List of the Insect Fauna of the County. 107 



insect selects, by instinct, the very part of the plant which 

 is adapted to its purpose. The provision made at the cost of 

 the plant is exactly adapted in quality to the welfare of the 

 insect or its offspring, and in quantity as well ; for both the 

 lodging and the food are made sufficient for any necessary 

 time— for days or weeks, often for many months ; in some 

 instances for two or even three years. Nay, more than this, 

 a gall, of which the growth has been provoked by the virus 

 of one insect, may be fit for the food and lodging of another, 

 which, when all seems complete, can penetrate the gall- 

 cavity, and there, as with theft or murder, obtain food and 

 lodging perfectly suited to itself or its progeny. And the 

 whole process in the plant, though it be one of disease, and, 

 in a sense, unnatural, is yet so regular, so constant and 

 specific, that the form and other characters of each gall or 

 other morbid product are usually as constant and charac- 

 teristic as are those of the insect itself, and the differences 

 among the galls are at least as great as those among the 

 insects. Is there, in all the range of natural history, a more 

 marvellous group of facts than may here be studied ? If you 

 would like to work out a problem in evolution, find how it 

 has come to be a part of the ordinary economy of nature 

 that a gall-insect compels some part of a plant to grow in a 

 manner which, while injurious to the plant, becomes useful 

 to one insect not yet born, and to another who will in due 

 time invade the gall and kill and feed upon its occupant, and 

 then may itself be invaded and eaten by a third. 



"But now of the relation between galls and our specific 

 diseases, such as our eruptive fevers, syphiUs, cancer, gout, 



and others. 



" In these galls 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 pecuhar to itself. The poison 

 may be merely deposited ; but, in the instances 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 gaUs, 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 httle wound 

 closes ; the virus, whether an oral or an ovarian secretion, 

 remains; and the result of its influence on the plant- 



