4 GENETICS 



life: the ejects of the different materials received from 

 their parents. These effects are commonly classified as 

 heredity, so that Genetics deals largely with heredity. 

 Heredity itself may be defined as the influence on the indi- 

 viduals of the materials which they receive from their 

 parents at the beginning of their lives. 



But the structures and qualities and activities of men and 

 other organisms do not depend exclusively on the materials 

 of which they are composed. They depend also — like the 

 structures and qualities and activities of an automobile or 

 other machine — on the way the materials are treated, on 

 the conditions to which they are subjected. The character- 

 istics, the methods of action, can be changed by changing 

 the conditions under which the organism develops and lives. 

 All this is often grouped under the effects of the environ- 

 ment; but it includes, as different aspects, what in man are 

 called training, education, experience. 



In trying to understand men or other organisms, one 

 must always keep in mind these two great classes of influ- 

 ences: — heredity, the influence of the different materials 

 with which different individuals begin life; and environ- 

 ment, or the Influence of the way these materials are 

 treated, the conditions to which they are subjected. Many 

 of the differences between organisms are due to the former, 

 many to the latter. It Is the province of Genetics to dis- 

 entangle so far as possible the action and effects of these 

 two classes of influences. Strictly considered, the province 

 of Genetics is the former: the effects of the different ma- 

 terials of which different Individuals are composed. But the 

 two classes of influences are so closely Intertwined, their 

 results so fully integrated, that one cannot be dealt with 

 separately from the other. 



