Studies in the Theory of Descent, Volume II — Themes and Context

Independence of Metamorphic Stages

Weismann’s central thesis—that larval and imaginal structures vary independently—is built on empirical cases. He notes that in all seasonally dimorphic species, caterpillars of butterflies “often widely different in the colour and mar” (the excerpt breaks off) remain unchanged, implying that adult variation does not ripple backward. He explicitly states that “no such cases have as yet been proved” of correlative structural change between stages. This independence is not absolute but “very high,” and he distinguishes it from indirect effects like nurse-breeding, where the larva’s parasitic lifestyle forces the adult to develop a sharp ovipositor. Here, external selection pressures, not internal growth laws, create the link.

Reversion vs. Larval Reproduction

Weismann takes issue with Duméril’s interpretation of the Triton (salamander) case, which Duméril had compared to Wagner’s Cecidomyia larva. Weismann insists that the Triton’s reproductive larvae represent “reversion to an older phyletic stage,” not true larval reproduction. In Cecidomyia, the larvae’s asexual propagation is a “secondary character” acquired later, as many congeneric species lack it. He emphasizes that these larvae “could never have played the part of the final stage of the ontogeny” nor possessed sexual reproduction. The distinction hinges on phyletic history: Triton larvae revert to an ancestral form, while Cecidomyia larvae are a contemporary adaptation, not a transitional stage.

The Precision of ‘Adaptation’

Weismann critiques Haeckel and others for loosening the term “adaptation” beyond Darwin and Wallace’s original sense. He argues that adaptation should denote “a gradual bodily transformation appearing in the course of generations” via natural selection, not a sudden functional shift. He cites Forel’s observation of freshwater Pulmonifera using lungs as gills at depth, with no morphological change—calling this “physiological adaptation” at best. Weismann warns that if such cases are labeled adaptation, the word loses its technical value. He insists that the Triton’s reversion and the Axolotl’s occasional transformation into Amblystoma are not true adaptations, as they involve no gradual selection-driven change.

Readers should attend to Weismann’s method: he builds arguments by dissecting individual cases (seasonal dimorphism, nurse-breeding, Triton, Cecidomyia) and then generalizing. His voice is precise, often qualifying claims with “almost” or “scarcely,” and he engages directly with contemporaries like Haeckel and Duméril. The translation by Meldola preserves this rigor, though the text’s dense paragraphs and footnoted references reward slow reading. Darwin’s prefatory notice signals the work’s significance, but Weismann’s own reasoning—especially his insistence on terminological clarity—remains the core.

That rainy afternoon, Weismann’s insistence that caterpillar and butterfly lead separate hereditary lives felt almost consoling—each stage bearing its own quiet logic. Later, I found myself reaching for the complete set, wanting to sit with the earlier arguments again. Studies in the Theory of Descent (Volumes 1 and 2) — Themes and Context turned the afternoon into a longer conversation, one I was in no hurry to end.