514 MISCELLANEOUS BOTANICAL LETTERS. [1876. 



for a long time, and had come to think that perhaps my no- 

 tion was mere fancy, but I had intended to begin experiment- 

 ing as soon as I returned home ; and now with your invaluable 

 letter about the position of the leaves of various plants dur- 

 ing rain (I have one analogous case with Acacia from South 

 Africa), I shall be stimulated to work in earnest. 



Variability. 



[The following letter refers to a subject on which my 

 father felt the strongest interest : — the experimental investi- 

 gation of the causes of variability. The experiments alluded 

 to were to some extent planned out, and some preliminary 

 work was begun in the direction indicated below, but the re- 

 search was ultimately abandoned.] 



C. Darwin to J. II. Gilbert* 



Down, February 16, 1876. 

 My dear Sir, — When I met you at the Linnean Society, 

 you were so kind as to say that you would aid me with ad- 

 vice, and this will be of the utmost value to me and my son. 

 I will first state my object, and hope that you will excuse a 

 long letter. It is admitted by all naturalists that no problem 

 is so perplexing as what causes almost every cultivated plant 

 to vary, and no experiments as yet tried have thrown any 

 light on the subject. Now for the last ten years I have been 

 experimenting in crossing and self-fertilising plants ; and one 

 indirect result has surprised me much ; namely, that by tak- 

 ing pains to cultivate plants in pots under glass during several 

 successive generations, under nearly similar conditions, and 

 by self-fertilising them in each generation, the color of the 

 flowers often changes, and, what is very remarkable, they be- 

 came in some of the most variable species, such as Mimulus, 

 Carnation, &c, quite constant, like those of a wild species. 



* Dr. Gilbert, F.R.S., joint author with Sir John Bennett Lawes of a 

 long series of valuable researches in Scientific Agriculture. 



