80 ACQUIRED CHARACTERS SEC. 



, Against the validity of the evidence afforded by Nageli's 

 experiments I would further, above all, bring into the field the 

 circumstance that they are artificial throughout, and that 

 they cannot therefore claim to be held as complete evidence 

 for the processes which occur under unrestricted natural 

 conditions. Not that I would attach no value at all to 



^ artificial experiments, but they must certainly be employed 

 with great caution. Their relatively slight importance depends 

 in my opinion on the fact that definite directions of evolution 

 become fixed in living beings by extremely long continu- 

 ance, and these can only be changed by continuous ex- 

 ternal influences acting for a long time. Thus an organism 

 is restricted long beforehand to a certain direction of modi- 

 fication. If an artificial experiment chance to promote 

 change in this direction, it soon results in the appearance of 

 permanent modifications : these will persist even after the 

 artificial external conditions have ceased to act. But not so 

 when art is in opposition to nature then the organism will 

 rather return to its former condition immediately after the 

 cessation of the artificial action, or its descendants will do so. 

 This last seems to have been the case in Nageli's experiments 

 with plants, which he placed under different and more favour- 

 able conditions of nutrition, and which were modified by the 

 change, but whose seeds sown in poorer soil again produced 

 plants of the old kind. 



An Alpine plant, for example, is necessarily covered with 

 snow for a long time in winter, and in summer is exposed to 

 strong heat from the sun and to intense light, and has acquired 

 through its peculiar conditions of life the power of very rapid 

 development and also special morphological characters. If 

 this plant is placed where these conditions are intensified, in 

 the far North for instance, these characters ought to be still 

 more strongly developed, and the advance ought to remain 

 permanent even in the offspring which are reared from their 



