1843—1882] SOCIAL PLANTS 43 1 



Mr. Watson to ask whether he knew of any plants not ranging Le tter -29 

 northward of Britain (say 55 ) which were in common, he 

 writes to me that he imagines there are very few ; with 

 Mr. Syme's assistance he found some 20 to 25 species thus 

 circumstanced, but many of them, from one cause or other, he 

 considered doubtful. As examples, he specifies to me, with 

 doubt, Chn'sosplenium oppositifolium ; Is>iardia palustris ; 

 Astragalus hypoglottis ; Thlaspi alpestre ; Arcnaria vcnia ; 

 LytJirum hyssopifolium. 



I hope that you will be inclined to work out for your next 

 paper, what number, of your 321 in common, do not range to 

 Arctic regions. Such plants seem exposed to such much 

 greater difficulties in diffusion. Very many thanks for all 

 your kindness and answers to my questions. 



P.S. — If anything should occur to you on variability of 

 naturalised or agrarian plants, I hope that you will be so 

 kind as to let me hear, as it is a point which interests me 

 greatly. 



Asa Gray to C. Darwin. Letter 330 



Cambridge, Mass., Sept. 23rd, 1856. 



Dr. Engelmann, of St. Louis, Missouri, who knew Euro- 

 pean botany well before he came here, and has been an 

 acute observer generally for twenty years or more in this 

 country, in reply to your question I put to him, promptly 

 said introduced plants are not particularly variable — are not 

 so variable as the indigenous plants generally, perhaps. 



The difficulty of answering your questions, as to whether 

 there are any plants social here which are not so in the Old 

 World, is that I know so little about European plants in 

 nature. The following is all I have to contribute. Lately, I 

 took Engelmann and Agassiz on a botanical excursion over 

 half a dozen miles of one of our seaboard counties ; when they 

 both remarked that they never saw in Europe altogether 

 half so much barberry as in that trip. Through all this 

 district B. vulgaris may be said to have become a truly social 

 plant in neglected fields and copses, and even penetrating 

 into rather close old woods. I always supposed that birds 

 diffused the seeds. But I am not clear that many of them 

 touch the berries. At least, these hang on the bushes over 



