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, and many even their genera and species companions, in the living world, but also those genera whose nearer relations are now extinct--as, for instance, the club-moss-trees, the trilobites, the ammonites, the belemnites, the sauria, the nummulites,--show still a very perceptible relationship with living genera, and can be quite accurately included in the botanical and zooelogical systems; nay, they even fill up gaps in it. The anatomical, morphological, and, so far as we can judge, the physiological and biological relationship of the fossil with living organisms, is so great and comprehensive that in the present state of science a systematic botany or zooelogy, that should only treat of the fossils or of living organisms alone, would be imperfect. But the relationship of the fossil organisms with the natural systems of botany and zooelogy is apparent not only in this respect, but also in the fact that the single species during the long periods of time which are shown by geology to have elapsed, came into existence in a series, which again pretty closely corresponds to the natural system of the organic kingdoms; and that the fossil representatives of all classes and families, the nearer they come to the present world, appear the more nearly related to the living organisms, so that the fauna and flora of the ante-human time are lost in those of the human period by transitions gliding from the one to the other. For instance, in the Miocene formation of the tertiary epoch {65} we find thirty per cent. of species still living to-day; in the Pliocene, even sixty to eighty per cent., and toward its end even about ninety-six per cent. of species which are identical with those now living.