America might be related to Eastern Asia (always excluding Arctic forms) by a genus having the same species confined to these two regions; or it might be related by the genus having different species, the genus itself not being found elsewhere.

The relation of the genera (excluding identical species) seems to me a most important element in geographical distribution often ignored, and I presume of more difficult application in plants than in animals, owing to the wider ranges of plants; but I find in New Zealand (from Hooker) that the consideration of genera with representative species tells the story of relationship even plainer than the identity of the species with the different parts of the world.

I should like to see the genera of the United States, say 500 (excluding Arctic and Alpine) divided into three classes, with the proportions given thus:-- 100/500 American genera; 200/500 Old World genera, but not having any identical species in common; 200/500 Old World genera, but having some identical species in common; Supposing that these 200 genera included 600 U.S.plants, then the 600 would be the denominator to the fraction of the species common to the Old World.

But I am running on at a foolish length.
There is an interesting discussion in De Candolle (about pages 503-514) on the relation of the size of families to the average range of the individual species; I cannot but think, from some facts which I collected long before De Candolle appeared, that he is on wrong scent in having taken families (owing to their including too great a diversity in the constitution of the species), but that if he had taken genera, he would have found that the individual species in large genera range over a greater area than do the species in small genera: I think if you have materials that this would be well worth working out, for it is a very singular relation.
With respect to naturalised plants: are any social with you, which are not so in their parent country?
I am surprised that the importance of this has not more struck De Candolle.

Of these naturalised plants are any or many more variable in your opinion than the average of your United States plants?
I am aware how very vague this must be; but De Candolle has stated that the naturalised plants do not present varieties; but being very variable and presenting distinct varieties seems to me rather a different case: if you would kindly take the trouble to answer this question I should be very much obliged, whether or no you will enter on such points in your essay.
With respect to such plants, which have their southern limits within your area, are the individuals ever or often stunted in their growth or unhealthy?
I have in vain endeavoured to find any botanist who has observed this point; but I have seen some remarks by Barton on the trees in United States.