Dear Darwin

On the back of this you have a list of 80 very anomalous genera selected by Bentham—² I appended 1) the number of species enumerated in Steudel Nomenclator ³ the only catalogue of plants & not a bad one for such purposes & 2) the number of species according to Benthams & my knowledge of the genera, which increases some, diminishes others, & also diminishes the total of species. I append Steudels ratio of species to genera of all plants.— I should suppose however that it would stand considerable reduction & that 1:8 would possibly be a fair estimate or even 1:6.—⁴

I am working up my Tasmanian Flora⁵ now, & at the same time making a running catalogue of the Australian plants. There is probably as great specific difference between East & West Australia, as between New Zealand & Australia, this is quite a guess, but right or wrong offers food for speculation. Suppose the sea to cut Australia longitudinally—& to leave species as they are, it would probably leave the northern ends of the two resultant Islands more Botanically alike than the Southern.— But allow the consequent equable climate upon both Islands produced by the intervening ocean to have its effect; & it would theoretically destroy more of the plants peculiar to the Southern (drier) than the Northern (wetter) halves of each, & thus approximate the Botanical features of the Southern parts of each proportionately more than the Northern. also we may assume that all after imports would succeed better in the two humid Islands than upon the one dry one & that the sea being as good as a transporter as the desert the tendency to uniformity would increase— “Under this view disruption produces similarity of Botanical features” & except you call in new creations I do not see how you are to produce any considerable amount of specific difference in two contiguous spots with the same climate consistently with Geological change having any effect at all. The prominent effect of geological change is change of climate & that as it appears to me tends to destroy species & especially peculiar genera—⁶ Granting even that the existing species are altered preexistent forms, it appears to me that the tendency is to obliteration of forms, for the number of species that will change under change of climate is indefinitely small compared to what will be killed by the same change of climate: the number of species was once infinitely greater than now, or we have new creations

Ever yrs | J D Hooker. diag Steudel Self

1 Ceratophyllum — 8 — 2

2 Cannabis — 1 — 1

3 Oldfieldia — 1 — 1

4 Gyrostemon — 3 — 3

5 Callitriche — 16 — 4

6 Batis — 1 — 1

7 Nepenthes — 7 — 10

8 Sabia — 4 — 7

9 Lacistema — 5 — 8 10 Moringa — 4 — 2 11 Fouquiera — 1 — 2 12 Trigonia — 8 — 8 13 Krameria — 9 — 9 14 Reaumuria — 2 — 2 15 Tetradiclis — 1 — 1 16 Canella — 2 — 1 17 Cneorum — 2 — 1 18 Suriana — 1 — 1 19 Hernandia — 4 — 2 20 Cassytha — 10 — 15 21 Schæffera — 5 — 3 22 Goupia — 2 — 1 23 Balanites — 1 — 1 24 Diapensia — 2 — 1 25 Stilbe — 6 — 6 26 Desfontainia — 1 — 1 27 Retzia — 3 — 1 28 Cyananthus — 3 — 5 29 Codon — 1 — 1 30 Ægiceras — 4 — 2 31 Salvadora — 5 — 1 32 Brunonia — 2 — 2 33 Phryma — 1 — 1 34 Bravaisia — 1 — 1 35 Columellia — 3 — 3 36 Xanthium — 8 — 4 37 Gyrocarpus — 4 — 2 38 Anisophylleia — 2 — 2 39 Gunnera — 8 — 10 40 Diclidanthera — 2 — 2 ————————

1161 1132 Ratio of species to genera in Steudel $\frac{6722}{78005}$ ramme