The Lamarckians were right when they maintained that the factor for which hitherto the name of natural selection had been exclusively reserved, viz., _personal_ selection, was insufficient for the explanation of the phenomena. They were also right when they declared that panmixia in the form in which until recently I held the theory was also insufficient to explain the degeneration of parts that had grown useless, but they {65} erred when they ascribed hereditary effects to the selection-processes which are enacted among the parts of the body (Wilhelm Roux) and which are rightly regarded as the results of functioning. And they did this, moreover, as they themselves admit, not because the facts of heredity directly and unmistakably required it, but because they saw no other possibility of explaining many phenomena of transformation. I am fain to relinquish myself to the hope that now after another explanation has been found, a reconciliation and unification of the hostile views is not so very distant, and that then, we can continue our work together on the newly laid foundations. That the application of the Malthusian principle was thoroughly justified is now clear. _The entire process of the development of living forms is guided by this principle._ The struggle for existence, _videlicet_, for food and propagation, takes place at all the stages of life between all orders of living units from the biophores recently disclosed upwards to the elements that are accessible to direct observation, to the cells, and still higher up, to individuals and colonies. Consequently, in all the divers orders of biological units lying between the two extremes of biophores and colonies, the modifications must be controlled by selective processes; therefore, these govern every change of living forms no matter what its significance, and bring it about that the latter fit their conditions of life as wax does the mould; and the various stages of these processes, as enacted between the divers orders of biological units, in all organisms not absolutely simple, are involved in incessant and mutual interaction. The three principal stages of selection, that of {66} _personal_ selection[23] as it was enunciated by Darwin and Wallace, that of _histonal_ selection as it was established by Wilhelm Roux in the form of a "struggle of the parts," and finally that of _germinal selection_ whose existence and efficacy I have endeavored to substantiate in this article--these are the factors that have co-operated to maintain the forms of life in a constant state of viability and to adapt them to their conditions of life, now modifying them _pari passu_ with their environment, and now maintaining them on the stage attained, when that environment is not altered.