When we look at one of the lowest forms of life, such as an Amœba or a single-celled Alga, and reflect that, according to the theory of evolution, the whole realm of creation as we see it now, with Man at its head, has evolved from similar or perhaps even smaller and simpler organisms, it seems at first sight a monstrous assumption, and one which quite contradicts our simplest and most certain observations. For what is more certain than that the animals and plants around us remain the same, as long as we can observe them, not through the lifetime of an individual only, but through centuries, and in the case of many species, for several thousand years? This being so, it is intelligible enough that the doctrine of evolution, on its first emergence at the end of the eighteenth century, was received with violent opposition, not on the part of the laity only, but by the majority of scientific minds, and instead of being followed up, was at first opposed, then neglected, and finally totally forgotten, to spring up anew in our own day. But even then a host of antagonists ranged themselves against the doctrine, and, not content with loftily ignoring it, made it the subject of the most violent and varied attacks. This was the state of affairs when, in 1858, Darwin's book on _The Origin of Species_ appeared, and hoisted the flag of evolution afresh. The struggle that ensued may now be regarded as at an end, at least as far as we are concerned--that is, in the domain of science. The doctrine of descent has gained the day, and we can confidently say that the Evolution theory has become a permanent possession of science that can never again be taken away. It forms the foundation of all our theories of the organic world, and all further progress must start from this basis. In the course of these lectures, we shall find at every step fresh evidence of the truth of this assertion, which may at first seem all too bold. It is not by any means to be supposed that the whole question in regard to the transformation of organisms and the succession of new forms of life has been answered in full, or that we have now been fortunate enough to solve the riddle of life itself. No! whether we ever reach that goal or not, we are a long way from it as yet, and even the much easier problem, how and by what forces the evolution of the living world has proceeded from a given beginning, is far from being finally settled; antagonistic views are still in conflict, and there is no arbitrator whose authoritative word can decide which is right. The _How?_ of evolution is still doubtful, but not the _fact_, and this is the secure foundation on which we stand to-day: The world of life, as we know it, has been evolved, and did not originate all at once.