46 THE ORIGIN OF SPECIES I 
in the great majority of cases, this touchstone if 
species is wholly inapplicable. . 
The constitution of many wild animals is so 
altered by confinement that they will not breed 
even with their own females, so that the negative 
results obtained from crosses are of no value; and 
the antipathy of wild animals of different species. 
for one another, or even of wild and tame members 
of the same species, is ordinarily so great, that it 
is hopeless to look for such unions in Nature. 
The hermaphrodism of most plants, the difficulty 
in the way of insuring the absence of their own 
or the proper working of other pollen, are obsta- 
cles of no less magnitude in applying the test to 
them. And, in both animals and plants, is super- 
added the further difficulty, that experiments 
must be continued over a long time for the purpose 
of ascertaining the fertility of the mongrel or 
hybrid progeny, as well as of the first crosses from 
which they spring. 
Not only do these great practical difficulties lie 
in the way of applying the hybridisation test, but 
even when this oracle can be questioned, its replies 
are sometimes as doubtful as those of Delphi, 
For example, cases are cited by Mr. Darwin, of 
plants which are more fertile with the pollen of 
another species than with their own; and there 
are others, such as certain Fuci, the male element 
of which will fertilise the ovule of a plant of 
distinct species, while the males of the latter 
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