Influence of Environment 217 



they are plainly extrinsic or environmental, but if they arise 

 within the organism they are said to be intrinsic though they 

 may be due to changes in the "internal environment." 



Stimuli are chiefly energy changes of a physical or chemical 

 nature. A stimulus to which an adult organism responds by 

 movements or other activities may call forth or inhibit develop- 

 mental responses when applied to germ cells or embryos. 



These developmental stimuli may be classed as : 



1. Physical stimuli including the following, (a) mechanical, 

 (b) thermal, (c) electrical, (d) radiant, (e) light, (f) density 

 of medium, (g) gravity and centrifugal force, etc. 



2. Chemical stimuli include the action of (a) substances found 

 in normal development, such as oxygen, carbonic acid, water, 

 food, secretions of ductless glands, etc. and (b) substances not 

 found in normal development, such as various salts, acids, alkalis, 

 alcohol, ether, tobacco, etc. 



3. General vs. Specific Stimuli. In general the action of these 

 stimuli during development does not call forth a perfectly specific 

 and definite response of the organism; various stimuli may pro- 

 duce the same result. Thus artificial parthenogenesis has been 

 produced by almost every stimulus named, and weakened or re- 

 tarded development is produced by many different stimuli. 



By the elimination of certain of these stimuli which are normally 

 present or by introducing stimuli which are not usually present 

 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 backbones are lacking, or in which the 

 various organs are not found in normal positions. In this way 

 dwarfs and giants and one-eyed monsters as well as all sorts 

 of double and partial embryos have been formed. In general 

 monstrous and defective forms of development are due to altera- 

 tions of the normal environment rather than to defective heredity. 



