2 70 The Ohio Naturalist. [Vol. II, No. 7, 



in its development before the separation of the two zones and the 

 nourishment remaining in the inner zone at the time of the separ- 

 ation is sufficient to complete its development. 



Adler and Stratton after describing similar modifications in the 

 European Cynipidae galls, say: "Besides these histological 

 differences, the outward characters are also of varying complex- 

 ity ; each infinitesimal improvement, which has been of service 

 as a protection against parasites, or has been successful in secur- 

 ing natural conditions favorable to the life and growth of the 

 larva, has been preserved, and has formed the starting point of 

 further beneficial variation. It is always that larva which has 

 been able to induce successful morphological abnormalition, which 

 is reproduced 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 

 Cynips ever sees its young, and none ever pricks a bud the sec- 

 ond 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 

 simultaneus 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 is instinctivel}^ proportionate to the space suitable for ovipo- 

 sition, to the size of the full}' grown galls, and to the food sup- 

 plies available for their nutrition." 



CONCLUSIONS. 



1. Galls may be classified into two general groups, viz., those 

 produced by mouth parts and those produced bj' oviposition. 

 Those produced by oviposition may be considered the more highly 

 developed. 



2. The family Cynipidae shows by far the highest develop- 

 ment of gall structures. 



3. The morphological character of the gall depends upon the 

 genus of the insect producing it rather than upon the plant on 

 which it is produced ; i. e. , galls produced by insects of a partic- 

 ular genus show great similarity of structure even though on 

 plants widely separated ; while galls on a particular genus of 

 plants and produced by insects of different genera show great 

 differences. This seems to indicate that the stimulus of a partic- 

 ular genus of insect is given to a particular part of the host plant 

 or is of a peculiar kind, characttristic of that genus. However, 

 if the stimulus of two different genera of insects be applied to the 

 same part of the plant the results may be similar. (See Part II. ) 



4. Within each family we find certain morphological resem- 

 blances ; e.£-., Aphididae. 



* Weismann, Essays on Heredity, Vol. I, p. 95. 



