I suspect (but am not sure) that in the genus Mus (the most mundane genus of all mammifers) the individual species have not wide ranges, which is opposed to my query.
I fancy, from a paper by Don, that some genera of grasses (i.e.Juncus or Juncaceae) are widely diffused over the world, and certainly many of their species have very wide ranges--in short, it seems that my question is whether there is any relation between the ranges of genera and of individual species, without any relation to the size of the genera.

It is evident a genus might be widely diffused in two ways: 1st, by many different species, each with restricted ranges; and 2nd, by many or few species with wide ranges.

Any light which you could throw on this I should be very much obliged for.

Thank you most kindly, also, for your offer in a former letter to consider any other points; and at some future day I shall be most grateful for a little assistance, but I will not be unmerciful.
Swainson has remarked (and Westwood contradicted) that typical genera have wide ranges: Waterhouse (without knowing these previous remarkers) made to me the same observation: I feel a laudable doubt and disinclination to believe any statement of Swainson; but now Waterhouse remarks it, I am curious on the point.

There is, however, so much vague in the meaning of "typical forms," and no little ambiguity in the mere assertion of "wide ranges" (for zoologists seldom go into strict and disagreeable arithmetic, like you botanists so wisely do) that I feel very doubtful, though some considerations tempt me to believe in this remark.