THEORIES OF GALL ORIGIN. 19 



the infection theory. Just as the germ of smallpox 

 inserted into a human subject gives issue to smallpox 

 and not scarlet fever — as each infectious disease 

 had its origin in its own specific germ, — so it was 

 assumed that each gall-fly was provided with a 

 specific poison peculiar to itself, which produced its 

 own specific gall-structure. 



Until recent years this theory was the one which 

 was held in most general repute. Darwin, apparently 

 on the strength of Lacaze-Duthiers' memoir, accepted 

 it without question. Thus (Animals and Plants under 

 Domest., ii, p. 384) he says, " As the poisonous secre- 

 tion of insects belonging to various orders has the 

 special power of affecting the growth of various plants ; 

 as a slight difference in the nature of the poison suffices 

 to produce widely different results ; and, lastly, as we 

 know that the chemical compounds secreted by plants 

 are eminently liable to be modified by changed condi- 

 tions of life, we may believe it possible that various 

 parts of a plant might be modified through the 

 agency of its own altered conditions". Again, he says, 

 "When we seethe symmetrical and complex outgrowths 

 caused by a minute atom of the poison of a gall insect, 

 we may believe that slight changes in the chemical 

 nature of the sap or blood would lead to extraordi- 

 nary modifications of structure" (Origin of Species, 

 (p. 572). In the same work (p. 9) he says further, " as 

 the complex and extraordinary outgrowths which in- 

 variably follow from the insertion of a minute drop 

 of poison by a gall-producing insect show us what 

 singular modifications result, in the case of plants, from 

 a chemical change in the nature of sap." 



Prof. Kiley accepts also this view. In the article 

 " Gall Insects," in Johnson's Cyclopaedia, p. 422, he 

 says, " With the ovipositor . . . the female pierces 

 the plant tissues, and therein consigns an egg, together 

 with a small quantity of a peculiar poisonous fluid. 

 Under the influence of this fluid the gall rapidly deve- 

 lops, and is generally fully formed before the egg 



