My dear Sir

I am not quite sure of understanding your question about “variable genera”.¹ To explain my uncertainty, I will endeavour to define or state the differences for choice.

1. Genera, of which the species are close, & difficult to distinguish by reason of their similarity;—but the species themselves not remarkably variable. Ex: Carex & Ranunculus (excluding Batrachium)

2. Genera, of which the quasi species are so close that it becomes highly difficult to say whether the genus is composed of a comparatively few extremely variable species, or of many very close species. Ex: Rubus & Hieracium.

3. Genera, the species of which are themselves so variable, & approximating, that it becomes difficult to say where one species ends & the next begins. Ex: Viola & Saxifraga, at least in certain sections or subgenera—

It seems to me that Dr. A. Gray may have inclined to the first, while you perhaps yes certainly intend 2 or 3. I will copy the three categories, that you maybe