470 HEREDITY AND ENVIRONMENT 



exercise a profound influence upon organisms, 

 especially in the course of their development. 

 Very slight changes in food, temperature, 

 moisture and atmospheric conditions may pro- 

 duce great changes in the developing organ- 

 ism, and these conditions are for the most 

 part entirely beyond the control of the indi- 

 vidual affected. 



In all organisms the potentialities of de- 

 velopment are much greater than the actuali- 

 ties. In many animals a small part of the 

 body is capable, when separated from the re- 

 mainder, of producing a whole body, though 

 this potenc)^ would never have become an ac- 

 tuality except under the stimulus of separa- 

 tion. In like manner a part of an egg may, 

 when separated from the remainder, give rise 

 to an entire animal. By modifying the con- 

 ditions of development animals may be pro- 

 duced which have one eye, many eyes or no 

 eyes; animals in which the body is turned in- 

 side out, or side for side; animals in which all 

 sorts of dislocation of organs have taken place ; 

 and the earlier the environmental forces act the 

 more profound are the modifications. 



