250 



ESSENTIALS OF ZOOLOGY 



in Mendel's peas, there are many more cases in which the Mendelian 

 principle is manifested as multiple allelomorphs, plural genes, modi- 

 fying factors, complementary factors, incomplete dominance, change- 

 able dominance, etc. There is a tendency among some to depreciate 

 our knowledge of human heredity on the ground that there is so much 

 that we do not know. It is scientific to admit the extent of our lack 

 of knowledge, but it is wise to give proper credit to our present store 

 of information and to take cognizance of the rapidity with which the 

 gaps in our knowledge are being filled in. One by one the complicated 

 problems of human heredity are being solved by patient investigators, 

 and by putting what we already know to use, we stimulate the acces- 

 sion of more data. 



Some Cases of Human Heredity. — To give a complete summary of 

 our knowledge of human heredity would be beyond the scope of this 

 chapter. The accompanying table gives some of the data that have 

 been accumulated: 



Data on Human Heredity 



