In the animal world, in the wild state, hybrids are rarely if ever
produced, and it is only from the experiments of the naturalists that the
law of hybridity has been explained.
We see the bipartites appear, when two kindred species mix together under
the influence of man, these animals partaking of the qualities of both.
The horse and the ass; the ass, zebra, and hermione; the wolf and the dog;
the dog and the jackal; the goat and the ram; the deer and the axis, &c.,
unite and breed; but these artificial species are not durable, and they
have only limited fecundity. "The mongrels of the dog and the wolf are
sterile from the third generation. The mongrels of the jackal and the dog
are so from the fourth. Moreover, if we unite these mongrels to one of the
two primitive species, they soon revert completely and totally to that
species.
"The mongrel of the dog and jackal contains more of the jackal than the
dog. It has the straight ears, the pendent tail; it does not bark; it is
wild. It is more jackal than dog. This is the first product of the crossed
union of the dog with the jackal. I continue to unite the successive
produce, from generation to generation, with one of the two primitive
roots,--with that of the dog, for example.
"The mongrel of the second generation does not bark yet, but it has the
ears pendent at the tip: it is less wild.
"The mongrel of the third generation barks: it has pendent ears, raised
tail: it is no longer wild. The mongrel of the fourth generation is
entirely dog. Four generations, then, have sufficed to restore one of the
two primitive types--the dog type; and four generations suffice also to
restore the other type--the jackal type. Thus, when the mongrels produced
from the union of two distinct species unite together, either become soon
sterile, or they unite with one of the two primitive stocks, and they soon
revert to this stock; in no case do they yield what may be called a new
species, that is, an intermediate, durable species.
"Whether, then, we consider the external causes,--the succession of time,
years, ages, revolutions of the globe, or internal causes,--that is to
say, the crossing of the species, the species do not alter, do not change,
nor pass from one to the other; the species is fixed." Such are the
conclusions of the admirable efforts of Flourens.
"The imprint of each species," says Buffon, "is a type, the principal
features of which are engraved in characters ineffaceable, and permanent
forever; but all the accessory touches vary; no individual perfectly
resembles another."