Notes oK mendels methods. 



13 



Some cliaracters showed dominance of one parent, some of the other 

 parent, while others were intermediate. 



When these several variations occurred in twenty different characters, the 

 possibilities of variations among the individual hybrids became very consid- 

 erable, so much so that the results became quite unmanageable. Since that 

 time the writer has been compelled to consider each single character on its own 

 merits. It is true that, in some cases, the correlation of characters tends to 

 modify this result to some extent, but in the case of the orchid hybrids in 

 question the correlation was not very evident. From this experience it follows 

 that in any statistics of inheritance a definite result can only be determined by 

 taking each single character separately as a distinct unit, completely ignoring, 

 for the time being, the individual plant made up of many characters. 



jMendel apparently was the iirst to see this clearlv. and acted upon it in his 

 experiments with Pisum, with remarkable results. 



(2) Constant Cliaracters. 

 Next comes the important question of ancestry. From the earliest times 

 it has been observed that in many instances offspring have resembled their 

 grandparents or their more remote ancestors, rather than their actual parents. 

 So that in experimental crossing, if two parents be chosen, each of whose 

 ancestry is unknown or perhaps consists of complicated factors, the resulting 

 offspring are either incomparable and incomprehensible, or they vary among 

 themselves in bewildering confusion. The result, in any case, is chaos, and 

 goes a long way to account for the many contradictory records which we find 

 in the experiments carried out in the old style. Mendel, in his experiments, 

 carefully and skilfully avoids this confusion by crossing together only constant 

 and fixed races, i. e., each parent has been the product of repeated self-fertili- 

 zation, so that its ancestry has been practically the same for many generations. 

 This effectually eliminates all the possible complications which might be 

 caused by the influence of the immediate ancestors at any rate, though how far 

 it affects the possible reversion to more remote ancestors is difficult to say. 

 The writer, in his experiments with orchids, has chosen distinct species only as 

 parents, and in this way, perhaps, reduces the possibilities of reversion still 

 more. De Vries, Correns, Tschermak and Bateson have all for the most part 

 followed or carried out Mendel's method by crossing constant races, jind it is 

 quite possible that some of their apparent exceptions to Mendel's results may 

 have been due to their crossing particular races which were not really so fixe*! 

 and constant as they believed them to be. 



As we have seen, Mendel carefully avoided this by selecting in the lirst 

 instance fixed parents of pure descent ; these he further tested for two years, 

 and satisfied himself as to their perfect constancy and fixity, and side by 

 side with his crossing experiments he was careful to carry out "control" ex- 

 periments with these original parents by still further testing their constancy 

 and fixity through all the generations. 



It is just possible that these precautions of Mendel may explain the 

 general uniformity of his results as compared with those of his disciples and 

 some of his critics. (Ref. 4.) 



(4) cf. Weldon in Biometrika, igo2, i., pp. 228—254. (For complete history, exposition 

 and Bibliography of tlae Mendelian question, see Bateson's admirable handbook on 

 "Mendel's Principles of Heredity, ' Cambridge University Press, 1902.) 



