|PHENOMENA OF INHERITANCE|
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 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.
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 various parts and organs of the body and even the organism, as a whole, is not absolutely independent, and yet there are varying degrees of independence in organisms, organs, cells, parts of cells, hereditary units and characters 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.
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 developed. For every inherited character there must have been a germinal 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