THEORIES OF HEREDITY 9 



off minute "gemmules" which are then dispersed through 

 the body where they multiply, and whence they are collected 

 into the germ-cells ; each of the latter thus being packets of 

 gemmules which later develop, each gemmule in its proper 

 place, into the somatic cells of the offspring. The other set 

 of theories supposes that the germ-plasm is not derived from 

 the cells of the soma, but directly from pre-existing germ- 

 plasm. Thus Weismann in his theory of the " Continuity of 

 the Germ-plasm " supposes that, when an individual develops 

 from a germ-cell, the germ-plasm contained in that cell 

 divides into two portions, from one of which, after undergoing 

 great changes and growth, is derived his soma, and from the 

 other his germ-cells. According to the first set of theories 

 the germ-plasm is formed anew in every individual from 

 materials derived from his tissues. According to the second 

 it is not formed anew, but is simply a reserved portion of the 

 germ-plasm from which his own soma was derived. 



15. None of these theories are capable of direct proof or 

 disproof. The hereditary substance, the germ-plasm, has 

 been identified with a high degree of probability with the 

 chromatin of the nuclei. When cells multiply the chromatin 

 of the parent cells is distributed to the nuclei of the daughter 

 cells. The continuity of the germ-plasm is therefore practic- 

 ally certain. But it is quite beyond the powers of our 

 microscopes to inform us whether the germ-plasm does, or 

 does not, receive additional germinal elements from the 

 soma. We know that it must receive non-living nutritive 

 material, but that is not the same thing. The question, 

 therefore, can be tested, only in an indirect way. If the first 

 set of theories be true, if the soma does contribute germinal 

 elements, and the various structures of the offspring are 

 derived in part at least from the corresponding structures of 

 the parent, then it follows that the child must tend to 

 reproduce the acquirements as well as the inborn traits of 

 the parent. On the other hand, if the structures of the child 

 are not derived from the corresponding structures of the 

 parent, then the inheritance of acquired characters is 

 evidently highly improbable. 



16. Formerly all the world believed in the transmission of 

 acquired characters. The discovery that offspring take origin 

 from single cells (generally fertilized ova) did not affect this 

 belief at first. Darwin's theory of " Pangenesis," for example, 

 expressed the popular faith in scientific terms. He tried to 

 explain how acquired characters are transmitted by supposing 

 that the hereditary substance in the gerin-cells is derived 



