448 Darwin, and after Darwin. 



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 

 plants, so that they might suffer a minimum of harm from the unavoid- 

 able attacks of insects. 



But here it may be questioned have we proof that internal feeders 

 tend to form galls ? 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 l . 



The paper then proceeds to give a number of individual cases. 

 No doubt the principal objection to which Mr. Cockerell's 

 hypothesis is open is one that was pointed out by Herr Wet- 

 terhan, 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 2 ." This difficulty, however, Mr. Cockerell 

 believes may be surmounted by the consideration that a growing 

 plant need not be regarded as a single individual, but rather 

 as an assemblage of such ''. 



NOTE C TO PAGE 394. 



The only remarks that Mr. Wallace has to offer on the 

 pattern of colours, as distinguished from a mere brilliancy of 

 colour, are added as an afterthought suggested to him by the 

 late Mr. Alfred Tylor's book on Colouration of Animals and 

 Plants (1886). But, in the first place, it appears to me that 

 Mr. Wallace has formed an altogether extravagant estimate of the 

 value of this work. For the object of the work is to show, 

 "that diversified colouration follows the chief lines of structure, 

 and changes at points, such as the joints, where function 

 changes." Now, in publishing this generalization, Mr. Tylor 

 who was not a naturalist took only a very limited view of the 



1 Entomologist, March, 1890. 2 Nature, vol. xli, p. 394. 



3 Ibid. vol. xli, pp. 559-560. 



