STRUCTURE OF THE GERM PLASM 13 



to exist in at least a dozen different modifications, in different 

 individuals. One of the modifications produces red eyes, 

 another white eyes, another buff eyes, and others produce 

 various shades of red, or mixtures of red and yellow^, or 

 slightly tinged, or ivory eyes. Similar modifications are 

 known for many other genes, each producing a slightly dif- 

 ferent effect; they probably occur in all the genes.* In this 

 way, by diverse modifications of the same gene, a great num- 

 ber of different types of characteristics are produced ; different 

 eye colors, different forms, different sizes, different degrees 

 of strength and vigor, different physiological activities. Every 

 structure and function of the organism, without exception, is 

 thus capable of being produced in different grades and 

 qualities, through diversities among the genes. 



If we look at any species of animal or plant as a unit, includ- 

 ing all the different individuals that compose it, we find then 

 the following situation as to genes: First, there is a great 

 number of different types of genes, strung up in a long 

 series. Each type has its special function, and each has its 

 definite place in the series; it is No. i or No. 40 or No. 200, or 

 the like. Any individual has two genes of each of the types. 

 But, second, each type has a number of different modifica- 

 tions, each modification performing the given function in a 

 somewhat different way. If the gene type is one that affects 

 the color of the eye, then the various modifications of that 

 type give in different individuals a whole series of different 

 eye colors. Since each individual has just two genes of a cer- 

 tain type, a single individual can contain only two of the 

 modifications of any type; other individuals may contain 

 other modifications. 



Thus what characteristics are shown by any particular indi- 

 vidual depends on what combinations of the various types 

 and modifications of the genes are present in his cells. And 



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