PHENOMENA OF INHERITANCE 425 



PHENOMENA OF INHEEITANCE 



By Professor EDWIN GRANT CONKLIN 



PRINCETON UNIVERSITY 



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 exceptions hypotheses 

 which were satisfactory at one stage of knowledge 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 relatively simple, but in attempting to apply 

 them to the many phenomena of inheritance now known it has become 

 necessary to modify or extend them in many ways. And yet the gen- 

 eral 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. 



1. The Principle of Unit Characters and Inheritance Factors. — 

 There has been much criticism on the part of some biologists of the 

 principle of unit characters. It is said that unit characters can not be 

 independent and discrete things ; the organism itself is a unity and 

 every one of its parts, every one of its characters, must influence more 

 or less every other part and every other character. Certainly unit 

 characters can not be absolutely independent of one another; the vari- 

 ous parts and organs of the body and even the organism, as a whole, is 

 not absolutely independent, and yet there are varying degrees of in- 

 dependence in organisms, organs, cells, parts of cells, hereditary units 

 and characters which make it possible for purposes of analysis to deal 

 with tbese things as if they were really independent, though we know 

 they are not. 



Of course characters of adult individuals do not exist as such in 

 germ cells, but there is no escape from the conclusion that in the case 

 of inherent differences between mature organisms there must have been 

 differences in the constitution of the germ cells from which they de- 

 veloped. For every inherited character there must have been a germi- 

 nal cause in the fertilized egg. This germinal cause, whatever it may 

 be, is often spoken of as a determiner of a character. But the character 

 in question is not to be thought of as the result of a single cause nor 



