There is nothing like one's own hobby-horse. I suspect it is the same case as of glacial migration, and of naturalised production--of production of greater area conquering those of lesser; of course the Indian forms would have a greater difficulty in seizing on the cool parts of Australia. I demur to your remarks (page 1), as not "conceiving anything in soil, climate, or vegetation of India," which could stop the introduction of Australian plants. Towards the close of the essay (page civ), you have admirable remarks on our profound ignorance of the cause of possible naturalisation or introduction; I would answer page 1, by a later page, viz. page civ.

Your contrast of the south-west and south-east corners is one of the most wonderful cases I ever heard of...You show the case with wonderful force. Your discussion on mixed invaders of the south-east corner (and of New Zealand) is as curious and intricate a problem as of the races of men in Britain. Your remark on mixed invading Flora keeping down or destroying an original Flora, which was richer in number of species, strikes me as EMINENTLY NEW AND IMPORTANT. I am not sure whether to me the discussion on the New Zealand Flora is not even more instructive. I cannot too much admire both. But it will require a long time to suck in all the facts. Your case of the largest Australian orders having none, or very few, species in New Zealand, is truly marvellous. Anyhow, you have now DEMONSTRATED (together with no mammals in New Zealand) (bitter sneer No. 3), that New Zealand has never been continuously, or even nearly continuously, united by land to Australia!! At page lxxxix, is the only sentence (on this subject) in the whole essay at which I am much inclined to quarrel, viz. that no theory of trans-oceanic migration can explain, etc. etc. Now I maintain against all the world, that no man knows anything about the power of trans-oceanic migration. You do not know whether or not the absent orders have seeds which are killed by sea-water, like almost all Leguminosae, and like another order which I forget. Birds do not migrate from Australia to New Zealand, and therefore floatation SEEMS the only possible means; but yet I maintain that we do not know enough to argue on the question, especially as we do not know the main fact whether the seeds of Australian orders are killed by sea-water.

The discussion on European Genera is profoundly interesting; but here alone I earnestly beg for more information, viz. to know which of these genera are absent in the Tropics of the world, i.e. confined to temperate regions. I excessively wish to know, ON THE NOTION OF GLACIAL MIGRATION, how much modification has taken place in Australia. I had better explain when we meet, and get you to go over and mark the list.

...The list of naturalised plants is extremely interesting, but why at the end, in the name of all that is good and bad, do you not sum up and comment on your facts? Come, I will have a sneer at you in return for the many which you will have launched at this letter. Should you have remarked on the number of plants naturalised in Australia and the United States UNDER EXTREMELY DIFFERENT CLIMATES, as showing that climate is so important, and [on] the considerable sprinkling of plants from India, North America, and South Africa, as showing that the frequent introduction of seeds is so important? With respect to "abundance of unoccupied ground in Australia," do you believe that European plants introduced by man now grow on spots in Australia which were absolutely bare? But I am an impudent dog, one must defend one's own fancy theories against such cruel men as you. I dare say this letter will appear very conceited, but one must form an opinion on what one reads with attention, and in simple truth, I cannot find words strong enough to express my admiration of your essay.

My dear old friend, yours affectionately, C. DARWIN.

P.S.--I differ about the "Saturday Review". ("Saturday Review", December 24, 1859. The hostile arguments of the reviewer are geological, and he deals especially with the denudation of the Weald.

Charles Darwin

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