The Causes of Individual Differences 17 



fluenced a certain physiological substance which is assumed 

 to reside in the reproductive regions of the organisms, 

 and that all those changes which have not yet reached 

 this germ-plasm are, therefore, lost, or die with the or- 

 ganisms. 



Do external influences produce permanent effects in 

 plants ? It is not necessary to discuss here the intri- 

 cate arguments in the time-honored controversy of the 

 permanent inheritance of external modifications. Such 

 violent modifications as traumatic injury do not affect its 

 germ cells and are not inherited. But it is the common 

 experience of gardeners that the modifications of the envi- 

 ronment of plants, such as changing food supply or changing 

 seed from one environment to another, produce changes 

 which eventually become hereditary. Whether these 

 changes of environment act directly upon the germ-plasm 

 to produce the change or whether they stimulate a ger- 

 minal change which was otherwise latent, is a question 

 which long and patient experimentation must decide. 

 Certain it is, that plants have gone through a profound 

 modification and it is easy to believe that environment has 

 played no little part in these changes. 



Weismann teaches that " acquired characters," or those 

 variations which first appear in the life-time of the indi- 

 vidual because of the influences of environment, are lost, 

 because they have not yet affected the reproductive sub- 

 stances ; but if these characters are induced by the effect 

 of impinging environment during two or more generations, 

 they may come to be so persistent that the plant cannot 

 throw them off, and they become, thereby, a part of the 

 hereditary and non-negotiable property of the species. 



