Evidences of Theory of Natural Selection. 293 



On the whole, it seems most probable that the fluid 

 is of the nature of an excretion, serving to carry off 

 waste products. Such, at all events, was the opinion 

 at which Darwin himself arrived, as a result of ob- 

 serving the facts anew, and in relation to his theory. 



The other instance to which I have alluded as 

 seeming at first sight likely to answer Darwin's 

 challenge is the formation of vegetable galls. The 

 great number and variety of galls agree in presenting 

 a more or less elaborate structure, which is not only 

 foreign to any of the uses of plant-life, but singularly 

 and specially adapted to those of the insect-life which 

 they shelter. Yet they are produced by a growth of 

 the plant itself, when suitably stimulated by the 

 insects' inoculation — or, according to recent observa- 

 tions, by emanations from the bodies of the larvae 

 which develop from the eggs deposited in the plant 

 by the insect. Now, without question, this is a most 

 remarkable fact ; and if there were many more of the 

 like kind to be met with in organic nature, we might 

 seriously consider whether the formation of galls should 

 not be held to make against the ubiquitous agency of 

 natural selection. But inasmuch as the formation of 

 galls stands out as an exception to the otherwise 

 universal rule of every species for itself, and for itself 

 alone, we are justified in regarding this one apparent 

 exception with extreme suspicion. Indeed, I think 

 we are ju.stified in regarding the peculiar pathological 

 effect produced in the plant by the secretions of the 

 insect as having been in the first instance accidentally 

 beneficial to the insects. Thus, if any other effect 

 than that of a growing tumour had been produced in 



