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Show 282 DEFINITE ACTION OF THE CHAP. XXIII. service to the two series in the Old and New W oriels ; therefore these peculiarities cannot have been naturally selected. Hence we are led to infer that they have been definitely caused Ly the long-continued action of the different climate of the two continents on the trees. Galls.-Another class of facts, not relating to cultivated plants, deserves attention. I allude to the production of galls. Every one knows the curious, bright-red, hairy productions on the wild rose-tree, and the various different galls produced by the oak. Some of the latter resemble fruit, with one face as rosy as the rosiest apple. These bright colours can be of no service either to the gall-forming insect or to the tree, and probably are the direct result of the action of the light, in the same manner as the apples of Nova Scotia or Canada are brighter coloured than English apples. The strongest upholder of the doctrine that organic beings are created beautiful to please mankind would not, I presume, extend this view to galls. Acc~ rcling to Osten Sacken's latest revision, no less than fifty-eight kmds of galls are produced on the several species of oak, by Oynips \vith its sub-genera; and Mr. B. D. Walsh 47 states that he can add many others to the list. One American species of willow, the Salix humilis, bears ten distinct kinds of galls. The leaves which spring from the galls of various English willows differ co~pl~tely in shape from the natural leaves. The young shoots of JUmpers and firs, when punctured by certain insects, yield monstrous growths like flowers and cones ; and the flowers of some plants become from the same cause wholly changed in appearance. Galls are produced in evE:ry quarter of the world· of several sent to me by Mr. Thwaites from Ceylon, some were a~ symmetrical as a composite flower when in bud, others smooth and spherical li~e a berry ; some protected by long spines, others cl~th ed w1th yellow wool formed of long cellular hairs, others w1th regularly tufted hairs. In some galls the internal structure is simple, but in others it is highly complex; thus M. Lucaze-Duthiers 48 has figured in the common ink-gall no less than seven concentric layers, composed of distinct tissue, 47 See l\fr. B. D. Walsh's excellent 48 See his admirable Histoire des papers in 'Proc. Entomolog. Soc. Phila- Galles, in 'Annal. des Sc. Nat. Bot.,' uelphia,' Dec.l8G6, p. 284. With respect 3rd series, tom. xix., 1853, p. 273. to the willow, see idem, 1864, p. 546. CIIAP. XXIII. CONDITIONS OF LIFE. 283 namely, the epidermic, sub-epidermic, spongy, intermediate, and the hard protective layer formed of curiously thickened woody cells, and, lastly, the central mass abounding with starchgranules on which the larvre feed. Galls are produced by insects of various orders, but the greater number by species of Cynips. It is impossible to read M. Lucaze-Duthier's discussion and doubt that the poisonous secretion of the insect causes the growth of the gall, and every one knows how virulent is the poison secreted by wasps and bees, which belong to the same order with Cynips. Galls grow with extraordinary rapidity, and it is said that they attain their full size in a few days ; 49 it is certain that they are almost completely developed before the larvre are hatched. Considering that many gall-insects are extremely small, the drop of secreted poison must be excessively minute; it probably acts on one or two cells alone, which, being abnormally stimulated, rapidly increase by a process of self-division. Galls, as Mr. vValsh 50 remarks, afford good, constant, and definite characters, each kind keeping as true to form as does any independent organic being. This fact becomes still more remarkable when we hear that, for instance, seven out of the ten different kinds of galls produced on Salix humilis are formed by gall-gnats ( Gecidomyidce) which, "though essentially distinct species, yet resemble one another " so closely that in almost all cases it is difficult, and in some "cases impossible, to distinguish the full-grown insects one from " the other." 51 For in accordance with a wide-spread analogy we may safely infer that the poison secreted by insects so closely allied would not differ much in nature; yet this slight difference is sufficient to induce widely different results. In some few cases the same species of gall-gnat produces on distinct species of willows galls which~ ~cannot be distinguished; the Oynips fecundatrix, also, has been known to produce on the 'furkish oak, to which it is not properly attached, exactly the same kind of gall as on the European oak.52 These latter facts apparently prove that the nature of the poison is a much more powerful 49 Kirby and Spence's 'Entomology,' 1818, vol. i. p. 450; Lucuze-Dutuiers, idem, p. 284. 50 ' Proc. Eutomolog. Soc. Philadelphia,' 1864, p. 558. 51 Mr. B. D. Walsh, idem, p. 633; and Dec. 1866, p. 275. fi2 Mr. B. D. Walsh, idPm, 1864, p, 545, 411, 4D5; and Dec. 1866, p. 27'::', See also Lucaze-Dutbiers. |
