Chap. XXIII. CONDITIONS OF LIFE. 273 



greater number by species of Cynips. It is impossible to read 

 M. Lacaze-Duthiers' 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 group with Cynips. Galls 

 grow with extraordinary rapidity, and it is said that they 

 attain their full size in a few days ; ^° it is certain that they 

 are almost completely develoj)ed before the larvae are batched. 

 Considering that many gall-insects are extremely small, the 

 drop of secreted poison must be excessively minute ; it pro- 

 bably acts on one or two cells alone, which, being abnor- 

 mally stimulated, rapidly increase by a process of self-division. 

 Galls, as Mr. Walsh^^ remarks, afford good, constant, and 

 definite characters, each kind keeping as true to foim 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 {Cecidomyidce) which " though essentially dis- 

 " tinct species, yet resemble one another so closely that in 

 " almost all cases it is difficult, and in most cases impossible, 

 " to distinguish the full-grown insects one from the other."^- 

 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 indiK e widely different results. In some few 

 cases the same specie.s of gall-gnat produces on distinct species 

 of willows galls which cannot be distinguished ; the Cynijjs 

 fecundatrix, also, has been known to produce on the Turkish 

 oak, to which it is not properly attached, exactly the same 

 kind of gall as on the European oak.^^ These latter facts 

 apparently prove that the nature of the poison is a more 

 powerful agent in determining the form of the gall than the 

 specific character of the tree which is acted on. 



As the poisonous secretion of insects belonging to various 

 orders has the special power of affecting the growth of various 



^0 KirbyandSpence's 'Entomology,' " Mr. B. D.Walsh, ibid., p. 633, 



1818, vol. i. p. 450; Lacaze-Duthiers, and Dec. 1866, p. 275. 

 ibid., p. 284. " Mr. B. D. Walsh, ibid., 1864, pj) 



"'Proc. Entomolog. Soc. Phila- 545, 411, 495 ; and Dec. 1866, p. 27« 



delphia,' 1864, p. 558. See also Lacaze-Duthiers. 



