Cambridge, Mass., September 23rd, 1856.
Dr.Engelmann, of St.Louis, Missouri, who knew European 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.