PROBLEMS OF HEREDITY, HORTICULTURAL IN^'ESTIGATIO^^ 57' 
These large classes of exceptions — to go no further —indicate that, 
as we might in any case expect, the principle is not of universal applica- 
tion, and will need various modifications if it is to be extended to more 
complex cases of inheritance of varietal characters. No more useful 
work can be imagined than a systematic determination of the precise 
" law of heredity " in numbers of particular cases. 
Until lately the work which Galton accomplished stood almost alone 
in this field, but quite recently remarkable additions to our knowledge of 
these questions have been made. In the present year Professor de Vries 
published a brief account* of experiments which he has for several years 
been carrying on, giving results of the highest value. 
The description is very short, and there are several points as to which 
more precise information is necessary both as to details of procedure 
and as to statement of results-t- Nevertheless it is impossible to doubt 
that the work as a whole constitutes a marked step forward, and the full 
publication which is promised will be awaited with great interest. 
The work relates to the course of heredity in cases where definite 
varieties differing from each other in some one definite character are 
crossed together. The cases are all examples of discontinuous variation : 
that is to say, cases in which actual intermediates between the parent 
forms are not usually produced on crossing. It is shown that the sub- 
sequent posterity obtained by self-fertilising these cross-breds or hybrids 
break up into the original parent forms according to fixed numerical rule. 
Professor de Vries begins by reference to a remarkable memoir by 
Gregor Mendel,t giving the results of his experiments in crossing varieties 
of Pisum sativum. These experiments of Mendel's v/ere carried out 
on a large scale, his account of them is excellent and complete, and 
the principles which he was able to deduce from them will certainly play 
a conspicuous part in all future discussions of evolutionary problems. 
It is not a little remarkable that Mendel's work should have escaped 
notice, and been so long forgotten. 
For the purposes of his experiments Mendel selected seven pairs of 
characters as follows 
1. Shape of ripe seed, whether round, or angular and wrinkled. 
2. Colour of " endosperm " (cotyledons), whether some shade of yellow,, 
or a more or less intense green. 
3. Colour of the seed-skin, whether various shades of grey and grey- 
brown, or white. 
4. Shape of seed-pod, whether simply inflated, or deeply constricted, 
between the seeds. 
5. Colour of unripe pod, whether a shade of green, or bright yellow. 
G. Shape of inflorescence, whether the flowers are arranged along on 
axis, or are terminal and more or less umbellate. 
7. Length of peduncle, whether about (> or 7 inches long, or about 
^ to 1^ inch. 
* Comptcs Rcndus, March 2G, 1900, and Bcr.d. Dciitsch.Bot. Ges., wiii. 1900, p.vS3. 
f For example, I do not understand in what sense de Tries considers that 
Mendel's law can be supposed to apply even to all " nionohybrids," for numerous 
cases are already known in which no such rule is obeyed. 
$ ' Versuche ilb. Pflanzenhybriden ' in the Verh. d. Naturf. Vcr. Briinn, iv. 18G5.. 
