xxxiv Introduction. 



growth of the larva, has been preserved, and has 

 formed the starting-point of further beneficial variations. 

 It is always that larva which has been able to induce 

 successful morphological abnormalities, which is re- 

 produced to continue the race ; the unsuccessful 

 perish. The ruling force is natural selection ; it is 

 impossible that intelligence or memory can be of any 

 use in guiding the Cynipidae ; no C3mips ever sees 

 its young, and none ever pricks buds a second 

 season, or lives to know the results that follow 

 the act. Natural selection alone has preserved an 

 impulse which is released by seasonally recurring 

 feelings, sights, or smells \ and by the simultaneous 

 ripening of the eggs within the fly. These set the 

 whole physiological apparatus in motion, and secure 

 the insertion of eggs at the right time, and in the 

 right place. The number of eggs placed is instinctively 

 proportionate to the space suitable for oviposition, to 

 the size of the fully grown galls, and to the food sup- 

 plies available for their nutrition. Dryophanta scutellaris 

 will only place from one to six eggs on a leaf which 

 Neuroteriis lenticularis would probably prick a hundred 

 times. The veins and petiole of the leaf carry onwards 

 water and salts derived from the soil, and return the 

 organic products of the leaf-cells ; and these food 

 currents render them desirable situations for gall- 

 growth. The under surface of the leaf is folded out- 

 wards in the bud (vernation conduplicate), so that 

 it is the first part reached, when buds are pricked. 

 When expanded leaves are pricked, the spongy meso- 

 phyll of the under surface is much more easily 

 penetrated than the upper surface, which is covered 

 with the cuticle of an epidermis, that rests on closely 



^ Weismann, Essays on Heredity^ vol. i p. 95. 



