296 FOUNDATIONS OF BIOLOGY 



distance between two genes is proportional to the percentage 

 of crossing-over which these genes show. (Fig. 151.) 



F. NATURE versus NURTURE 



From one viewpoint, then, the individual may be considered 

 as a composite of very many unit characters which behave in a 

 definite way in inheritance. " Expressed otherwise, and 



i n a jib JLC 



FIG. 151. Diagram to show a possible mechanism of crossing-over during 

 synapsis of homologous paternal and maternal chromosomes. The segments 

 indicate the assumed linear arrangement of the genes with allelomorphic genes 

 opposite each other. I, pair of chromosomes which have entered and emerged 

 from the synaptic state without any crossing-over; I la, chromosomes winding 

 about each other at synapsis; 116, separation of these chromosomes, involving 

 breaking at the points of crossing; lie, their emergence from synapsis with the 

 members of the pairs of allelomorphic genes interchanged. (After Wilson.) 



somewhat fancifully, individuals are simply temporary 

 kaleidoscopic combinations of the various determiners (genes) 

 belonging to the species; the act of reproduction, especially 

 the reduction division and subsequent fusion, providing the 

 new turn of the kaleidoscope." But since the life of an 

 organism is one continuous series of reactions with its sur- 

 roundings, it follows that nurture plays an immensely im- 

 portant part in molding the individual on the basis of its 

 heritage. This is especially true in the case of Man. Devel- 

 opment is a form of behavior, and how a child develops 



