SOMATIC MODIFICATIONS NOT TRANSMISSIBLE 139 



in the constitution of the surrounding cells. Now, although 

 certain trees — oak-trees especially — are every year regularly 

 visited by great numbers of gall insects, the gaU itself is never 

 inherited; a fact which shows that considerable modification 

 may be efEected by the direct action of environmental conditions 

 in each individual life, and yet remain purely somatic and 

 without in any way affecting the germ-cells. 



The experiments of Nageli on different species and varieties 

 of Hieracium (Hawkweed) teach the same lesson. For thirteen 

 years he cultivated 2,500 varieties of these Alpine plants in 

 the botanical gardens of Munich, and observed in every case 

 that the characteristic modifications due to the change of habitat 

 which were effected during the first year reappeared afresh in 

 each successive generation. Experiments showed that they were 

 not inherited, but simply acquired anew. It is thus evident 

 that these changes, due to climatic influence, considerably 

 modified the soma of all the individuals placed under the same 

 conditions, but did not affect the reproductive cells. The 

 experiment of Nageli is valuable and interesting in showing 

 that profound modifications may affect the soma, yet not affect 

 the germ-cells ; and here agaiu a great breach — ^if another 

 breach be needed — ^is made in the Lamarckian theory. Never- 

 theless, Nageli's experiment is not to be held as conclusively 

 demonstrating that modifications due to environing conditions 

 are never in any circumstances transmissible. Detner has shown 

 that the cherry-tree of European climes, when transported to 

 Ceylon, becomes transformed into a tree having persistent 

 foliage, and that this transformation is hereditary. As we have 

 said, profound changes in temperature or in the nutrition of 

 the organism are, a friori, very likely to affect the reproductive 

 cells, although it is probable that not all, but only certain 

 specific determinants of the germ-plasm are thus modified. 



The experiments of Weismann on Polyommatus PMceas, a 

 butterfly of the family of Lyccmidw, show that temperature 



