An Introduction to a Biology 



himself. If this is carefully done, no difficulty will be en- 

 countered in establishing the invalidity of Mendel's Law 

 as facts accumulate. 



To discover a definition of the second class is not so 

 easy. To my mind, there are two perfectly distinct things 

 included under the one term Mendelism. One is belief in 

 the existence of character-units in the germ, and in the 

 thesis that these units are pure in respect of the characters 

 which they represent. The other is the method by which 

 the extent, separateness, and transmission of these units 

 is discovered. The first may be called the Mendelian theory, 

 the second the Mendelian method which is the application 

 of the experimental method to the study of heredity. I 

 think that both these things are implied when the term 

 Mendelism is used ; and whether they are or not and it 

 does not in the least matter I believe that the Mendelian 

 method will do as great service, in accounting for the pheno- 

 mena of heredity, as that particular theory which Mendelians 

 happen to be employing at the moment. 



For I think that it must be evident to anyone who has 

 followed closely the Mendelian work of the last few years 

 that, while the method which workers of that school have 

 employed has remained the same, the actual theories, by 

 testing the validity of which they have sought to attain 

 their end, have been from time to time considerably modified. 

 This procedure, of course, makes it difficult for those who 

 wish to criticise or base statistical calculations on the theory 

 itself. Pearson comments on it in these words : " The 

 original Mendelian theory has been replaced by what are 

 termed ' Mendelian Principles.' In this aspect of investi- 

 gation the fundamental principles propounded by Mendel 

 are given up, and for each individual case a pure gamete 

 formula of one kind or another is suggested as describing 

 the facts. This formula is then emphasised, modified, or 

 discarded, according as it fits well, badly, or not at all with 

 the growing mass of experimental data. It is quite clear 

 M 177 



