Refutation of Von Hartmann’s views, 645. Variability, 646. The assumption of unlimited variability no postulate of the selection theory, 647. The acknowledgment of a fixed and directed variability does not necessitate the assumption of a phyletic vital force, 647. Heredity, 657. Useful modifications do not occur only singly, 657. New characters appearing singly may also acquire predominance, 659. A mechanical theory of heredity is as yet wanting, 665. Haeckel’s “Perigenesis of the Plastidule,” 667. Correlation, 670. The “specific type” depends upon the physiological equilibrium of the parts of the organism, 671. The theoretical principles of the doctrine of selection are thus mechanical, 675. Importance of the physical constitution of the organism in determining the quality of variations, 676. All individual variability depends upon unequal external influences, 677. Deduction of the limitability of variation, 682. Deduction of local forms, 686. Parallelism between the ontogenetic and the phyletic vital force, 687. The two are inseparable, 690. II. _Mechanism and Teleology_, p. 694. Von Baer’s exaction from the theory of selection, 694. Justification of his claim, but the impossibility of the co-operation of a metaphysical principle with the mechanism of Nature, 695. _Per saltum_ development (heterogeneous generation), 698. Weakness of the positive basis of this hypothesis, 699. The latter refuted by the impossibility of the co-operation of “heterogeneous generation” with natural selection, 702. The interruption by a metaphysical principle cannot be reconciled with gradual transformation, 705. The metaphysical (teleological) principle can only be conceived of as the ultimate ground of the mechanism of Nature, 709. Value of this knowledge for the harmonious conception of the Universe, 711. Explanation of the spiritual by the assumption of conscious matter, 714. The theory of selection does not necessarily lead to Materialism, 716. INDEX p. 719. STUDIES IN THE THEORY OF DESCENT. Part I. ON THE SEASONAL DIMORPHISM OF BUTTERFLIES. I. THE ORIGIN AND SIGNIFICANCE OF SEASONAL DIMORPHISM. The phenomena here about to be subjected to a closer investigation have been known for a long period of time. About the year 1830 it was shown that the two forms of a butterfly (_Araschnia_) which had till that time been regarded as distinct, in spite of their different colouring and marking really belonged to the same species, the two forms of this dimorphic species not appearing simultaneously but at different seasons of the year, the one in early spring, the other in summer. To this phenomenon the term “seasonal dimorphism” was subsequently applied by Mr. A. R. Wallace, an expression of which the heterogeneous composition may arouse the horror of the philologist, but, as it is as concise and intelligible as possible, I propose to retain it in the present work.