20 THEORIES OF GALL ORIGIN. 



hatches ; " but this latter remark does not hold with 

 the CynipidaB, as has been shown already. 



Sir James Paget likewise supports the infection 

 theory. Thus he says, " In these and other similar 

 diseases in plants we have, it seems, hundreds of 

 specific diseases due to as many hundreds of specific 

 morbid poisons; for the most reasonable, if not the 

 only reasonable theory of these diseases is, that each 

 insect infects or inoculates the leaf or other structure 

 of the chosen plant with a poison peculiar to itself. 

 The poison may be merely deposited ; but in the in- 

 stances best for study it is inserted in the plant 

 structure, whether leaf or any other : and the wound 

 for inserting it, the poisoned wound, may be made 

 either with part of the oral apparatus, or, as in most 

 of the true galls, with the ovipositor, through which 

 one or more eggs are passed with the virus, and are 

 left among actively living structures of the plant. 

 The little wound closes; the virus, whether an oral 

 or an ovarian secretion, remains ; and the result of 

 its influence on the plant structures and their con- 

 tained protoplasm is the formation of the gall or other 

 morbid product." Further on he says, " We find 

 hundreds of different forms of galls, and we may be 

 nearly sure that there are as many kinds of morbid 

 poisons produced by gall insects, each form answering 

 toadifferent insect" (An Address on Elementary Patho- 

 logy, London, 1880). 



Now in considering this question it must be borne 

 in mind that very many insects, of divers orders and 

 modes of life, cause the formation of galls. Hymenop- 

 tera as represented by Cynipidse, Tenthredinidas, and 

 Chalcididse. The order of Diptera furnishes many expo- 

 nents ; some beetles ; the Hemiptera, especially the 

 plant lice, all form galls. Further, a large number of 

 galls are caused by minute four-footed mites (Phytop- 

 tus) ; and, again, some are formed by Anguillidae, and 

 even by Rotifera. And Fungi originate more or less 

 clearly denned galls. Unquestionably it may be con- 



