23. It excludes Panmixia and thus prevents disused 291 organs from becoming rudimentary 24. Final considerations 294 APPENDICES. I. Further considerations which oppose Nägeli’s 298 explanation of Transformation as due to internal causes II. Nägeli’s Explanation of Adaptation 300 III. Adaptations in Plants 308 IV. On the Supposed Transmission of Acquired 310 Characters 1. Brown-Séquard’s experiments on Guinea-pigs 310 2. A case which at first sight appears to prove 320 the transmission of acquired characters V. On the Origin of Parthenogenesis 323 VI. W. K. Brooks’ Theory of Heredity 326 ------------------------------------------------------------------------ V. THE SIGNIFICANCE OF SEXUAL REPRODUCTION IN THE THEORY OF NATURAL SELECTION. During the quarter of a century which has elapsed since Biology began to occupy itself again with general problems, at least one main fact has been made clear by the united labours of numerous men of science, viz. the fact that the Theory of Descent, the idea of development in the organic world, is the only conception as to the origin of the latter, which is scientifically tenable. It is not only that, in the light of this theory, numerous facts receive for the first time a meaning and significance; it is not only that, under its influence, all the ascertained facts can be harmoniously grouped together; but in some departments it has already yielded the highest results which can be expected from any theory, it has rendered possible the prediction of facts, not indeed with the absolute certainty of calculation, but still with a high degree of probability. It has been predicted that man, who, in the adult state, only possesses twelve pairs of ribs, would be found to have thirteen or fourteen in the embryonic state: it has been predicted that, at this early period in his existence, he would possess the insignificant remnant of a very small bone in the wrist, the so-called _os centrale_, which must have existed in the adult condition of his extremely remote ancestors. Both predictions have been fulfilled, just as the planet Neptune was discovered after its existence had been predicted from the disturbances induced in the orbit of Uranus.