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beneficial to the plant itself. That is, I suppose, an incontrovertible axiom. Odoriferous glands, especially if imbedded in the leaf, act as a protection against leaf-cutting ants, and (to some extent) also against catterpillars. I can remember no instance of seeing insects attracted to a plant, to aid in its fertilization, or for any other purpose, by their presence. The glands on which some insects feed are (so far as I know) always exposed, either in the shape of cups on the petioles, involucres &c., or of hairs with dilated and hollow bases, and of sessile or stalked cysts, on the leaves, petioles, pedicels &c.; and the secretion is either tasteless or slightly sweet, but inodorous—to our senses at least.
“Trees with aromatic leaves abound in the plains of equatorial America. Those which have the aromatic (and often resinous) secretion imbedded in distinct cysts include all Myrtacea, Myrsineæ, Sanydeæ, and many Euphorbiaceæ, Compositæ etc. The leaves of very few of these are, when growing, ever touched by leaf-cutting ants. In the few cases, however, where the secretion is slightly but pleasantly bitter, and wholesome, as in the Orange, the leaves are quite to their taste. At a farm house on the Trombetas 1 I was shown orange-trees which had been entirely denuded in a single night by Saúba ants. Various expedients are resorted to by the inhabitants of Saúba-infested lands to protect their fruittrees, such as a small moat, kept constantly filled with water, around each tree; or wrapping the base of the trunk with cotton kept soaked with andiroba oil, etc.
[NOTE.—Leaf-cutters in the vicinity of man work chiefly by night, taught doubtless by painful experience of his vicious propensity to interfere with their operations. But in the depths of the forest I have often caught them at work, some up a tree cutting off leaves and even slender young branches, others on the ground sawing them up and carrying them off. When at San Carlos,? I one day went into the forest to gather a Securidaca (woody Polygaleous twiner) I had seen coming into flower a few weeks before. I found it in full
" A northern tributary of the Amazon above Santarem.
flower, but the little tree on which it grew-a Phyllanthus, with slightly milky and quite innocuous juice, had been taken possession of by a horde of ants, and I had to wait until they had stripped it of every leaf before I could pull down my Securidaca, which they had left quite untouched. It was probably preserved by its drastic properties from sharing the fate of the Phyllanthus.]
“Many odoriferous leaves seem destitute of special oilglands, and their essential oil probably exists in nearly every cell, along with the chlorophyll as I have found it in several aromatic Hepatics. Many Laurineæ and Burseraceæ (Amyrideæ of Lindley) are in this case. The latter are eminently resiniferous, and yield the best native pitch (the brea branca) of the Amazon valley. I have never seen their leaves mutilated by ants, and I think never by catterpillars. Oil-glands indeed exist in many plants where they are either so deeply imbedded or so minute as only to be detected by close scrutiny. Their presence was denied in the Nutmegs (see Lindley, etc.)? until I found them in the American species, and one species has them so conspicuous that I have called it Myristica punctata.
“In nearly all these plants, however, when the essential oil has been wholly or in part dissipated by drying, the leaf-cutters find the leaves apt material for their purposewhatever that
be. They once fell on some of my dried specimens, and first cut up a Croton-a genus I had never seen them touch in the living state. It reminded me of our cows in England, which cautiously avoid the fresh foliage of Buttercups, but eat it readily when made into hay. The acrid principle in these and many other plants, odorous and inodorous, is known to be highly volatile.
“Where aromatic plants most abound is in the dry—often nearly treeless—mountainous parts of southern Europe and Western Asia, especially in the sierras of Spain. When I
* In Lindley's “ Vegetable Kingdom" (3rd ed.) he gives among the characters of the Order Myristicaceæ, “Leaves not dotted.”—A. R. W.
? The ants store these leaves in extensive underground cavities, where fungi grow on them on which the ants feed (see Bates and Belt).-A. R. W.
was with Dufour at St. Sever, in April, 1846, he received a large parcel of plants recently gathered in the Sierra Guadarama by Prof. Graells, of Madrid. A very large proportion were aromatic, and many of them Labiates.
“I cannot make out that plants with scented leaves abound more in the tropics than in mid-Europe: nor does there seem to be a larger proportion of them in any zone of the equatorial Andes than in the Amazonian plain ; although, as hill-plants are often gregarious, and those of hot plains very rarely so, odoriferous plants may seem more prevalent in the high Andes than on the Amazon.
“Plants growing nearest eternal snow in the Andes are, however (so far as I have observed them), all scentless; but some acquire an aroma in drying, as, for example the thick roots of the Valerians that abound there.
