So it is with numerous plants; for the pollen is often
mature and shed, or is mechanically protruded, before
the flower’s own stigma is ready; and such flowers ab-
solutely require the presence of another hermaphro-
dite for sexual union. But with the cowslip and various
other species of Primula there is this wide difference,
that one individual, though it can fertilise itself im-
perfectly, must unite with another individual for full
fertility ; it cannot, however, unite with any other in-
dividual in the same manner as an hermaphrodite
plant can unite with any other one of the same species ;
or as one snail or earth-worm can unite with any other
hermaphrodite individual. On the contrary, an indi-
vidual belongmg to one form of the cowslip in order
to be perfectly fertile must unite with one of the other
form, just as a male quadruped must and can unite
only with the female.
I have spoken of the legitimate unions as being
fully fertile; and I am fully justified in doing so, for
flowers artificially fertilised in this manner yielded
rather more seeds than plants naturally fertilised in
a state of nature. The excess may be attributed to
the plants having been grown separately in good soil.
With respect to the illegitimate unions, we shall best
appreciate their degree of lessened fertility by the


- following facts. Gartner estimated the sterility of the


unions between distinct species,* in a manner which
allows of a strict comparison with the results of the


* ‘Versuche iiber die Bastarderzeugung,’ 1819, p. 216.
