154 HEREDITY AND ENVIRONMENT 



ble though still demonstrable. These differ- 

 ences may not be detectable by chemical or 

 physical tests, and yet they may be demon- 

 strated physiologically and developmentally. 

 The most delicate of all tests are physiological, 

 as is shown by the Weidal test in typhoid 

 fever, the Wassermann reaction in syphilis, 

 the reactions of irmnxmized animals to different 

 toxins, etc. Lillie has recently shown that 

 egg cells give off a substance which he calls 

 fertilizin, which can be detected only by the 

 way in which spermatozoa react to it. No 

 chemical or physical test can distinguish be- 

 tween the different eggs or spermatozoa pro- 

 duced by the same individual, but the reactions 

 of these cells in development prove that they 

 are different. Undoubtedly chemical and 

 physical differences are here present but no 

 chemical methods at present available are 

 sufficiently delicate to detect them. The de- 

 velopmental test proves that there must be as 

 many kinds of germs as there are different 

 kinds of individuals which come from germs. 

 It is one of the marvellous facts of biology 

 that every individual which has been produced 



