Phenomena of Inheritance 99 



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, so that at present it is practically certain that 

 there is no other kind of inheritance than Mendelian. 



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, 

 characters are not units, and while the conception of "unit char- 

 acters" has served a useful purpose it cannot any longer be re- 

 garded as wholly accurate. 



Inheritance Factors are Differential Causes. Of course char- 

 acters of adult individuals do not exist as such in germ cells, but 

 there is no escape from the conclusion that in the case of inherited 

 differences between mature organisms there must have been dif- 

 ferences in the constitution of the germ cells from which they 

 developed. For every inherited character there must have been a 

 germinal cause in the fertilized egg. This germinal cause, what- 

 ever it may be, is often spoken of as a determiner of a character. 



