From Asa Gray 7 July 1857
July 7th. 1857.
My Dear Mr. Darwin
Your letter of June 18th came last week. Surely I need not say your letters are always most welcome. Among the great amount and variety of letters I receive yours and Dr. Hooker’s are almost the only ones that set me thinking. I am sorry I can make only such insignificant returns.
By a private hand, a passenger by sailing ship to Liverpool I last week sent addressed to you (ready stamped for the Post. at Liverpool) an envelope containing, your Watson correspondence:1 —also a copy of my pamphlet from Silliman’s Journal—. the same you have sheets of already.—2 and a copy of my little hornbook for beginners, the First Lessons in Botany.3
I have not the least remembrance of what I wrote last to you. I am interested in what you say about extinction as giving the key to many of your ingenious views, & the clue to the probable explanation of a good many otherwise inexplicable things.4
I accept it as best explaining disjoined species. I see that the same cause must have reduced many species of great range to small, and that it may have reduced large genera to small, and so of families. But why is it not just as likely that there were as many small genera (nearly) at first as now, and as great a disproportion in the number of their species? There is Sarraceniaceæ of three genera—belonging to E. U. States, California, & Guiana, two of them of one known species each, and all of narrow ranges Is it philosophical, is it quite allowable, to assume (without evidence from fossil plants) that the family or any of the genera was once larger and wide spread? and occupied a continuous area? The tree of narrowest range in U.S. is Cladrastis: must I suppose it ever had a wide range, or even a wider range than it has now? That its area was once somewhat larger is extremely probable. But, granting you lots of species which have been greatly reduced by extinction, must you not admit that there are and were many more that never succeeded in getting a wide range? Would it not be pretty sure to be so of the later-comers? And there must have been late comers, comparatively. What chance would a new species, of average powers, have had of getting a foot-hold, still less of ever getting a wide range, if introduced locally into the forest of N. America while yet untouched by man?
Of any given local species, it seems to me a priori quite as likely (to say the least) that it never had a wide dominion, as that it had, and is on the road to extinction:— That is of species of continents.— As to islands, when you know their geological history, the presumption may be quite the other way, no doubt.
That is the way the question strikes an outsider like me,—a cautious and slow one,—just at first view. If I had ever thought over the matter, and investigated it as you have, very likely I should think quite differently. But it is just such sort of people as I that you have to satisfy and convince, and I am a very good subject for you to operate on, as I have no prejudice nor preposessions in favor of any theory at all.
I never yet saw any good reason for concluding that the several species of a genus must ever have had a common or continuous area. Convince me of that, or show me any good grounds for it (beyond the mere fact that it is generally the case),— i.e. show me why it ought to be so, and I think you would carry me a good way with you—as I dare say you will, when I understand it.
About crossing of individuals of a species, I have as you know a strong inclination to suppose it so, and that it plays an important part in repressing variation, and perhaps even in ensuring the perpetuation of species.5 That it is a vera causa I surely believe; but how much operation it has is the question. Perhaps you may overestimate it. As you say, Papilionaceæ are dead against you. There the blossoms are self-fertilized—no doubt. I will give you a stronger case still, in Fumariaceæ! Here it cannot be otherwise.6 But are Fumariaceæ sportive? Fumaria off. seems to be. Corydalis aurea runs to variation. All the rest of ours are remarkably steady; and that this is, on the whole, decidedly so in the family, See Hook. & Thom. Fl. Ind. p. 259.7
As to Gramineæ I must have taken for granted (in my last letter) that what has of late been so positively asserted was true. But I have just been looking at the principal Grasses around my door. Festuca elatior, Triticum repens, Dactylis, Arrhenatherum, Phleum, Alopecurus, Panicum clandestinum, Phalaris: not a soul of them shed a grain of pollen till the anthers and the stigmas hang out:—and the conspicuous way they hang out in anthers should have prevented anybody from talking of fertilization in the bud till they had observed it.8 So I can confirm your own observations here.
As to Compositæ my hasty statement may need equal correction. It was based on the fact that the anther-cells are empty of pollen as soon as the corolla opens, or very soon after, the style as it elongates pushing out the pollen. But as the branches do not diverge until after protrusion, I will not swear that any pollen reaches the stigmatic surface while in the bud.—probably not generally.—9 But I think I know some cases where the fertilization occurs in the bud. I have not time now to look at some common plants.— What you say about the pollen carried to a female Holly so abundantly, and to such a distance is very remarkable.
Ever most cordially Yours | Asa Gray
Believes, with CD, that extinction may be an important factor in explaining plant distributions, but sees no reason why the several species of a genus must ever have had a common or continuous area. "Convince me of that, or show me any good grounds for it … and I think you would carry me a good way with you". It is just such people as AG that CD has to satisfy and convince.
Feels that the crossing of individuals is important in repressing variation and perhaps in perpetuating the species, but instances some plants in which it cannot, apparently, take place.