HETEROGENEITY OF GERMINAL SUBSTANCE 43 



geneous germinal substance, which Herbert Spencer, in particular, 

 has advocated as against Weismann's view. For, overlooking 

 the fact that the development of the heterogeneous from the 

 homogeneous is a phenomenon more than difficult of explana- 

 tion, the independent variability of one particular part of the 

 organism would be impossible under these conditions. For 

 the variation of one part would, on the supposition of the homo- 

 geneity of the hereditary substance, lead to a concomitant 

 variation of aU the other parts ; whereas such a case as that 

 of a man possessing a tuft of white hair among a mass of black 

 hair is enough of itself to show that variations of particular 

 parts can take place without in any way affecting the whole. 

 This fact is only explicable on the supposition that the hereditary 

 substance is essentially heterogeneous in its nature — that, for 

 instance, in the case mentioned, the particular tuft of white 

 hair is represented in the hereditary substance by a particular 

 determinant or group of determinants. All those parts which 

 have imdergone independent variation in the course of generations 

 must be independently heritable. The theory of a homogeneous 

 germinal substance, however complex its combinations, would 

 imply that this substance, which goes through many thousands 

 of changes in regular succession, in order that all the hetero- 

 geneous parts of the finished organism may be developed, can 

 alter its constitution countless numbers of times without altering 

 the whole organism. This, however, would be an impossibility, 

 and is contradicted by the facts. 



In certain famiHes a small scar, not larger than a pin's head, 

 is observable behind the ear ; this scar is capable of being trans- 

 mitted, as has been observed, from grandmother to son and 

 grandson. The explanation of this fact can only be found in 

 the supposition that a determinant exists in the germ-plasm of 

 these particular individuals which does not exist in the germ- 

 plasm of other people, and which causes in successive generations 

 the reappearance of this tiny'abnormality. 



