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. Trees seem in this respect to behave rather
differently from other plants.

It would be a very curious point, but I fear you would think it out of
your essa