My dear Sir

I must thank you for your letter to me very valuable. You have put the case far clearer than I have ever done to myself, by your figure ² 1, 2, 3, 4 5 6 1 3 6 &c If it could be ascertained whether the extremes were more or less different the question w^d be answered much & more satisfactorily than by my means but to resolve such a point, by me at least, is quite impossible.

With respect to the genera, families, being quite artificial, I always endeavour (perhaps not always with success) to keep this steady before my mind; & in the comparison, which I sent you, I merely meant by genera a certain number of species, in the eyes of most Botanists, more like each other than the average run of plants.— I should think (& I formerly had to allow [illeg]) that it is in small genera than the more dissimilar species are aggregated together; & if it were possible to divide all groups with absolute truth according to their amount of structural difference, (quite disregarding numbers of species,) the small genera would have to be more split up, than the larger; for it is probable that all systematists gladly seize on any character by which to break up the larger genera;— So that if genera could be made of absolute equal value in structural difference I imagine the large genera would often have to be made larger, & small genera often smaller, & this would tend to increase the difference in the averages in my comparison. Is not so?—but I am not at all sure I have made clear; what I mean.

From being accustomed to look at the world under a geological point of view I view the view the whole existing Fauna & Flora as a mere fragment; & therefore I have been inclined to look at (for instance) the Flora of England (or even smaller isld.) as a fragment, certainly much smaller but not essentially different, as standard of comparison for any numerical results with respect to the English plants alone. In the Flora of world, as many (or more) links are missing from extinction, as in the case of England, inhabits other geographical regions; the existing flora of the world bears the same sort of relation to the past, as flora of England to all whole rest of globe. Suppose the whole N. hemisphere submerged except Great Britain, then the Flora of England w^d almost necessarily have to be compared numerically with itself,—(Australia & S. America & S. Africa not, I presume, greatly affecting the result), & this imaginary case I look at as only a strong illustration of what has always being going on.—

Your letters are most interesting & valuable to me, but I beg you earnestly on no account to write without you feel inclined, for I have not the least right to cause you to waste your time on me.— Since writing I have worked out Asa Gray list,³ which was a very long one, & the average of the genera with “close species” is 6.37 & the same standard of comparison as used with your list is 4.67 per genus = 1.7 in excess.⁴ It is an odd chance that with the London Cat. as marked by you, the excess is so nearly the same viz 1.68.⁵

with many thanks | Yours very sincerely | C. Darwin

I am quite delighted to hear that you can perhaps illustrate apparent rule, about difference between species & variation of the individual.—

I now wish I had told you my object, before you marked the list but judging for myself I sh^d have f^d it impossible to have kept the size of genus, out of consideration whilst considering any doubtful case.—