XV. Double adaptations. 430 Analogy between double adaptations and anomalous middle races. _Polygonum amphibium_. Alpine plants. _Othonna crassifolia_. Leaves in sunshine and shadow. Giants and dwarfs. Figs and ivy. Leaves of seedlings. E. MUTATIONS. XVI. Origin of the peloric toad-flax. 459 Sudden and frequent origin in the wild state. Origin in the experiment-garden. Law of repeated mutations. Probable origin of other pelories. [xv] XVII. The production of double flowers. 488 Sudden appearance of double flowers in horticulture. Historical evidence. Experimental origin of _Chrysanthemum segetum plenum_. Dependency upon nourishment. Petalody of stamens. XVIII New species of _Oenothera_. 516 Mutations of _Oenothera lamarckiana_ in the wild state near Hilversum. New varieties of _O. laevifolia_, _O. brevistylis_, and _O. nanella_. New elementary species, _O. gigas_, _O. rubrinervis_, _albida_, and _oblonga_. _O. lata_, a pistillate form. Inconstancy of _O. scintillans_. XIX. Experimental pedigree-cultures. 547 Pedigree of the mutative products of _Oenothera lamarckiana_ in the Botanical Garden at Amsterdam. Laws of mutability. Sudden and repeated leaps from an unchanging main strain. Constancy of the new forms. Mutations in all directions. XX. Origin of wild species and varieties. 576 Problems to solve. _Capsella heegeri_. _Oenothera biennis cruciata_. _Epilobium hirsutum cruciatum_. _Hibiscus Moscheutos_. Purple beech. Monophyllous strawberries. Chances of success with new mutations. XXI. Mutations in horticulture. 604 _Chelidonium majus lacinatum_. Dwarf and spineless varieties. Laciniate leaves. Monophyllous and broom-like varieties. [xvi] Purple leaves. _Celosia_. Italian poplar. Cactus dahlia. Mutative origin of _Dahlia fistulosa_, and _Geranium praetense_ in the experiment-garden. XXII. Systematic atavism. 630 Reappearance of ancestral characters. _Primula acaulis umbellata_. Bracts of crucifers. _Zea Mays cryptosperma_. Equisetum, _Dipsacus sylvestris torsus_. Tomatoes. XXIII. Taxonomic anomalies. 658 Specific characters occurring in other cases as casual anomalies. _Papaver bracteatum monopetalum_. _Desmodium gyrans_ and monophyllous varieties. Peltate leaves and ascidia. Flowers on leaves. Leaves. _Hordeum trifurcatum_. XXIV. Hypothesis of periodical mutations. 686 Discovering mutable strains. Periods of mutability and constancy. Periods of mutations. Genealogical trees. Limited life-time of the organic kingdom. F. FLUCTUATIONS.