60 JOUENAL OF THE EOYAL HORTICULTURAL SOCIETY. 
stated also it can only be true where reciprocal crossings lead to the- 
same result. Moreover, it can only be tested when there is no sensible 
diminution in fertility on crossing. 
Upon the appearance of de Vries' papers announcing the ''redis- 
covery " and confirmation of Mendel's law and its extension to a great 
number of cases two other observers came forward and independently 
describe series of experiments fully confirming Mendel's work. Of these 
papers the first is that of Correns,* who repeated Mendel's original 
experiment with Peas having seeds of different colours. The second is a 
long and very valuable memoir of Tschermak,t which gives an account 
of elaborate researches into the results of crossing a number of varieties 
of Pisum sativum. These experiments were in many cases carried out 
on a large scale, and prove the main fact enunciated by Mendel beyond 
any possibility of contradiction. Both Correns (in regard to Maize) and 
Tschermak in the case of P. sativum have obtained further proof that 
Mendel's law holds as well in the case of varieties differing from each 
other in ttvo characters, one of each being dominant, though of course a 
more complicated expression is needed in such cases. t 
That we are in the presence of a new principle of the highest im- 
portance is, I think, manifest. To what further conclusions it may lead 
us cannot yet be foretold. But both Mendel and the authors who have 
followed him lay stress on one conclusion, which will at once suggest itself 
to anyone who reflects on the facts. For it will be seen that the results 
are such as we might expect if it is imagined that the cross-bred plant pro- 
duced pollen grains and ovules, each of which bears onlyo7z<3 of the alter- 
native varietal characters and not both. If this were so, and if on the 
average the same number of pollen grains and ovules partook of each of 
the two characters, it is clear that on a random assortment of pollen 
grain and ovules Mendel's law would be obeyed. For 25 per cent, of 
" dominant " pollen grains would unite with 25 per cent. " dominant " 
ovules ; 25 per cent. " recessive " pollen grains would similarly unite with 
25 per cent, "recessive" ovules; while the remaining 50 per cent, of 
each kind would unite together. It is this consideration which leads 
both de Vries and Mendel to assert that these facts of crossing prove 
that each ovule and each pollen grain is pure in respect of each character 
to which the law applies. It is highly desirable that varieties differing 
in the form of their pollen should be made the subject of these experi- 
ments, for it is quite possible that in such a case strong confirmation of 
this deduction might be obtained. 
As an objection to this deduction, however, it is to be noted that 
though true intermediates did not occur, yet the degrees in which the 
characters appeared did vary in degree, and it is not easy to see how the 
hypothesis of perfect purity in the reproductive cells can be supported in 
such cases. Be this, however, as it may, there is no doubt we are be- 
ginning to get new lights of a most valuable kind on the nature of 
* Ber. cUut. Bot. Cos., 1900, xviii. p. 158. 
t Zeitschr. f. d. landio. Versnchswesen in Oesierr., 1900, iii. p. 465. 
J Tschermak's investigations were besides directed to a re-examination of the 
question of the absence of beneficial results on cross-fertilising P. sativum, a subject 
already much investigated by Darwin, and upon this matter also important further 
evidence is given in great detail. 
