PKOBLEMS OF HEREDITY, HORTICULTUEAL JNVESTIGATIOX. 59 
shown to be made up of pure dominants and cross-breds in the same 
proportion of one dominant to two cross-breds. 
The process of breaking up into the parent forms is thus continued in 
each successive generation, the same numerical law being followed so far 
.as has yet been observed. 
Mendel made further experiments with Pisum sativum, crossing pairs 
of varieties which differed from each other in tivo characters, and the 
results, though necessarily much more complex, showed that the law 
exhibited in the simpler case of pairs differing in respect of one character 
operated here also. 
Professor de Vries has worked at the same problem in some dozen 
species belonging to several genera, using pairs of varieties characterised 
by a great number of characters : for instance, colour of flowers, stems, or 
fruits, hairiness, length of style, and so forth. He states that in all these 
cases Mendel's law is followed. 
The numbers 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 approxi- 
mation to the numbers demanded by the law is greatest in those cases 
where the largest numbers were used. When we consider, besides, that 
Tschermak and Correns announce definite confirmation in the case of 
Pisum, 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 harmony 
with the facts of hybridisation as hitherto known ; and, if all this is true, 
how is it that others who have so long studied the phenomena of 
hybridisation have not long ago perceived this law "? The answer to this 
question is given by Mendel at some length, and it is, I think, satisfactory. 
He admits from the first that there are undoubtedly cases of hybrids and 
cross-breds which maintain themselves pure and do not break up. Such 
examples are plainly outside the scope of his law\ Next he points out, 
what to anyone who has rightly comprehended the nature of discontinuity 
in variation is well known, that the variations in eacli character must be 
separately regarded. In most experiments in crossing, forms are taken 
which differ from each other in a multitude of characters — some con- 
tinuous, others discontinuous, some capable of blending with their 
•contraries, while others are not. The observer on attemptmg to perceive 
.any regularity is confused by the complications thus introduced. Mendel's 
law, as he fairly says, could only appear in such cases by the use of 
overwhelming numbers, which are beyond the possibilities of practical 
experiment. 
Both these answers should be acceptable to those who have studied 
the facts of variation and have appreciated the nature of Species in the 
light of those facts. That different species should follow difierent laws, 
and that the same law should not apply to all characters alike, is exactly 
what we have every right to expect. It will also be remembered that 
the principle is only declared to apply to discontinuous characters. As 
