106 Tke Galls of Essea- ; a Contrihutlini to a 



slijiped and variously constructed growths of cellular paren- 

 chymatous tissue and cork, and, more sparingly, of woody 

 tissue or of cells wdiose thick walls become as hard as wood. 

 Besides, some of the monstrous growths of parts of plants, 

 and some of the viviparous variations, and of the undue 

 metamorphoses of leaves, are to be referred to the influence 

 of parasites. 



"It may seem bold to speak of so many hundreds of widely 

 various morbid processes as having any essential character 

 in common, or as fit to be included under one name ; yet I 

 think we may regard the whole of these afj being such as, in 

 our pathology, we should call inflammatory hypertrophies 

 or hyperplasife. They all show a rapid increase of lowly 

 organised structures, by derivation from, and in continuity 

 with, those pre-existing. There is, as in the products of our 

 inflammations, a general likeness among these new structures, 

 whatever be the part of the plant from which they are 

 derived, and all bear a general likeness to the structures 

 formed after injuries of actively growing parts. In the 

 morbid growths formed by these new structures, the deflection 

 fL'om the natural shape and construction of the part, in con- 

 tinuity with which they have grown, is often not complete ; 

 they often retain marks of characteristic normal forms, aiid 

 sometimes acquire marks of natural variation from the 

 species. Moreover, all these morbid growths have their 

 origin in wdiat may justly be called 'irritation' of the part 

 on which they grow; and in all of them, I think, we may 

 note signs of degeneracy from natural conditions, either in 

 the absence of stomata or similar structures, or in the 

 presence of the red, or yellow, or other colours commonly 

 noticed in decay. 



"Here, I believe, are reasons enough for regarding all these 

 galls and gall-like products of disease, generated in plants by 

 insects, as analogous with a large group of the products of 

 inflammation which we study in our own pathology ; and 

 the analogy is not the less because neither group can be 

 circumscribed with any exact definition. 



"I will not be tempted to speak long, but I beg you to 

 think long, of the marvellous facts of natural adjustment 

 which we have here, in this intense example of the ^sic vos, 

 non vohis.' Here are the bare facts. Each sj)ecies of these 

 parasitic insects can compel some part of a i^lant into such 

 disease as shall supply good food, or well-built and well- 

 placed lodging, or both, for itself or for its eggs and larvae, or 

 even for part of the life of its complete offspring. Each 



