SUPPOSED TRANSMISSION OF ACQUIRED CHAR.4CTERS 407 



external influence affects only the fully-formed organ, — such as 

 a muscle which has become enlarged by exercise ; for the influ- 

 ence consists in the increased activity of the organ, which takes 

 place within it alone ; the germ-plasm of the germ-cells, and 

 even the determinant in the germ-plasm for the muscle in ques- 

 tion, are not thereby affected. In all cases of functional hyper- 

 trophy or atrophy, the external influence affects none of the 

 determinants, but only the fully-formed organs, — i,e.^ groups of 

 specific cells produced from determinants. In my opinion it is 

 very probable that such twofold modifying influences of environ- 

 ment as we meet with in P. phlceas, can only occur when the 

 determinants which have not yet been transformed into the 

 organ, as well as the germ-cells, have been affected by the modi- 

 fying influence. And this will be most likely to happen in those 

 structures which, like the scales on the wings of butterflies, 

 are formed at a later stage of the animaPs existence, and the 

 detertnmajits of which are consequently stored in an undeveloped 

 condition in theidants of certain somatic cells during a s^r eat part 

 of the ontogefiy. The wings of the butterfly arise as outgrowths 

 from the hypodermis of the caterpillar. Before these outgrowths 

 can be formed, the determinants for the wing-scales must be 

 contained in the idioplasm of some of the cells of the hypo- 

 dermis, but after their appearance they would be found in some 

 of the cells in the rudiments of the wings. The wings at first 

 are small, and contain by no means so large a number of cells 

 as when full-grown, so that inactive determinants for several 

 wing-scales must be contained in the idants oi one nucleus. At 

 a certain period in the course of further growth, however, the 

 number of cells increases to such an extent that each determinant 

 constitutes the idioplasm of a particular cell, and the modifying 

 external influences seem then to have the greatest effect on these 

 determinants. By means of experiments it may be possible to 

 ascertain exactly when this occurs. 



It might have been expected that in this section I should enter 

 into the whole question of the possibility of the transmission of 

 acquired variations, about which there has been ro much dispute 

 of late years, and that all the arguments and facts which have 

 been put forward in favour of the theory should be discussed. 

 But, as I have already remarked, this seems to be out of place 

 in a theory of heredity, the object of which is to show whether 

 this form of transmission is or is not possible from a theoretical 



