producing
offspring which are of a totally different character from themselves,--
the researches of two centuries have led to a different result. That the
grubs found in galls are no product of the plants on which the galls
grow, but are the result of the introduction of the eggs of insects into
the substance of these plants, was made out by Vallisnieri, Reaumur, and
others, before the end of the first half of the eighteenth century. The
tapeworms, bladderworms, and flukes continued to be a stronghold of the
advocates of Xenogenesis for a much longer period. Indeed, it is only
within the last thirty years that the splendid patience of Von Siebold,
Van Beneden, Leuckart, Kuechenmeister, and other helminthologists, has
succeeded in tracing every such parasite, often through the strangest
wanderings and metamorphoses, to an egg derived from a parent, actually
or potentially like itself; and the tendency of inquiries elsewhere has
all been in the same direction. A plant may throw off bulbs, but these,
sooner or later, give rise to seeds or spores, which develop into the
original form. A polype may give rise to Medusae, or a pluteus to an
Echinoderm, but the Medusa and the Echinoderm give rise to eggs which
produce polypes or glutei, and they are therefore only stages in the
cycle of life of the species.

But if we turn to pathology, it offers us some remarkable approximations
to true Xenogenesis.

As I have already mentioned, it has been known since the time of
Vallisnieri and of Reaumur, that galls in plants, and tumours in cattle,
are caused by insects, which lay their eggs in those parts of the animal
or vegetable frame of which these morbid structures are outgrowths.
Again, it is a matter of familiar experience to everybody that mere
pressure on the skin will give rise to a corn. Now the gall, the tumour,
and the corn are parts of the living body, which have become, to a
certain degree, independent and distinct organisms. Under the influence
of certain external conditions, elements of the body, which should have
developed in due subordination to its general plan, set up for themselves
and apply the nourishment which they receive to their own purposes.

From such innocent productions as corns and warts, there are all
gradations to the serious tumours which, by their mere size and the
mechanical obstruction they cause, destroy the organism out of which they
are developed; while, finally, in those terrible structures kn