GENESIS, HEREDITY, AND VARIATION. 357 



special to itself, and capable of developing into similar cells. 

 We may here, in passing, note that this view implies a funda- 

 mental distinction between unicellular organisms and the 

 component cells of multicellular organisms, which are other- 

 wise homologous with them. For while in their essential 

 structures, their essential internal changes, and their essential 

 processes of division, the Protozoa and the component units 

 of the Metazoa are alike, the doctrine of Pangenesis implies 

 that though the units when separate do not give off invisible 

 gemmules the grouped units do. 



Much more recently have been enunciated the hypotheses 

 of Prof. Weismann, differing from the foregoing hypotheses 

 in two respects. In the first place it is assumed that the frag- 

 ment of matter out of which each organism arises consists of 

 two portions one of them, the germ-plasm, reserved within 

 the generative organ of the incipient individual, representing 

 in its components the structure of the species, and gives origin 

 to the germs of future individuals; and the other of them, 

 similarly representative of the specific structure, giving origin 

 to the rest of the body, or soma, but contains in its compo- 

 nents none of those latent powers possessed by those of the 

 germ-plasm. In the second place the germ-plasm, in com- 

 mon with the soma-plasm, consists of multitudinous kinds 

 of units portioned out to originate the various organs. Of 

 these there are groups, sub-groups, and sub-sub-groups. 

 The largest of them, called " idants," are supposed each to 

 contain a number of " ids " ; within each id there are numer- 

 ous " determinants " ; and each determinant is made up of 

 many " biophors " the smallest elements possessing vitality. 

 Passing over details, the essential assumption is that there 

 exists a separate determinant for each part of the organ- 

 ism capable of independent variation; and Prof. Weismann 

 infers that while there may be but one for the blood and 

 but one .for a considerable area of skin (as a stripe of the 

 zebra) there must be a determinant for each scale on a butter- 

 fly's wing: the number on the four wings being over two 

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