298 FOUNDATIONS OF BIOLOGY 



Although we are wofully ignorant of the cause of variations, 

 the difficulty is more apparent than real and arises from our 

 absolute ignorance of what genes really are. We may 

 conceive them to be chemical molecules, and if so they can 

 change only by an alteration of their chemical constitution. 

 And for all we know, this may occur. Or, without any 

 change in the genes themselves, their expression the 



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HERITAGE 



FIG. 152. Scheme to illustrate the contributions of nature and nurture to the make- 

 up of the individual. The triangles represent various types of individuals which may 

 be produced by the same germ cells (heritage) if environment and training are variable. 

 The foundation of the "triangle of life" is heritage. (After Conklin.) 



chemical effects which they produce may change by the 

 alteration of other substances with which they react. If we 

 interpret such phenomena as recombinations, they are 

 profoundly more subtle and far-reaching than are called to 

 mind by our simile of a kaleidoscope. They may be essen- 

 tially infinite in number and infinite in potentialities for varia- 

 tions in the germ plasm and therefore for heritable variations 

 expressed in the soma. Again it is possible, perhaps probable, 

 that inheritable variations are often the result of chromosomes 

 'accidentally' losing or gaining one or more genes during 



