134 FOEEST ENTOMOLOGY. 



of the prolonged egg-stalk. Adler discovered that in two species gall- 

 formation commenced just as the larva was ahout to escape from the 

 egg. It is, of course, difficult in fact, in most cases impossible to 

 see the egg within the gall, but the same observer found that as the 

 larva grew and fed, so the gall increased in size. In other words, no 

 gall-formation took place until just immediately before hatching, but 

 the growth of the gall proceeded simultaneously with the development 

 of the larva. Hence it may be inferred that oak-gall formation is the 

 result of the excitatory action of the larva acting in conjunction with 

 the vitality of the vegetable cell. It must be borne in mind that this 

 special method of gall- formation is only applicable to oak galls, as galls 

 formed by some species of Hymenopterous insects, as, for example, 

 in the case of the common bean-like gall Nematus gallicola formed 

 on both sides of the willow leaves, is more or less of an amorphous 

 structure, and practically fully developed before any embryological 

 changes take place within the egg. 



In consideration of the variety of gall-structure, and the comparative 

 sameness of larval anatomy, the question may be asked, Is there a 

 mechanical difference in the direct action of the larva or a chemical 

 difference in the composition of the excitatory emanations'? In other 

 words, as the larvae are exactly similar in structure and action, what 

 are the factors which produce variations of structure, size, and colour 1 

 Thus in point of anatomical structure the larva? of the Cynipidae are 

 practically identical, and, as previously remarked, the galls vary in 

 size, colour, and complex structure. 



The question of gall-formation, as considered in relation to the 

 theory of evolution, is somewhat difficult to understand, but, at the 

 same time, is extremely interesting to the philosophical scientist. 

 Darwin somewhere in his writings remarks that if it could be proved 

 that any one creature worked for the good of another creature, his 

 theory would fall to the ground. Of course, this remark is only illus- 

 trative of the general principle of natural selection and the " survival 

 of the fittest," and must not be confounded with the principle of 

 altruism, as, for example, the care of the parent for the offspring. 

 Now, in oak-gall formation, and all its complexity of morphological 

 structure, we cannot believe that the oak would form these galls in a 

 disinterested way for the good of the insect, nor, on the other hand, 

 as being of any advantage to the oak itself, considered as a vegetable 

 organism. The oak gall is, therefore, a distinctly differentiated struc- 



