58 THE FIRST PRINCIPLES OF HEREDITY 



presses it, if two parts had only one determinant, any 

 change of this determinant would alter both parts simul- 

 taneously. The very fact that both parts can change 

 independently from each other leads to the conclusion that 

 they must each be represented by a separate determinant. 

 Thus, for instance, we could imagine the whole skin repre- 

 sented by one determinant only ; but as a minute portion of 

 the skin may vary in its character {e.g., a wart may exist 

 on one side of the face), we must imagine that portion of 

 the skin bearing the wart represented in the germ by a 

 separate determinant ; and as all parts of the skin may 

 thus vary, each of them must have a separate determinant. 

 In the same way the minute coloured scales on the wings 

 of butterflies, or the different patterns of feathers in birds, 

 are each and all extremely variable, and must therefore 

 be determined by separate units. It is otherwise, on the 

 other hand, with the red blood-corpuscles ; these do not 

 seem to vary from each other, and may therefore be 

 represented by a single determinant. 



All the determinants necessary for the development of 

 the organism are gathered together in a higher unit, which 

 is called the " Id," also with the power of growth and self- 

 propagation. These Ids are identical with the " micro- 

 somes " or " chromomeres," the small globular bodies of 

 which the chromosomes can sometimes be seen to consist. 

 There must therefore be a still higher unit than the Id, 

 the Chromosome, or, as Weismann has called it, in con- 

 formity with his nomenclature, the " Idant." We see, 

 therefore, that each germ-cell having many Idants (chromo- 

 somes), each consisting of a number of Ids, contains the 

 representative units of many individuals ; each single Id, 

 with its determinants, standing for a complete individual. 



The idioplasm which is contained in the germ-cell has 

 been specially named by Weismann the " germ-plasm." It 

 comprises, as has been seen, the constituents for the con- 

 struction of the new being. Weismann holds it as an 

 uncontroverted fact that the body-cells cannot, as, for 



