ON THE FINAL CAUSES OF TRANSFORMATION (_continued_). =III.= THE TRANSFORMATION OF THE MEXICAN AXOLOTL INTO AMBLYSTOMA. _Introduction_, p. 555. _Experiments_, 558. Significance of the facts, 563. The Axolotl rarely or never undergoes metamorphosis in its native country, 565. North American Amblystomas, 570. Does the exceptional transformation depend upon a phyletic advancement of the species? 571. Theoretical bearing of the case, 574. Differences between Axolotl and Amblystoma, 575. These are not correlative results of the suppression of the gills, 578. Explanation by reversion, 581. Cases of degeneration to a lower phyletic stage: Filippi’s sexually mature “_Triton_ larvæ,” 583. Analogous observations on _Triton_ by Jullien and Schreibers, 591. The sterility of the artificially produced Amblystomas tells against the former importance of the transformation, 594. It is not opposed to the hypothesis of reversion, 596. Attempted explanation of the sterility from this point of view, 597. Causes which may have induced reversion in the hypothetical Mexican Amblystomas, 600. Saltness of the water combined with the drying up of the shores by winds, 604. Consequences of the reversion hypothesis, 609; Systematic, 609; an addendum to the “fundamental biogenetic law,” 611; General importance of reversion, 612. _Postscript_; dryness of the air the probable cause of the assumed reversion of the Amblystoma to the Axolotl, 613. _Addendum_, 622. =IV.= ON THE MECHANICAL CONCEPTION OF NATURE. _Introduction_, p. 634. Results of the three foregoing essays: denial of a phyletic vital force, 634. Application of these results to inductive conclusions with reference to the organic world in general, 636. The assumption of such a force is opposed to the fundamental laws of natural science, 637. The “vital force” of the older natural philosopher, 640. Why was the latter abandoned? Commencement of a mechanical theory of life, 642. I. _Are the Principles of the Selection Theory Mechanical?_ p. 645.