Bentham has stated this in a striking way: "Fixed and immovable as is the individual plant, there is no class in which the race is endowed with greater facilities for the widest dispersion...Plants cast away their offspring in a dormant state, ready to be carried to any distance by those external agencies which we may deem fortuitous, but without which many a race might perish from the exhaustion of the limited spot of soil in which it is rooted." (Pres. Addr.(1869), "Proc. Linn. Soc." 1868-69, pages lxvi, lxvii.)I have quoted this passage from Bentham because it emphasises a point which Darwin for his purpose did not find it necessary to dwell upon, though he no doubt assumed it. Dispersal to a distance is, so to speak, an accidental incident in the life of a species. Lepidium Draba, a native of South-eastern Europe, owes its prevalence in the Isle of Thanet to the disastrous Walcheren expedition; the straw-stuffing of the mattresses of the fever-stricken soldiers who were landed there was used by a farmer for manure. Sir Joseph Hooker ("Royal Institution Lecture", April 12, 1878.)tells us that landing on Lord Auckland's Island, which was uninhabited, "the first evidence I met with of its having been previously visited by man was the English chickweed; and this I traced to a mound that marked the grave of a British sailor, and that was covered with the plant, doubtless the offspring of seed that had adhered to the spade or mattock with which the grave had been dug."Some migration from the spot where the individuals of a species have germinated is an essential provision against extinction. Their descendants otherwise would be liable to suppression by more vigorous competitors. But they would eventually be extinguished inevitably, as pointed out by Bentham, by the exhaustion of at any rate some one necessary constituent of the soil. Gilbert showed by actual analysis that the production of a "fairy ring" is simply due to the using up by the fungi of the available nitrogen in the enclosed area which continually enlarges as they seek a fresh supply on the outside margin. Anyone who cultivates a garden can easily verify the fact that every plant has some adaptation for varying degrees of seed-dispersal. It cannot be doubted that slow but persistent terrestrial migration has played an enormous part in bringing about existing plant-distribution, or that climatic changes would intensify the effect because they would force the abandonment of a former area and the occupation of a new one. We are compelled to admit that as an incident of the Glacial period a whole flora may have moved down and up a mountain side, while only some of its constituent species would be able to take advantage of means of long-distance transport.
I have dwelt on the importance of what I may call short-distance dispersal as a necessary condition of plant life, because I think it suggests the solution of a difficulty which leads Guppy to a conclusion with which I am unable to agree. But the work which he has done taken as a whole appears to me so admirable that I do so with the utmost respect. He points out, as Bentham had already done, that long-distance dispersal is fortuitous. And being so it cannot have been provided for by previous adaptation. He says (Guppy, op. cit. II. page 99.): "It is not conceivable that an organism can be adapted to conditions outside its environment." To this we must agree; but, it may be asked, do the general means of plant dispersal violate so obvious a principle? He proceeds: "The great variety of the modes of dispersal of seeds is in itself an indication that the dispersing agencies avail themselves in a hap-hazard fashion of characters and capacities that have been developed in other connections." (Loc. cit. page 102.) "Their utility in these respects is an accident in the plant's life." (Loc. cit. page 100.) He attributes this utility to a "determining agency," an influence which constantly reappears in various shapes in the literature of Evolution and is ultra-scientific in the sense that it bars the way to the search for material causes. He goes so far as to doubt whether fleshy fruits are an adaptation for the dispersal of their contained seeds. (Loc. cit. page 102.) Writing as I am from a hillside which is covered by hawthorn bushes sown by birds, I confess I can feel little doubt on the subject myself. The essential fact which Guppy brings out is that long-distance unlike short-distance dispersal is not universal and purposeful, but selective and in that sense accidental. But it is not difficult to see how under favouring conditions one must merge into the other.
Guppy has raised one novel point which can only be briefly referred to but which is of extreme interest. There are grounds for thinking that flowers and insects have mutually reacted upon one another in their evolution.
Guppy suggests that something of the same kind may be true of birds. Imust content myself with the quotation of a single sentence. "With the secular drying of the globe and the consequent differentiation of climate is to be connected the suspension to a great extent of the agency of birds as plant dispersers in later ages, not only in the Pacific Islands but all over the tropics. The changes of climate, birds and plants have gone on together, the range of the bird being controlled by the climate, and the distribution of the plant being largely dependent on the bird." (Loc.cit. II. page 221.)