REPRODUCTION 85 



body, and which are all descendants from the 

 single original egg cell, falls naturally into two 

 classes ; first, somatic cells, such as those which 

 form the skin, muscles, bone, nerve, and other 

 parts of the adult body, and, secondly, the re- 

 productive cells, the egg cells in the female 

 and the sperm cells in the male, which are des- 

 tined to give rise to new individuals. Of these 

 two sets of cells the one which by its collective 

 arrangement and activity exhibits those as- 

 pects of the child wherein it shows its likeness 

 to its parents are the somatic cells, and the 

 discovery of the way in which these cells be- 

 come impressed with the parental traits is one 

 of the chief problems of genetics. 



Before 1900 scarcely anything of value in 

 genetics could be said to have been in the 

 hands of the working biologist, but with that 

 year the discovery of Mendel's writings, lost 

 for about a generation, gave to the study of 

 heredity such an impetus as it had never ex- 

 perienced before. The Mendelian principles, 

 to which this revival is due, are already popu- 

 larly known. You are aware that inheritance 

 in accordance with these principles is accom- 

 plished by the association of characteristics re- 

 lated in pairs and that these characteristics 



