PLAY AND INSTINCT. 53 



weakened this position by the admission that the germ 

 plasm may have only a " very great " but not absolute 

 persistence. I do not refer to his granting the inherit- 

 ance of diseases (these are, after all, only pollutions of 

 the stream that may not essentially alter it), but to his 

 admitting the possibility of modifying the germ plasm 

 by changing nutriment and temperature.* 



Next in order is his essay on External Influences as 

 Aids to Development (1894), where he shows that he is 

 not blind to the importance of external conditions. He 

 here concedes that the development of germ plasm itself 

 may be modified by means of changes in nutriment and 

 temperature, while predispositions that remain latent 

 under ordinary circumstances may be stimulated to 

 activity by such " external aids." The fact that this is 

 not the cause but only the occasion of the modification 

 is especially emphasized, the cause being always the pre- 

 disposition latent in the germ. That the persistent qual- 

 ity of the germ plasm was only relative had already been 

 clearly intimated, however, in his more important work, 

 Das Keimplasma, 1892, p. 526. Speaking of a butter- 

 fly, which has bright or dark wings, according to the 

 climate, he goes on to say: " The modifying influence, 

 here temperature, affects in each individual both the 

 fundament of the wings — that is, a portion of the soma 

 — and also the germ plasm contained in germ cells of the 

 organism. In the wing-fundament the same determi- 

 nants change as in the germ-cells — namely, those of the 

 wing-scales. The first modification can not influence 

 the germ cells, and so affects only the colour of the wings 



the inheritance of acquired characters for at least the lowest orders, 

 for the mixing presupposes some given differences.) 



* This appears in the early essay Ueber die Vererbung, Jena, 

 1883, p. 49. 



