THE GERM -PLASM 77 



cell its inherited specific character. Each id, in every stage, has 

 its definitely inherited architecture ; its structure is a complex 

 but perfectly definite one, which, originating in the id of germ- 

 plasm, is transferred by regular changes to the subsequent idic 

 stages. The structure exhibited in all these stages exists poten- 

 tially in the architecture of the id of germ-plasm : to this architec- 

 ture is due, not only the regular distribution of the determinants, 

 — that is to sav, the entire construction of the body from its 

 primary form to that in which its parts attain their final arrange- 

 ment and relation, — but also the fact that the determinant, of a 

 small spot on a butterfly's wing, for example, reaches exactly the 

 right place ; and that, to take another instance, the determinant 

 of the fifth segment in the feeler of a Gammarus reaches this 

 particular segment. The determination of the character of the 

 individual cell depends on the biophors which the correspond- 

 ing determinant contains, and which it transmits to the cell. 



6. The Mechanism for the Phyletic Variations in 

 THE Germ-plasm 



The causes of phyletic development will be treated of in the 

 chapter on Variation : the present section merely gives an 

 account of the mechanism existing in the idioplasm in connec- 

 tion with this process. I shall here attempt to show how^ the 

 phyletic changes in the idioplasm follow mechanically from its 

 assumed ultimate strvicture. 



Since all parts of the organism are determined from the germ 

 onwards, permanent variations in these parts can only originate 

 from variations in the germ. Each phyletic variation must 

 therefore be due to a variation in the structure of the id of 

 germ-plasm. If we suppose, with Darwin, that the transforma- 

 tion of species is a gradual one, originating in individual varia- 

 tions which become increased and directed by selection, it 

 follows that the corresponding process in the idioplasm cannot 

 be due to a sudden and complete variation in the entire id, 

 but must begin with changes in the individual biophors or 

 in individual determinants and groups of determinants also, 

 and must then extend gradually to more numerous groups, until 

 finally the nature of the id becomes entirely, or to a great 

 extent, changed. 



