160 HEREDITY AND ENVIRONMENT 



pig and another to a white one, though both 

 may be produced by the same parents? Why 

 does one child differ from another in the same 

 family? Why does one cell give rise to a gland 

 and another to a nerve, one to an egg and an- 

 other to a sperm? If these differences are not 

 due to environmental causes, and the evidence 

 shows that they are not, they must be due to 

 differences in the structures and functions of 

 the cells concerned. 



Many differences in the material substances 

 of cells are visible, and many more are invisi- 

 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 immunized 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- 



