ioo Heredity and Environment 



II. MODIFICATIONS AND EXTENSIONS OF MENDELIAN 

 PRINCIPLES 



It is a common experience that natural phenomena are found 

 to be more complex the more thoroughly they are investigated. 

 Nature is always greater than our theories, and with few excep- 

 tions hypotheses which were satisfactory at one stage of knowl- 

 edge have to be extended, modified or abandoned as knowledge 

 increases. This observation is well illustrated in the case of the 

 Mendelian theory. The principles proposed by Mendel were rela- 

 tively simple, but in attempting to apply them to the many phe- 

 nomena of inheritance now known it has become necessary to 

 modify or extend them in many ways. And yet the general and 

 fundamental truth of these principles has been established in a 

 surprisingly large number of cases, and they have been extended 

 to forms of inheritance where at first it was supposed that they 

 could not apply. 



I. The Principle of Unit Characters and of Inheritance Fac- 

 tors. There has been much criticism on the part of some biolo- 

 gists of the principle of unit characters. It is said that unit char- 

 acters cannot be independent and discrete things; the organism 

 itself is a unity and every one of its parts, every one of its char- 

 acters, must influence more or less every other part and every 

 other character. Certainly unit characters cannot be absolutely 

 independent of one another ; the various parts and organs of the 

 body, and even the organism as a whole, are not absolutely inde- 

 pendent, and yet there are varying degrees of independence in 

 organisms, organs, cells, parts of cells, hereditary units and char- 

 acters which make it possible for purposes of analysis to deal 

 with these things as if they were really independent though we 

 know they are not. But the most serious objection to the doctrine 

 of unit characters is not against their independence but against 

 their unity. Every character is complex, many factors enter into 

 its development, and since the combination of these factors is 

 variable the character itself cannot be constant. Strictly speaking. 



