ther from
them. On the other hand, the theory appears to us all the more plausible,
that every new species came into existence on that stage which is the most
nearly related to it, and which was already in existence. If we add
further, that the two old maxims of the natural scientists, _omne vivum ex
ovo_ and _omne ovum ex ovario_, have not been invalidated, in spite of all
the searching for a _generatio aequivoca_, and that, even if the origination
of the lowest organisms out of the inorganic could in future be {63}
proved, yet the truth of these maxims for all the higher organized
individuals is established as a fact without exception. Moreover, if we
take into consideration the fact that we can not at all imagine either the
origin or the first development of a higher animal or a human organism
without the protecting integument and the nourishing help of a mother's
womb, we may venture to say that each and every attempt to render the
origin of the first individuals of the higher species conceivable, leads of
necessity to the descent theory. We have either to reject, once for all,
such an attempt, as an unscientific playing with impossibilities, or to
accept the idea of descent. It is certainly the lasting merit of Darwin,
even if his whole structure of proofs should in the course of time show
itself weak, that he not only had the courage (as others had before him),
but also inspired scientists with the courage to trace the idea of a
descent of species in a scientific way.

To be sure, so long as we have no other proof of the descent theory than
the circumstance that we can imagine it, it will continue to be nothing
more than an ingenious hypothesis. We have, therefore, to look to the realm
of nature for more direct proofs; and we are there furnished with them.
They are presented to us by geology in connection with the botanical and
zooelogical systems, by geology in connection with vegetable and animal
geography, by comparative anatomy, and by the history of the embryonic
development of animals.

_Geology_ finds in the strata of the crust of the globe a large number of
extinct plants and animals of extraordinary variety; but all of them,
however much they {64} may differ from the organisms of to-day, are
completely in harmony with the _botanical_ and _zooelogical systems_ in
which we divide the still living organisms. Not only have by far the most
of the now extinct genera and species their family and stem-companions,