Sept. 26, 1889] 



NATURE 



527 



mode of life, and sometimes in the tendency to particular diseases. 

 This transmission of characters from parent to offspring is 

 summarized in the well-known expre-sion that "like begets 

 like," and it rests upon a phy-ical basis. 



The size of the particles which are derived from the parents, 

 called the male and female pronuclei, the potentiality of which 

 is so utterly out of proportion to their bulk, is ahnost inconceiv- 

 ably small when compared with the magnitude of the adult 

 body. Further, by the continual process of division of the 

 cells, the substance of the segmentation nucleus is diffused 

 throughout the body of the new individual produced through its 

 influence, so that each cell contains but an infinitesimal particle 

 of it. The parental dilution, if I may so say, is so attenuated 

 as to surpass the imagination of even the most credulous believer 

 in the attenuation of drugs by dilution. And yet these particles 

 are sufficient to stamp the characters of the parents, of the 

 grandparents, and of still more remote ancestors on the offspring, 

 and to preserve them throughout life, notwithstanding the 

 constant changes to which the cells forming the tissues and 

 organs of the body are subjected in connection with their use 

 and nutrition. So marvellnus, indeed, is the whole process, 

 that even the exact contributions to recent knowledge on the 

 fusion of the two pronuclei, instead of diminishing our wonder, 

 have intensified the force of the expression " magnum henditatis 

 mystirium." 



In considering the question of how new individuals are pro- 

 dured, one must keep in mind that it is not every cell in the 

 body which can act as a centre of reproduction for a new 

 generation, but that certain cells, which we name germ-cells 

 and sperm-cells, are set aside for that purpose. These cells, 

 destined for the production of the next generation, form but 

 a small proportion of the body of the animal in which they 

 are situated. They are, as a rule, marked off" from the rest 

 of the cells of its body at an early period of development. 

 The exact stage at which they become specially differentiated 

 for reproductive purposes varies, however, in different organ- 

 isms. In some organisms, as is said by Balbiani to be the case 

 in Chironomus, they apparently become isolated before the 

 formation of the germinal layers is completed ; but, as a rule, 

 their appearance is later, and in the higher organisms not until 

 the development of the body is relatively much more advanced. 



The germ-cells after their isolation take no part in the growth 

 of the organism in which they arise, and their chief association 

 with the other cells of its body is that certain of the latter are of 

 service in their nutrition. The problem, therefore, for consider- 

 ation is the mode in which these germ or reproductive cells 

 become influenced, so that after being isolated from the cells 

 which make up the bulk of the body of the parent they 

 can transmit to the offspring the characters of the parent 

 organism. Various speculations and theories have been ad- 

 vanced by way of explanation. The well-known theory of 

 Pangenesis, which Charles Darwin with characteristic modera- 

 tion put forward as merely a provisional hypothesis, assumes 

 that gemmules are thrown off" from each different cell or unit 

 throughout the body which retain the characters of the cells from 

 which they spring ; that the gemmules aggregate themselves 

 either to form or to become included within the reproductive 

 cells ; and that in this manner they and the characters which 

 they convey are capable of being transmitted in a dormant state 

 to successive generations, and to reproduce in them the likeness 

 of their parents, grandparents, and still older ancestors. 



In 1872, and four years afterwards, in 1876, Mr. Francis 

 Gallon published most suggestive papers on Kinship and 

 Heredity (Proc. Roy. Soc. Lond., 1872, and Journ. Anthrop. 

 Inst , vol. v., 1876). In the latter of these papers he developed 

 the idea that " the sum-total of the germs, gemmules, or what- 

 ever they may be called," which are to be found in the newly 

 fertilized ovum, constitute a stirp, or root ; that the germs which 

 make up the stirp consist of two groups — the one which develops 

 into the bodily structure of the individual, and which constitutes, 

 therefore, the personal structure ; the other, which remains latent 

 in the individual, and forms, as it were, an undeveloped residuum ; 

 that it is from these latent or residual germs that the sexual 

 elements intended for producing the next generation are derived, 

 and that these germs exercise a predominance in matters of 

 heredity ; further, that the cells which make up the personal 

 structure of the body of the individual exercise only in a very 

 faint degree any influence on the reproductive cells, so that any 

 modifications acquired by the individual are barely, if at all, 

 inher.ted by the offspring. 