“ Aromatic plants grow in the Andes up to, perhaps, 13,000 feet, and consist chiefly of Composites, Myrtles, Labiates and Verbenas. I know a hill-side at about 9000 feet, which at this time of year is one mass of odoriferous foliage and flowers, chiefly of a Labiate undershrub (Gardoquia fasciculata, Bth.). Another slope of far wider extent is much gayer with varied colour mainly of the blue flowers of Dalea Mutisii H. B. K.-a papilionaceous shrub allied to the Indigos—and of the red-purple foxglove-like flowers of Lamourouxia virgata H. B. K. (which is parasitic on the roots of the Dalea) mingled with the yellow flowers of the Quitenian broom (Genista Quitensis, L.), and of many other herbs and shrubs with flowers of various shades of colour; but aromatic plants are almost unrepresented except by scattered bushes of a Salvia and a Eupatorium. Analogous contrasts are common enough in our own country.
"In those parts of the Peruvian and Quitenian Andes I have explored, I have not found odoriferous plants more abundant than in some parts of England and the Pyrenees; yet they are quite as much so as in the Amazonian plain, and often belong to the same Natural Orders. Now leafcutting ants are unknown in the Andes; whence I infer that, although the presence of a pungent smell and taste may be
protective to leaves in hot forests where such ants do exist, it has not been acquired originally to provide the requisite protection.
“I much doubt the correctness of Mr. Belt's theory that the ants which inhabit leaf-sacs protect the leaves from leaf-cutting ants; for the leaves of such plants are almost invariably thin and dry; whereas the Saúba always selects leaves that are more or less coriaceous, and if it really wanted the sacciferous leaves I fancy it would make short work of their frail inhabitants. Besides, there are numbers of Melastomes, allied to Tococa and Myrmidone, which the Saúba never touches, although they have no protective (?) sacs; but it cuts up readily the coriaceous leaves of other Melastomes, such as various Bellucias, Henrietteas, &c.
This letter was written in pencil lying on a couch, to which he was confined the greater part of the day during the latter years of his life, and I have much pleasure in printing it here, because it serves to show my friend's acuteness of observation, and the great interest he took, not only in the structure, but in the whole life and nature of the plants he loved so well, and in their relations to the animal world. I have no doubt but that his objections to Belt's theory that the small stinging ants protected the leaves of the trees or shrubs they inhabited from the very powerful and destructive Saúba ants, are quite sound, and that his many years' observations in the Amazonian forests are to be trusted on this point; yet I believe that Belt was right in their being protective, and there are many devourers of leaves that are as destructive as the leaf-cutting ants. Shrubs which always had colonies of stinging ants would probably be avoided by the tapir and by deer, while they would almost certainly check the ravages of caterpillars, locusts, and the large leaf- and stick-insects.
There is another point that this letter illustrates : the wonderful complexity and adaptability of organization of all living things leading to that infinite variety of form and structure, of colour and motion, which constitute the greatest
charm of the study of nature. People continually ask, “If scented leaves are such a protection, why do not all plants have them? If so many can do without them they cannot be of any use.” And the same objection is made to all the other wonderful modes of protection by concealing colours or patterns, by resembling uneatable or dangerous species, by the production of spines or various kinds of armour. “Why are not all protected ?” they say. “You admit that the majority are without these kinds of protection, yet they all continue to exist. The whole idea is therefore a delusion.” And they think they have thus destroyed a large part of Darwin's theory. But all this shows that they are either ignorant of, or forget, the main facts on which that theory is founded—the enormous rate of possible increase of all organisms, the intensity therefore of the struggle to exist, since only the few best adapted of these enormous numbers can survive to produce offspring ; and also the undoubted fact that species vary enormously in population, some being common over large areas, some comparatively scarce, others confined to very limited areas, others again only existing in such small numbers and in such restricted areas that they are very rarely found. Now, if some great change of climate comes on slowly, such a mixed population of species will be affected in different ways and will require different modifications to become adapted to it. Some will become extinct, some will be adapted in one way, some in quite a different way, depending partly on the kind and amounts of variation that occurs in each species. Some will therefore become more numerous in individuals, others less; and when the complete change of climate has been effected, we should find a new set of species, some differing very little, others very greatly from the former inhabitants of the district, but all fairly well adapted to live under the new conditions. Taking the one case of the protected leaves, it would be only those which were in some danger of extermination by insect and other enemies that would develop the various forms of protection by oil-glands, or hairs, or spines, or by attracting stinging ants; while many which existed in great numbers and over wide areas, and which