the
following facts. When fertility is lost from a wholly different cause,
namely, from hybridism, there is a strong tendency, as Gaertner[436]
affirms, for flowers to become double, and this tendency is inherited.
Moreover it is notorious that with hybrids the male organs become sterile
before the female organs, and with double flowers the stamens first become
{172} foliaceous. This latter fact is well shown by the male flowers of
dioecious plants, which, according to Gallesio,[437] first become double.
Again, Gaertner[438] often insists that the flowers of even utterly sterile
hybrids, which do not produce any seed, generally yield perfect capsules or
fruit,--a fact which has likewise been repeatedly observed by Naudin with
the Cucurbitaceae; so that the production of fruit by plants rendered
sterile through any other and distinct cause is intelligible. Koelreuter
has also expressed his unbounded astonishment at the size and development
of the tubers in certain hybrids; and all experimentalists[439] have
remarked on the strong tendency in hybrids to increase by roots, runners,
and suckers. Seeing that hybrid plants, which from their nature are more or
less sterile, thus tend to produce double flowers; that they have the parts
including the seed, that is the fruit, perfectly developed, even when
containing no seed; that they sometimes yield gigantic roots; that they
almost invariably tend to increase largely by suckers and other such
means;--seeing this, and knowing, from the many facts given in the earlier
parts of this chapter, that almost all organic beings when exposed to
unnatural conditions tend to become more or less sterile, it seems much the
most probable view that with cultivated plants sterility is the exciting
cause, and double flowers, rich seedless fruit, and in some cases
largely-developed organs of vegetation, &c., are the indirect
results--these results having been in most cases largely increased through
continued selection by man.

* * * * *

{173}

CHAPTER XIX.

SUMMARY OF THE FOUR LAST CHAPTERS, WITH REMARKS ON HYBRIDISM.

ON THE EFFECTS OF CROSSING--THE INFLUENCE OF DOMESTICATION ON
FERTILITY--CLOSE INTERBREEDING--GOOD AND EVIL RESULTS FROM CHANGED
CONDITIONS OF LIFE--VARIETIES WHEN CROSSED NOT INVARIABLY FERTILE--ON
THE DIFFERENCE IN FERTILITY BETWEEN CROSSED SPECIES AND
VARIETIES--CONCLUSIONS WITH RESPECT TO HYBRIDISM--LIGHT THROWN ON