1 1 8 THE SOCIAL LIFE OF ANIMALS 



presentation in detail except to highly specialized 

 readers. 



The environment is in a state of constant flux and 

 its progressive changes, whether slow or fast, make 

 the well-adapted types of the past generations into 

 misfits under present conditions. The result may be 

 rectified either by the extinction of the species, if it 

 is not sufficiently plastic, or through reorganization 

 of the hereditary types. In such a reorganization the 

 simple Lamarckian reactions apparently do not op- 

 erate; that is to say, when confronted with new, 

 critical conditions, species cannot go to work and 

 produce needed changes to order. The reactions are 

 much more complicated than that. 



To present the modern interpretation of this re- 

 organization I need three technical terms which I 

 shall define before using. Genes are bits of proto- 

 plasm too small to be seen through the microscope, 

 which are located in all cells and which are thought 

 to be the bearers of heredity. They behave as indi- 

 visible units, that is to say, a gene if present in an 

 organism is either transmitted as a whole or not at 

 all. Gene frequency is the term applied to the fre- 

 quency with which a given gene is found in a popu- 

 lation, relative to the total possible frequency (two 

 in every individual). By mutation is meant a large or 

 small hereditary change which appears suddenly, 



