y at precisely the same time. Even
where such synchronism does prevail, so that a cross impregnation is
effected, the chances are very numerous against the establishment of a
hybrid race.

If we consider the vegetable kingdom generally, it must be recollected
that even of the seeds which are well ripened, a great part are either
eaten by insects, birds, and other animals, or decay for want of room
and opportunity to germinate. Unhealthy plants are the first which are
cut off by causes prejudicial to the species, being usually stifled by
more vigorous individuals of their own kind. If, therefore, the relative
fecundity or hardiness of hybrids be in the least degree inferior, they
cannot maintain their footing for many generations, even if they were
ever produced beyond one generation in a wild state. In the universal
struggle for existence, the right of the strongest eventually prevails;
and the strength and durability of a race depend mainly on its
prolificness, in which hybrids are acknowledged to be deficient.

_Centaurea hybrida_, a plant which never bears seed, and is supposed to
be produced by the frequent intermixture of two well-known species of
Centaurea, grows wild upon a hill near Turin. _Ranunculus lacerus_, also
sterile, has been produced accidentally at Grenoble, and near Paris, by
the union of two Ranunculi; but this occurred in gardens.[832]

_Mr. Herbert's experiments._--Mr. Herbert, in one of his ingenious
papers on mule plants, endeavors to account for their non-occurrence in
a state of nature, from the circumstance that all the combinations that
were likely to occur have already been made many centuries ago, and have
formed the various species of botanists; but in our gardens, he says,
whenever species, having a certain degree of affinity to each other, are
transported from different countries, and brought for the first time
into contact, they give rise to hybrid species.[833] But we have no
data, as yet, to warrant the conclusion, that a single permanent hybrid
race has ever been formed, even in gardens, by the intermarriage of two
allied species brought from distant habitations. Until some fact of this
kind is fairly established, and a new species, capable of perpetuating
itself in a state of perfect independence of man, can be pointed out, it
seems reasonable to call in question entirely this hypothetical source
of new species. That varieties do sometimes spring up from
cross-breeds, in a natural