HYMENOPTERA OAK GALLS. 135 



tare, apparently formed for the good of the insect, which, as remarked, 

 would be against the principle of the theory of evolution ; and in 

 view, therefore, of the extreme interest of this question, I have taken 

 the liberty of quoting the following on " Vegetable Galls " by Mr 

 G. J. Romanes in ' Darwin and after Darwin.' Mr Eomanes says 



" 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 insect's 

 inoculation ; or, according to recent observations, by emanations from 

 the body of the larvae which develop from the eggs deposited in the 

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

 able 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 justified 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 the first 

 instance, or if the needs of the insect progeny had not been such as 

 to have derived profit from being enclosed in such a tumour, then, of 

 course, the inoculating instinct of these animals could not have been 

 developed by natural selection. But given these two conditions, and 

 it appears to me there is nothing very much more remarkable about 

 an accidental correlation between the effects of a parasitic larva on a 

 plant and the needs of that parasite, than there is between the simi- 

 larly accidental correlation between a hydatid parasite and the nutri- 

 tion furnished to it by the tissues of a warm - blooded animal. 

 Doubtless the case of galls is somewhat more remarkable, inasmuch 

 as the morbid growth of the plant has more concern in the correlation 

 being, in many instances, a more specialised structure on the part 

 of a host than occurs anywhere else, either in the animal or vegetable 



