THE SCOPE AND METHODS OF GENETICS 297 



to heredity; (J) theories as to the mechanism that brings about the 

 observed regularity in heredity, including theories of linkage, cross- 

 overs, and other phases of neo-Mendelian heredity, . 



HEREDITY, ENVIRONMENT, AND TRAINING 



"Every individual," says D. F. Jones,' "is the resultant of the 

 action of three forces: inheritance, environment, and training. In- 

 heritance is what the organism receives at birth ; it can not ordinarily 

 he changed thereafter. Environment includes all the influences which 

 have their origin outside of the body, whether favorable or injurious. 

 Of these, food, temperature, light, and parasitism are the most obvious. 

 Training is what the organism does for itself by using its inheritance 

 and environment. Thus the speed of horses is developed by running. 

 Skill of any kind comes largely from practice." 



When we say that certain characters are inherited, such, for ex- 

 ample, as blue eyes or brown hair, we do not mean that the germ cell 

 of the parent has blue eyes or brown hair, for germ cells do not have 

 any of the distinguishing characters of the adult. All that the germ 

 cell possesses is some protoplasmic complex, probably wholly chemical 

 in character, that has the capacity of giving rise to an individual with 

 certain specific structural or functional peculiarities under certain 

 normal environmental conditions and with the requisite amount of 

 training. If we change the character of the environment or interfere 

 with the functioning or training of an embryo, we can no longer get 

 the so-called normal hereditary characters, but a different kind of indi- 

 vidual, some sort of "monster," will result. The "monster," however, 

 is the normal expression of the heredity under a changed environment. 

 The particular expression of the character we call normal is merely 

 that which results under the prevailing environmental conditions. 

 The expression of heredity differs according to the environment and 

 training, but the hereditary material itself remains practically un- 

 altered, as will be shown later. The situation, according to Jones, 

 "is like an exposure on a photographic film. The picture is there. 

 No developer can change its inherent character, but proper develop- 

 ment may make of it a beautiful picture while careless handling or the 

 use of wrong solutions may mar or ruin it. But the best developers 

 and the greatest skill can not make a good picture out uf a poor 

 exposure." 



Heredity is what we are inherently. Environment and training 



' Donald F. lones, Genetics in Plant and Animal Improvement (1925). 



