FRIENDS AND ACQUAINTANCES 65 



illustrate my memoir. It is the inability to do this which 

 has kept me from writing on many subjects which engaged 

 my attention during the course of my travels. . . . 



" The ants cannot be said to be useful to the plants, any- 

 more than fleas and lice are to animals. They make their 

 habitation in the melastomas, etc., and suck the juice of the 

 sweet berries ; and the plants 'have to accommodate to their 

 parasites as they best may. But even an excrescence may be 

 turned into a ' thing of beauty,' as witness the galls of the 

 wild rose. 



" That diseased structures may become inherited — even 

 in the human subject — there is plenty of evidence to prove. 

 Some curious instances are given in Dr. Elam's ' Physician's 

 Problems.' " 



At this period Dr. Spruce was, of course, not aware of 

 the very strong evidence against the inheritance of acquired 

 characters of any kind, nor had he the advantage of Kerner's 

 wonderful series of observations on the nature of protective 

 plant-structures against enemies of various kinds — " unbidden 

 guests." Nor was he aware of Belt's remarkable explanation 

 of the use to the plant of one of the most remarkable of these 

 ant-structures — the'bull's-horn thorns of a species of acacia. 

 He shows that the ants encouraged by these structures to 

 inhabit the plants are stinging species, are very pugnacious, 

 and thus protect the foliage both from browsing mammals, 

 from other insects, and even from the large leaf-cutting ants. 1 

 In a later letter, however, Dr. Spruce adopts utility to the 

 plant as a general principle. 



In a letter, dated Coneysthorpe, Malton, Yorkshire, July 

 28, 1876, he writes as follows : — 



" I can hardly say that I have ever speculated on the 

 purport of the odours of leaves, but I have (at your in- 

 stance) rummaged in my notes and my memory, for such 

 evidence as I possess on that head, and will lay it before 

 you. 



" Every structure, every secretion, of a plant is (before all) 

 beneficial to the plant itself. That is, I suppose, an incontro- 



1 " The Naturalist in Nicaragua," pp. 218, 223. 



