110 The Galls of Essex ; a Contribution to a 



"Very rarely, the same insect may produce on one leaf 

 different forms of galls. 



"In all these points you may, I think, find liel^^ in the study 

 of specific diseases. I will add only one more. Usually, the 

 gall begins to grow directly after the deposit of the egg ; but 

 sometimes there is a long delay, a long period of suspense, an 

 " Eirulie," which may last for many months before the growth 

 begins. What is going on during this time ? I believe we 

 may see here an instance of events very difficult to study in 

 our own pathology, in which two or more conditions must 

 concur to the production of some disease, and one of them 

 must wait for the complete efficiency of the rest. In the case 

 of these long-delayed galls, either the Qgg, after being laid, 

 requires a long time for the completion of changes ending in 

 the production of the necessary morbid poison, or the plant- 

 structure in which it is laid requires the time for changes to 

 make it susceptible of the poison ; or both Qgg and plant may 

 need to change. So, in us, two or more conditions must 

 concm*. A tendency to gout may be inherited, and the blood 

 may have slowdy acquired the necessary morbid condition ; 

 but no structure may be susceptible of gouty disease tiU a 

 blow, or a strain, or some disturbance of nervous force makes 

 it so. So with cancer; a general tendency may be inherited, 

 but it must wait till the material of some structure is, by age, 

 or injury, or long-continued 'h'ritation,' changed into fitness 

 for concurrence in morbid action with the material on which 

 the general tendency depends. * * '■' '•' In the growth 

 of these galls, the comparison may seem less far-fetched. At 

 least, it may be difficult to suggest any nearer comparison 

 for a process in which the meeting of two living materials 

 fi'om different organisms is immediately followed by such a 

 change in the method of life of one of them, as ends in the 

 production of a definite new growth exactly adapted to the 

 method and purpose of the life of the other. 



"But it is more than time that I should have done with 

 galls. If I have been tedious, let me assure you that I am 

 myself ashamed to have gathered so little from the rapidly 

 increasing records concerning them to which the botanists, 

 and still more the entomologists, of our time are contributing. 

 And, even for that little, I feel as if I deserved to be 

 compared with one of those burglars of whom I spoke as 

 feeding on the results of other's labours. Let it be my 

 apology, that I believe I have taken nothing that those others 

 would have used. I have only taken from their rich stores of 

 lacts some that may be much more useful in pathology than 



