THE MECHANISM OF HEREDITY 75 



of only a limited amount of development could bear as 

 large an ear as if it were as a whole capable of greater size 

 development. Thus it must not be expected that theoreti- 

 cal possibilities are always expressed perfectly in nature, 

 any more than it should be expected that theoretical 

 pnysicai calculations concerning known laws should agree 

 perfectly with experimental data. Plants and animals do 

 indeed seem to have in their reproductive cells a mosaic 

 of independently transmissible factors, but a plant or 

 animal is certainly not to be described as a mosaic of in- 

 dependent unit characters. These factors that appear to 

 be independent in heredity act and react upon one another 

 in complex ways during their development. 



Hundreds of studies on quantitative characters have 

 been made. They all have the same result. Mendelian 

 inheritance rules. Plants appear to be less complex than 

 animals. A size complex in an animal seems to be the 

 result of the interaction of a large number of factors, a 

 size complex in a plant appears to be the result of the 

 interaction of a small number of factors. But the mode 

 of inheritance is always the same. It is the result of the 

 behavior of the chromosomes, and one can picture it with 

 the greatest ease with the simple diagram of the reduc- 

 tion division at gametogenesis if he fancies to himself that 

 the chromosomes are carrying bodies for the unit factors 

 of heredity, that the arrangements within them are a near 

 approach to perfection, that exchanges of contents may 

 be made only with regularity and precision so no essen- 

 tial feature of the mechanism shall break down. 



In thus visualizing the process of heredity, one must 

 not be so overcome by the beauty of the picture that he is 

 unable to realize just what has been done. He must not 



