108 TJie Galls of Essex : a Contribution to a 



structures and their contained protoplasm is the formation of 

 the gall or other morbid product. The whole process may be 

 compared with the local consequences of the insertion of 

 vaccine lymph, or any such morbid poison, in ourselves or 

 other animals. I say the local consequences, for we have no 

 clear evidence of what might be called general infection or 

 constitutional disease in the gall-forming plants. In the 

 absence of quickly moving fluid, such as lymph or blood, the 

 virus infects only the part in and very near to which it is 

 inserted. A single oak-leaf may have fifty ' spangle ' galls 

 on its under-surface, but the structures between them may be 

 quite healthy ; and when in any instance a general damage 

 is done to a plant by gall-growths, it seems to be only as a 

 remote consequence of the spoiling of considerable portions 

 of its structures. And this appears to be true, even though 

 the virus may continue active for a long time, as in the galls 

 which begin to grow soon after the insertion of the virus 

 with the ovum, and continue to increase during the whole — 

 sometimes long — development of the larvae ; or even, in a few 

 instances, after the larvae have deserted them. 



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"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 the gall-insects, each form answering to 

 a difterent virus. This may suggest that we may be too 

 grudging in thinking of the number of morbid poisons, or of 

 their modifications in the blood, to which diseases in ourselves 

 may, at least in part, be due. 



"It is true that the galls are produced by many species of 

 insects on many species of plants ; and that the differences 

 among these species may be as wide as those between 

 ourselves and any other Mammalia. But, even among 

 closely allied species, there are many and very difierent 

 forms of galls. Mayr, ten years ago, described and figured 

 ninety- six kinds of galls found on the oaks of Central Europe, 

 all but two of them being produced by different species of 

 gall-wasps. Of those ninety-six kinds, thirty-two are formed 

 on the leaves alone ; and even on similar parts of one oak-leaf 

 it is not rare to find three or four different forms of galls. 



"\Ye have, thus, clear evidence of a very large number of 

 morbid poisons, each of Avhich is capable of producing, in an 

 appropriate subject, a distinct specific disease with a charac- 

 teristic morbid structure. 



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