Dimorphism of one stage only, 402. Independent variability of the stages (heterochronic variability), 403. Constancy and variability are not inherent properties of certain forms of marking, 407. Heterochronic variability is not explained by assuming a phyletic vital force, 410. Rarity of greater variability in pupæ. Greater variability more common among caterpillars than among the imagines. Causes of this phenomenon, 412. Apparent independent variability of the single larval stages. Waves of variability, 416. _Saturnia Carpini_ an instance of _secondary_ variability, 419. Causes of the exact correlation between the larval stages and its absence between the larva and imago, 429. II. _Does the Form-relationship of the Larva coincide with that of the Imago?_ p. 432. Family groups, 432. Families frequently completely congruent, 435. Exception offered by the _Nymphalidæ_, 435. In transitional families the larvæ also show intermediate forms, 441. Genera; almost completely congruent; the Nymphalideous genera can be based on the structure of the larvæ, 444. So also can certain sub-genera, as _Vanessa_, 445. Incongruence in _Pterogon_, 450. Species; incongruence very common; _S. Ocellatus_ and _Populi_, 451. Species of _Deilephila_ show a nearer form-relationship as imagines than as larvæ, 454. Systemy not only the expression of morphological relationship, 455. Varieties; incongruence the rule; seasonal dimorphism; climatic varieties; dimorphism of caterpillars; local varieties of caterpillars, 456. Result of the investigation, 458. Causes of incongruence, 460. A phyletic vital force does not explain the phenomena, 461. This force is superfluous, 464. III. _Incongruences in other Orders of Insects_, p. 481. Hymenoptera. The imagines only possess ordinal characters, 481. Double incongruence: different distance and different group-formation, 483. Diptera, 488. The larvæ form two types depending on different modes of life, 489. The similarity of the grub-like larvæ of Diptera and Hymenoptera depends upon convergence, 494. These data again furnish strong arguments against a phyletic vital force, 496. The tribe _Aphaniptera_, 498. Results furnished by the form-relationship of Diptera and Hymenoptera, 499. Difference between typical and non-typical parts transient, 501. IV. _Summary and Conclusion,_ p. 502. First form of incongruence, 503. Second form of incongruence, 506. General conclusion as to the elimination of a phyletic vital force, 511. Parallelism with the transformation of systems of organs, 513. _Appendix I._, p. 520. Additional notes on the Ontogeny, Phylogeny, &c., of Caterpillars. Ontogeny of _Noctua_ larvæ, 520. Additional descriptions of Sphinx-larvæ, 521. Retention of the subdorsal line by ocellated larvæ, 529. Phytophagic variability, 531. Sexual variation in larvæ, 534. _Appendix II._, p. 536. _Acræa_ and the _Maracujà_ butterflies as larvæ, pupæ, and imagines, 536. _Explanation of the Plates_, p. 546. =Part III.=