80 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY, 
is found in South Africa and Madagascar, species of the latter are natives 
of the Malay Archipelago, and E. Asia ; so that in this case constitutional 
affinity does not correspond with morphological resemblances. 
That Streptocarpus should fail to be crossed by Gesnera was more 
likely, as, besides being natives of different hemispheres, they are distantly 
located in the ‘Genera Plantarum.’ 
Of sub-genera that failed to cross, Mimulus x Diplacus may be 
mentioned. 
Though Hpidendrum radicans has been successfully crossed as male 
with Sophronitis grandiflora female, yet S. violacea has been crossed with 
E. radicans and E. Obrienianum without result. 
CoNSTITUTIONAL AFFINITY AND SteRILITy.—Admitting the fact that 
the closer agreement there may be between the forms of two species, the 
more likely is it that they will cross, yet it is not universally true; so 
that, leaving ‘morphological affinity’ out of the question, we have to 
depend on, for want of a better expression, what one may call “ constitu 
tional affinity,’’ cautioning the reader that this is a phrase which covers 
our profound ignorance of the true nature of physiological aftinity ! 
Dean Herbert observes: ‘Some crosses are sterile and some quite 
fertile, without any apparent reason, except the greater or less approxima- 
tion of constitution in the parents; and that the cross-bred plant which 
has seemed for a long course of years to be absolutely sterile becomes 
under some circumstances productive.” * 
The last sentence is important, for it introduces another fact, that 
sterility and fertility are not absolute features, but vary in the same 
plant according to circumstances; and it applies to self-fertilisation as 
well as crosses and hybrids. Thus, Darwin found that the dimorphic 
forms of Linwm perenne were self-sterile ; but Mr. T. Meehan, of German- 
town, Philadelphia, had one form only in his garden, which never set 
seed for fourteen years; yet, then, one branch bore flowers which became 
homomorphic and immediately fruited. Under cultivation Primulas of 
various kinds, as P. sinensis, can become self-fertile iv a similar way.T 
Eschscholtzia californica was self-sterile in Brazil, bus acquired great 
self-fertility in England in three years—nearly 87 per cent. + 
Dean Herbert says elsewhere: ‘ Experiments have confirmed the 
view to such a degree as to make it almost certain that the fertilisation 
of the hybrid or mixed offspring depends more upon the ¢oustitution than 
the closer botanical affinity of the parents.” $ 
He illustrates this by the genus Crinwm, showing that while certain 
nearly—i.e. morphologically—allied forms are difficult to produce fertile 
hybrids, others so distinct as to have been placed in different genera do 
so, the interpretation being that the latter were aquatic plants, and 
therefore presumably of the same constitution ; whereas in the former 
case the ineffectual cross was made between an aquatic and a terrestrial 
form frequenting dry localities. 
This hybrid between C. capense (aquatic) and C. scabrum continued 
for sixteen years to be sterile, but ‘‘ produced one good seed in 1884, and 
* Amaryllidacee, p. 340. 
t Origin of Floral Structures, p. 209. 
g 
Ibid. p. 320. 
Amaryllidacee, p. 342. 
