1 88 1.] BOTANICAL LETTERS. 345 



environment ; but whether I said so I know not, for without 

 being asked I should have thought it presumptuous to have 

 criticised your book, nor should I now say so had I not during 

 the last few days been struck with Professor Hoffmann's 

 review of his own work in the * Botanische Zeitung,' on the 

 variability of plants ; and it is really surprising how little effect 

 he produced by cultivating certain plants under unnatural 

 conditions, as the presence of salt, lime, zinc, &c., &c., during 

 several generations. Plants, moreover, were selected which 

 were the most likely to vary under such conditions, judging 

 from the existence of closely-allied forms adapted for these 

 conditions. No doubt I originally attributed too little weight 

 to the direct action of conditions, but Hoffmann's paper has 

 staggered me. Perhaps hundreds of generations of exposure 

 are necessary. It is a most perplexing subject. I wish 

 I was not so old, and had more strength, for I see lines 

 of research to follow. Hoffmann even doubts whether 

 plants vary more under cultivation than in their native home 

 and under their natural conditions. If so, the astonishing 

 variations of almost all cultivated plants must be due to 

 selection and breeding from the varying individuals. This 

 idea crossed my mind many years ago, but I was afraid to 

 publish it, as I thought that people would say, " how he does 

 exaggerate the importance of selection." 



I still must believe that changed conditions give the impulse 

 to variability, but that they act in most cases in a very 

 indirect manner. But, as I said, it is a most perplexing pro- 

 blem. Pray forgive me for writing at such length ; I had no 

 intention of doing so when I sat down to write. 



I am extremely sorry to hear, for your own sake and for 

 that of Science, that you are so hard worked, and that so much 

 of your time is consumed in official labour. 



Pray believe me, dear Professor Semper, 



Yours sincerely, 

 CHARLES DARWIN. 



