direct continuity between 

 the successive sets of reproductive products lies the 

 solution of the main problem of heredity. The 

 germ-cells which give nse to offspring are nnique 

 in their continuity with those which gave rise to 

 the parents and it is this continuity or the involved 

 sameness of material which explains the production 

 of like by like. In the simplest animals or Proto- 

 zoa, organism A buds and hands on a fraction of 

 its living matter to AI, which, being so really part 

 and parcel of A, must grow up into a similar 

 adult Protozoon. With higher animals the same 

 holds true, though the continuity, as expressed in 



Fig. 2. The Relation between Reproductive Cells and 



the Body : 



The continuous chain of dotted cells at first represents a roc- 

 cession of Protozoa ; further on, it represent* the ova from 

 which the ' bodies ' ( undotted ) are produced. At each gener- 

 ation a spermatozoon fertilising the liberated ovum is also 

 indicated. 



the figure, is less direct. At various levels of 

 analysis suggestions have been made which attempt 

 to render the fact of continuity more luminous. 

 Thus, Professor Hering and Samuel Butler sug- 

 gested about the same time a psychical aspect of 

 hereditary continuity, according to which memory 

 is regarded as a general function of organised 

 matter, and the reproduction of parental likeness 

 as due to an unconscious recollection of the past. 

 Haeckel also emphasised the luminous metaphor of 

 'organic memory,' but sought to express this in 

 terms of molecular motion. The invisible activity 

 of the organic molecules he compares to a complex 

 wave-motion, harmonious ana persistent from 

 generation to generation, though capable of in- 

 corporating the results of fresh experience. The 

 periodic wave-motion of the molecules he de- 

 scribes characteristically as 'the perigenesis of 

 the plastidules.' In metaphorical language, the 

 molecules remember or persist in the rhythmic 

 dance which they have learned. Most naturalist*, 

 however, have been content to express the con- 

 tinuity in terms of the cells or of the nuclei, or of 

 yet smaller elements. Galton and Jiiger, Brooks 

 and Nussbaum, Hertwig and Herdman, Niigcli 

 and Weismann, and others have all contributed 

 to making the fact of continuity more preci-c. 

 Hopeful also are the suggestions of Jiiger, Bert- 

 hold, Gautier, and Geddes, which make towards 

 a chemical expression of the continuity between 

 germ and germ. Within present limits it is im- 

 possible to criticise any of the above elaborations. 

 Behind all the suggestions, whether of ' organic 

 memory,' 'persistent wave-motion,' 'stable germ- 

 plasma,' or 'constancy of chemical processes,' 



