5Gr) THE PROBLEMS OP HEREDITY AND THEIR SOLUTION. 



The details of these experiments and of others like them mad(> with 

 three pairs of dilferentiating characters are all set out in Mendel's 

 memoir. 



Professor de Vries has worked at the same problem in some dozen 

 species belonging to several genera, using pairs of varieties charac- 

 terized by a great num])er of chai'acters: for instance, color of Howers, 

 stems, or fruits, hairiness, length of style, and so forth. He states 

 that in all these cases Mend(d's principles are followed. 



The iuiml)ers with which Mendel worked, though large, were not 

 large enough to give really smooth results;" but with a few rather 

 marked exceptions the observations are remarkably consistent, and 

 the approximation to the numbers demanded by the law is greatest in 

 those cases where the largest numbers were used. When we consider, 

 l^esides, that Tschermak and Correns announce detinite contirmation 

 in the case of PiHum^ and de Vries adds the evidence of his long series 

 of observations on other species and orders, there can be no doubt that 

 Mendel's law is a substantial reality; though whether some of the 

 cases that depart most widely from it can be brought within the terms of 

 the same principle or not, can only be decided by further experiments. 



One may naturally ask, How can these results be brought into hai'- 

 mony with the facts of hybridization hitherto known; and if all this is 

 true, how is it that others who have carefully studied the phenomena 

 of hybridization have not long ago perceived this law? The answer 

 to this question is given by Mendel at some length, and it is, I think, 

 satisfactor3\ He admits from the first that there are undoul)tedly 

 cases of hybrids and cross-breds which maintain themselves pure and 

 do not break up. Such examples ai'e plainly outside the scope of his 

 law. Next he points out, what to anyone who has rightly compre- 

 hended the nature of discontinuity in variation is well known, that the 

 variations in each character must be separately regarded. In most 

 experiments in crossing, forms are taken which diiier from each other 

 in a multitude of cli:iracters — some continuous, others discontinuous, 

 some capaljle of blending with their contraries, while others are not. 

 The observer on attempting to perceive any regularity is confused by 

 the complications thus introduced. INIenders law, as he fairly says, 

 could only appear in such cases by tli(^ use of overwhelming num])ers, 

 which arc beyond the possibilities of practical experiment. Lastly, no 

 previous observer had applied a strict statistical jnethod. 



Both these answers should be acceptable to those Avho have studied 

 the facts of variation and have appreciated the naturt^ of species in the 

 light of those facts. That dili'erent species should follow different 



" ProfeHsoi- Woklou (p. 232) takes great exception to this statement, ■\vhicli he 

 considerately attributes to "some writers." After examining the conchisions he 

 ol)taine(1 l)y algebraical study <if ^hMidel's ligurcs I am disixised tn tliiid< my state- 

 ment not very far ont. 