Subsequent to the publication of Mr. Galton's essays, valuable 

 contributions to the subject of Heredity have been made by 

 Profs. Brooks, Jaeger, Naegeli, . Nussbaum, Weismann, and 

 others. Prof. Weisniann's theory of Heredi'y embodits the 

 same fundamental idea as that propounded by Mr. Galton ; but 

 as he has employed in its elucidation a phraseology which is 

 more in harmony with that generally used by biologists, it has 

 had more immediate attention given to it. As Weismann's 

 essays have, during the present year, been translated for and 

 pubHshed by the Clarendon Press (Oxford, 1889), under the 

 editorial superintendence of Messrs. Poulton, Schiinland, and 

 Shipley, they are now readily accessible to a'l En;4;lish readers. 



Weismann asks the fundamental questi ^n, " How is it that 

 a single cell of the body can contain within itself all the 

 hereditary tendencies of the whole organism ? " He at once 

 discards the theory of pangenesis, and states that in his belief 

 the germ-cell, so far as its essential and characteristic substance 

 is concerned, is not derived at all from the body of the indi- 

 vidual in which it is produced, but directly from the parent 

 germ-cell from which the individual has also arisen. He calls 

 his theory the contimtity of the germ-plasm, and he bases it upon 

 the supposition that in each individual a portion of the specific 

 germ-plasm derived from the germ-cell of the parent is not used 

 up in the construction of the body of that individual, but is 

 reserved unchanged for the formation of the germ-cells of the 

 succeeding generation. Thus, like Mr. Gallon, he recOijnizes 

 that in the stirp or germ there are two classes of cells declined 

 for entirely distinct purposes : the one for the development of 

 the soma or body of the individual, which class he calls the 

 soviatic cells ; the other for the perpetuation of the species, i.e. 

 for reproduction. 



In further exposition of his theory Weismann goes on to say, 

 as the process of fertilization is attended by a conjugation of the 

 nuclei of the reproductive cells — the pronuclei referred to in an 

 earlier part of this address — that the nuclear substance must be 

 the sole bearer of hereditary tendencies. The two uniting 

 nuclei would contain the germ-plasms of the parents, and 

 this germ-plasm also would contain that of the grandparents as 

 well as that of all previous generations. 



To make these somewhat abstract propositions a little more 

 clear, I have devised the following graphic mode of repre- 

 sentation : — 



J^ j[±> W J±^ 



Let the capital letters A, B, C, D, &c. , express a series of 

 successive generations. Suppose A to be the starting-point, 

 and to represent the somatic or personal structure of an indi- 

 vidual ; then a may stand fir the reproductive cells, or germ- 

 plasm, from which the offspring of A, viz. B, is produced. 

 B, like A, has both a personal structure and reproductive cells 

 or germ-plasm, the latter of which is represented by the letters 

 ab, which are intended to show that whilst belonging to B they 

 have a line of continuity with A. C stands for an individual of 

 the third generation, in which the reproductive plasm is indi- 

 cated by abc, to express that, though within the body of C, the 

 germ-plasm is continuous with that of both b and a. D also 

 contains the reproductive cells, abed, which are continuous 

 with the germ-plasm of the three preceding generaticns, and 

 so on. 



It follows, therefore, from this theory that the germ-plasai 

 possesses throughout the same complex chemical and molecular 

 structure, and that it would pass through the same stages when, 

 the conditions of development are the same, so that the same 

 final product would arise. Each successive generation would 

 have therefore an identical starting-point, so that an identical 

 product would arise from all of them. 



Weismann does not absolutely assert that an organism cannot 

 exercise a modifying influence upon the germ-cells within it ; 

 yet he limits this influence to such slight effect as that which 

 would arise from the nutrition and growth of the individual, and 

 the reaction of the germ-cell upon changes of nutrition caused 

 by alteration in growth at the periphery, leading to some change 

 in the size, number, and arrangements of its molecular units. 

 But he throws great doubt upon the existence of such a re- 

 action, and he, more emphatically than Mr. Galton, argues 

 against the idea that the cells which make up the somatic or 

 personal structure of the individual exercise any influence oa 



