rable to the ideas of descent and of evolution,
rejecting only the selection theory, are at one in silent or open
acknowledgment of this limit of our knowledge, be it permanent or
temporary.

But now the question arises: does the search after these agencies
henceforth remain the exclusive task of natural science, and have we
therefore simply to wait and see whether it will succeed in finding them?
or have we to look for the answer to these questions, which natural science
can no longer give, in another science--namely, philosophy? The first
question we will have to answer in the affirmative, the second in the
negative. It is certainly understood that _metaphysical_ principles must
underlie all _physical_ appearances; and the right to define these
principles, so far as they can be known, is willingly conceded to
philosophy by the scientists, with the exception of those of materialistic
and naturalistic tendencies. This mutual re-approaching of philosophy and
natural science is one of the most gratifying, and, to both, most fruitful
evidences of the intellectual work of the present generation. But these
metaphysical principles themselves become cognizable only when the physical
effects, whose cause they are, become accessible to our knowledge; and
every attempt to find them _a priori_, or only to extend them _a priori_,
will always fail through the opposition of empirical facts; or even if this
attempt accommodates itself to the existing state of knowledge at a given
time, it will always be overcome by the {110} progress of the empirical
sciences. In the most favorable case, it can claim the value of a
hypothesis which has to be put to the proof, whether it can be empirically
confirmed and whether we can successfully operate with it in knowing the
world of realities. But herewith it leaves the realm of pure philosophy,
and makes the question of its right to exist dependent upon the decision of
natural science.

Since the decline of the doctrines of nature held by Schelling, Steffens,
and Hegel, there has come to our knowledge, from the domain of philosophy,
but one earnest attempt to explain the origin and development of organisms
down to the concrete differences between single types, classes, and even
orders and families, from one single metaphysical principle; and this
attempt has been made by an antagonist of the descent doctrine. K. Ch.
Planck, in "Seele und Geist, oder Ursprung, Wesen und Thaetigkeitsform der
physis