INFLUENCE OF ENVIRONMENT 309 



such as oxygen, carbonic acid, water, food, 

 secretions of ductless glands etc., and (b) sub- 

 stances not found in normal development, 

 such as various salts, acids, alkalis, alcohol, 

 ether, tobacco, etc. 



3. In general the action of these stimuli 

 during development does not call forth a per- 

 fectly specific and definite response of the or- 

 ganism ; various stimuli may produce the same 

 result. Thus artificial parthenogenesis has 

 been produced by almost every stimulus named, 

 and weakened or retarded development is pro- 

 duced by many different stimuli. 



By the elimination of certain of these 

 stimuli which are normally present or by in- 

 troducing stimuli which are not usually pres- 

 ent very important and even profound 

 changes in development may be produced. In 

 this way animals have been formed which are 

 turned inside out, or side for side, or in which 

 heads or nervous systems or muscles or back- 

 bones are lacking, or in which the various or- 

 gans are not found in normal positions. In 

 this way dwarfs and giants and one-eyed 

 monsters as well as all sorts of double and par- 



