THE THREE-LEVEL ANALYSIS 



no doubt be equally productive in the study of animals, especially 

 the important mammals and their most dangerous enemies and 

 parasites, and in the study of the lower plants. Secondly, the appli- 

 cation of genetics to man is begimiing to show results which are 

 as important for man as was expected, but more important for 

 genetics than could have been expected, owing to the great exten- 

 sion which man's special character gives to the visible spectrum of 

 variation. We are only begiiming to acquire confidence in the 

 application of genetics to pathology, psychology, and sociology. 

 As we do so, however, the new social science that will develop will 

 increase in power more than in proportion to its continually 

 increasing predictive capacity. 



The Three-Level Analysis 



Looking back at our account we can now see why it was that 

 so many false starts were made with genetics. Many very different 

 kinds of study have contributed to our conclusions. We have 

 observed structures at many different levels of organization, but we 

 have not been able to ascend in simple order from the gene to the 

 chromosome, from the chromosome to the nucleus and so on to 

 the cell, the organism, the family, and the population, the species 

 and the whole world of evolution. Or even to descend in opposite 

 order. The first two parts of our account were, to be sure, concerned 

 with the family and smaller units, and the third part with larger 

 units. Yet our beginning had to be made in the middle, with 

 individuals and families. It was Mendel who discovered that 

 heredity, a property of the family, had to be studied from the 

 relationships of individuals in families, and it was with Mendel 

 that we had to begin working. 



From the individual we have to proceed downwards largely with 

 the aid of the microscope to the cell, the nucleus, the chromosome, 

 the gene and eventually the protein molecule. At this lower level 

 heredity and variation remain our basic notions, but they acquire 

 new senses. For they begin to depend on the capacity of self- 

 propagation of nucleo-proteins, a capacity which we fmd exists both 

 inside and outside the nucleus. The differentiation between the 

 nucleus and the cytoplasm then appears as a differentiation in method 



Element!, uf Genetics T. (j^ ^^ 



