132 Heredity and Environment 



6. Heredity and Development. The germ cells are individual 

 organisms and after the fertilization of the egg the new individual 

 thus formed remains distinct from every other one. Further- 

 more, from its earliest to its latest stage of development it is one 

 and the same organism ; the egg is not one being and the embryo 

 another and the adult a third, but the egg of a human being is a 

 human being in the one-celled stage of development, and the char- 

 acteristics of the adult develop out of the egg and are not in some 

 mysterious way grafted upon it or transmitted to it. 



Parents do not transmit their characters to their offspring, but 

 their germ cells in the course of long development give rise to 

 adult characters similar to those of the parents. The thing which 

 persists more or less completely from generation to generation 

 is the organization of the germ cells which differentiate in similar 

 ways in successive generations if the extrinsic factors of develop- 

 ment remain similar. 



Definitions. In short, heredity may be defined as the particu- 

 lar germinal organization which is transmitted from parents to 

 offspring. Heritage is the sum of all those qualities which are 

 determined or caused by this germinal organisation. Develop- 

 ment is progressive and coordinated differentiation of this ger- 

 minal organisation, by which it is transformed into the adult 

 organisation. Differentiation is the formation and localisation of 

 many different kinds of substances out of the germinal substance, 

 of many different structures and functions out of the relatively 

 simple structures and functions of the oosperm. 



This germinal organization influences not merely adult charac- 

 ters but also the characters of every stage from the egg to the adult 

 condition. For every inherited character, whether embryonic or 

 adult, there is some germinal basis. We receive from our parents 

 germ cells of a particular kind and constitution, and under given 

 conditions of environment these cells undergo regular transforma- 

 tions and differentiations in the course of development, which 

 differentiations lead to particular adult characteristics. In the 



