HYMENOPTERA OAK GALLS. 137 



then, were miners in the roots, stems, twigs, or leaves, such as occur 

 very commonly at the present day. These miners are excessively 

 harmful to plant-life, and form a class of the most destructive insect- 

 pests known to the farmer : they frequently cause the death of the 

 whole or part of the plant attacked. Now, we may suppose that 

 the secretions of certain of these insects caused a swelling to appear 

 where the larvae lived, and on this excrescence the larvae fed. It is 

 easy to see that the greater the excrescence, and the greater the 

 tendency of the larvae to feed upon it, instead of destroying the 

 vital tissues, the smaller is the amount of harm to the plant. Now, 

 the continued life and vitality of the plant is beneficial to the larvae, 

 and the larger and more perfect the gall, the greater the amount of 

 available food. Hence natural selection will have preserved and 

 accumulated the gall-forming tendencies, as not only beneficial to the 

 larvae, but as a means whereby the larvae can feed with least harm to 

 the plant. So far from being developed for the exclusive benefit of 

 the larvae, it is easy to see that, allowing a tendency to gall-formation, 

 natural selection would have developed galls exclusively for the 

 benefit of the plant, so that they might suffer a minimum of harm 

 from the unavoidable attacks of insects. 



"But here it may be questioned, Have we proof that internal feeders 

 tend to form galls 1 In answer to this I would point out that gall- 

 formation is a peculiar feature, and cannot be expected to arise in 

 every group of internal feeders. But I think we can afford sufficient 

 proof that wherever it has arisen it has been preserved ; and further, 

 that even the highly complex forms of galls are evolved from forms 

 so simple that we hesitate to call them galls at all (' Entomologist,' 

 March 1890). The paper then proceeds to give a number of in- 

 dividual cases. No doubt the principal objection to which Mr 

 Cockerell's hypothesis is open is one that was pointed out by Herr 

 Wetterhan viz., ' the much greater facility afforded to the indirect 

 action through insects, by the enormously more rapid succession of 

 generations with the latter than with many of their vegetable hosts 

 oaks above all' ('Nature,' vol. xli. p. 394). This difficulty, 

 however, Mr Cockerell believes may be surmounted by the con- 

 sideration that a growing plant need not be regarded as a single 

 individual, but rather as an assemblage of such" (vol. xli. pp. 

 559, 560). 



With regard to the insects producing oak galls, the transformations 



