544 ON HEREDITY. 



aud states that iu bis belief the germ cell, so far as its essential and 

 characteristic substance is concerned, is not derived at all from the body 

 of the individual in which it is produced, but directly from the parent 

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

 the continuity 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 th» construction of 

 the body of that individual, but is reserved unchanged for the forma- 

 tion of the germ-cells of the succeeding generation. Thus like Mr. 

 Galton, he recognizes that in the stirp or germ there are two classes of 

 cells destined for entirely distinct purposes: the one for the develoi)- 

 mentof the soma or body of the individual, which class he calls the so- 

 matic cells; the other for the perpetuation of the species, /. e., for re- 

 l>roduction. 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 rei)roductive cells (the pronuclei referred to in an earlier 

 part of this address), that the nuclear substance must be the sole bearer 

 of hereditary tendencies. Each of the two uniting nuclei would con- 

 tain the germ-plasm of one parent, and this germ-plasm also would 

 contain that of the grandparents as well as that of all ])revious gen- 

 erations. - - - 



It follows therefore from this theory that the germ-plasm possesses 

 throughout, the same complex, chemical, and molecular structure, and 

 that it would passs 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 can not exer- 

 cise a modifying influence upon tlie germ-cells within it; yet he limits 

 this influence to such slight eflect 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 arrange- 

 ments of its molecular units. But ho throws great doubt upon the 

 existence of such a re-action, and he, more emphatically than Mr. Gal- 

 ton, argues against the idea that the cells which make up the somatic 

 or personal structure of the individual exercise any influence on the 

 rei)roductive cells. From his point of view the structural or other 

 properties which characterize a family, a race, or a species, are derived 

 solely from the reproductive cells through continuity of their germ- 

 plasm, and are not liable to modification by the action on them of the 

 organs or tissues of the body of the individual organism in which they 

 are situated. 



The central idea of heredity is permanency; that like begets like, or 

 as Mr. Galton more fitly puts it, tliat " like tends to produce like." But 

 though the oftsi)ring conform with their parents in all their main char- 



