Systematic and Sexual Relationship. 597 
species of Diplacus (Mimulus) do not seem to be mor- 
phologically more remote from one another than Tro- 
pacolnm ma jus and Tr. minus, Nicotiana latissima and 
N. Maryland-lea, N. rustica and N. Texana or Pisitm sa- 
tiviim and P. arvense. Nevertheless the results of cross- 
ing in the former cases exhibit all the characters of hy- 
brids, but in the latter those of mongrels." FOCKE sum- 
marizes his conclusions on this subject in the thesis that 
''systematically probable" crosses often miscarry, whilst 
improbable ones sometimes succeed (loc. cit., p. 457). 
Even regarded from this point of view the difference 
between Mendelian and uni-sexual crosses offers itself 
as a criterion for distinguishing between species and 
varieties. 
But sexual affinity does not always give reliable in- 
dications. In the first place GARTNER frequently found 
that fertility, as measured by the number of seeds ripen- 
ing in a capsule, is different in reciprocal crosses. This 
shows that it is not merely determined by the degree of 
relationship, but obviously by some other causes besides. 
The best known of these is the length of the style; and 
the recent investigations of BURCK on the concentration 
and stimulating properties of the fluid secreted by the 
stigma have thrown much valuable light on this subject. 1 
In extreme cases one of the crosses succeeds well, whilst 
the other does not at all, as for instance Mirabilis Jalapa 
X longiflora, Geum urbanum X rivale, Sophronitis X 
Cattleya, and so forth. In the second place some crosses 
do not succeed in spite of a very close apparent relation- 
shin, as for instance between Anagallis arvensis and coe- 
rulea (GARTNER). 
1 W. BURCK, Over de "bewcging der stempels by Mimidus en To- 
ren'ia. Sitzungsber. d. Kon. Akad. d. Wet., Amsterdam, 1901, and 
in previous articles. 
