denies the existence of natural classifications of other kinds.

And, after all, is it quite so certain that a genetic relation may not
underlie the classification of minerals? The inorganic world has not
always been what we see it. It has certainly had its metamorphoses, and,
very probably, a long "Entwickelungsgeschichte" out of a nebular
blastema. Who knows how far that amount of likeness among sets of
minerals, in virtue of which they are now grouped into families and
orders, may not be the expression of the common conditions to which that
particular patch of nebulous fog, which may have been constituted by
their atoms, and of which they may be, in the strictest sense, the
descendants, was subjected?

It will be obvious from what has preceded, that we do not agree with
Professor Koelliker in thinking the objections which he brings forward
so weighty as to be fatal to Darwin's view. But even if the case were
otherwise, we should be unable to accept the "Theory of Heterogeneous
Generation" which is offered as a substitute. That theory is thus
stated:--

"The fundamental conception of this hypothesis is, that, under the
influence of a general law of development, the germs of organisms
produce others different from themselves. This might happen (1) by
the fecundated ova passing, in the course of their development,
under particular circumstances, into higher forms; (2) by the
primitive and later organisms producing other organisms without
fecundation, out of germs or eggs (Parthenogenesis)."

In favour of this hypothesis, Professor Koelliker adduces the well-known
facts of Agamogenesis, or "alternate generation;" the extreme
dissimilarity of the males and females of many animals; and of the
males, females, and neuters of those insects which live in colonies: and
he defines its relations to the Darwinian theory as follows:--

"It is obvious that my hypothesis is apparently very similar to
Darwin's, inasmuch as I also consider that the various forms of
animals have proceeded directly from one another. My hypothesis of
the creation of organisms by heterogeneous generation, however, is
distinguished very essentially from Darwin's by the entire absence
of the principle of useful variations and their natural selection;
and my fundamental conception is this, that a great plan of
development lies at the foundation of the origin of the whole
orga