degree, a mystery.

Hybrids.--It is a well-known law of biology that no progeny result from
union of animals of different species. Different varieties of the same
species may in some cases form a fertile union, the result of which
is a cross between its two parents, possessing some of the qualities
of each. The mule is the product of such a union between the horse and
the ass. A curious fact is that the offspring of such unions are
themselves sterile almost without exception. The reason of this is that
they do not produce mature elements of generation. In the mule, the
zoosperms are either entirely absent or else very imperfectly
developed; hence the fact that a colt having a mule for its sire is
one of the rarest of curiosities, though a few instances have been
reported. This is a wise law of nature to preserve the purity of species.

Law of Sex.--If there is a law by which the sex of the developing embryon
is determined, it probably has not yet been discovered. The influence
of the will, the predominant vitality of one or the other of the parents,
and the period at which conception occurs, have all been supposed to
be the determining cause. A German physician some time since advanced
the theory that the two testicles and ovaries produce elements of
different sexual character, the right testicle forming zoosperms
capable of producing only males, and the right ovary producing ova with
the same peculiarity. The left testis and the left ovary he supposed
to form the female elements. He claimed to have proved his theory by
experiments upon animals. Even if true, this theory will not be made
of practical importance. It is, in fact, nothing more than a revival
of an old theory held by physicians who flourished more than two
thousand years ago.

More recently another German physician has advanced the theory that
the sex may be controlled at will by observing the time of fecundation.
He asserts that when fecundation occurs shortly after menstruation,
the result will be a female; but if impregnation occurs later in the
month, and prior to the three or four days preceding the next menstrual
period, a male will almost certainly be produced. This theory was
proposed by Prof. Thury of the academy of Geneva, who claims to have
thoroughly tested it in a great variety of ways, and always with an
affirmative result. Dr. Heitzman, of New York, an instructor in
pathological histology, and an eminent physiologist, informs us that
he h